[PATCH bpf-next v3 0/5] File descriptor interface for BPF streams

[PATCH bpf-next v3 0/5] File descriptor interface for BPF streams

[Kumar Kartikeya Dwivedi]

BPF program stdout and stderr streams can currently be consumed only
through BPF_PROG_STREAM_READ_BY_FD. This requires repeated bpf() calls and
provides no way to block for output or integrate with poll-based event
loops.

Add BPF_PROG_STREAM_OPEN to return a read-only, close-on-exec descriptor
for a program stream. Reads block by default, with an option for
non-blocking mode. poll and epoll report readable data and report hangup
when the program is freed, while allowing buffered data to be drained
before EOF. Stream descriptors retain the stream storage without retaining
the program itself. Waiters are notified through irq_work only when a
publication turns an empty stream readable, as bpf_ringbuf does, so a
program whose stream nobody drains pays for a single notification.

Expose the command through libbpf and add a -w/--wait option to bpftool
prog tracelog that blocks for new output until the program is unloaded or
bpftool is interrupted. The default dump keeps using
BPF_PROG_STREAM_READ_BY_FD and exits once buffered output is drained, so
existing scripts behave the same on old and new kernels. The legacy
command remains supported.

The first patch fixes a pre-existing issue where empty stream writes
allocate elements that escape capacity accounting; the readiness tracking
added later relies on every element carrying data.

See commit logs for details.

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20260924162641.1922423-1-memxor@gmail.com

 * Queue the wakeup irq_work only when a publication turns an empty stream
   readable instead of on every write, as bpf_ringbuf does. (Alexei)
 * Make waiting for stream output opt-in through a new bpftool -w/--wait
   option; the default dump keeps using BPF_PROG_STREAM_READ_BY_FD and
   exits as before. (Alexei)
 * Exit from the bpftool signal handlers like the trace pipe command instead
   of checking a stop flag that a signal arriving before read() blocks would
   miss, and drop the disposition save and restore. (Alexei)
 * Fail bpftool --wait with a clear error on kernels without
   BPF_PROG_STREAM_OPEN instead of degrading into a dump.
 * Test that output published after a full drain notifies an edge-triggered
   epoll waiter.
 * Rebase on bpf-next.

v1 -> v2
v1: https://lore.kernel.org/bpf/20260830093514.4105972-1-memxor@gmail.com

 * Fold the irq_work and readable-counter patches into the interface patch
   so no intermediate state wakes waiters directly or derives readiness
   from the capacity counter. (BPF CI)
 * Accept only BPF_F_STREAM_NONBLOCK; BPF_F_RDONLY is no longer accepted
   since the descriptor is always read-only.
 * Allocate streams only for programs loaded through BPF_PROG_LOAD, not for
   classic BPF filters, JIT subprograms or shim programs.
 * Add a patch that skips zero-length stream writes instead of allocating
   elements that bypass capacity accounting. (Emil, Sashiko)
 * Report EOF only when the stream was already dead before it was found
   empty, so data published right before program teardown is not lost.
   (Sashiko)
 * Document that POLLHUP follows program destruction, not the caller's own
   release, and that it may lag the final reference drop.
 * Drop the llseek operation so lseek fails with ESPIPE like other stream
   descriptors. (Emil)
 * Drop the unreachable length check in the file read path; the VFS caps
   read sizes below INT_MAX.
 * Let __bpf_prog_free() clean up after a failed stream allocation instead
   of freeing the streams twice, and drop redundant zeroing of the stream
   counters after kzalloc().
 * Explain why wakeups are always deferred through irq_work and drop the
   batching rationale. (Emil)
 * Skip irq_work_sync() at teardown for streams that never queued a
   notification; on PREEMPT_RT it waits for an RCU grace period that every
   program free, including cBPF filters, would otherwise pay. (Sashiko)
 * Qualify the POLLIN followed by EAGAIN claim to a single reader. (BPF CI)
 * Handle SIGINT, SIGHUP and SIGTERM in bpftool so a followed stream exits
   cleanly, and document the follow behavior. (Emil, BPF CI)
 * Drop bpftool's program reference once the stream is open so following
   ends with EOF when the program is unloaded, and restore the previous
   signal dispositions afterwards for batch mode. (Sashiko)
 * Report bpftool read errors on the fallback path as well.
 * Test lseek rejection and empty stream writes.
 * Retry the NMI test write on a later sample if the first attempt fails.
 * Drop Emil's Reviewed-by from the patches that changed.
 * Rebase on bpf-next.

Kumar Kartikeya Dwivedi (5):
  bpf: Skip zero-length stream writes
  bpf: Add file descriptor interface for program streams
  libbpf: Add bpf_prog_stream_open()
  bpftool: Add option to wait for program stream output
  selftests/bpf: Test program stream file descriptors

 include/linux/bpf.h                           |  14 +-
 include/uapi/linux/bpf.h                      |  39 ++
 kernel/bpf/core.c                             |   8 +-
 kernel/bpf/stream.c                           | 220 +++++++-
 kernel/bpf/syscall.c                          |  29 ++
 .../bpftool/Documentation/bpftool-prog.rst    |  12 +-
 tools/bpf/bpftool/bash-completion/bpftool     |   2 +-
 tools/bpf/bpftool/main.c                      |   7 +-
 tools/bpf/bpftool/main.h                      |   1 +
 tools/bpf/bpftool/prog.c                      |  62 ++-
 tools/include/uapi/linux/bpf.h                |  39 ++
 tools/lib/bpf/bpf.c                           |  19 +
 tools/lib/bpf/bpf.h                           |  24 +
 tools/lib/bpf/libbpf.map                      |   1 +
 .../testing/selftests/bpf/prog_tests/stream.c | 486 ++++++++++++++++++
 tools/testing/selftests/bpf/progs/stream.c    |  20 +
 16 files changed, 946 insertions(+), 37 deletions(-)


base-commit: 51455305b0d6ab5a3432e0d7858f4e73f397cc95
-- 
2.53.0

[PATCH bpf-next v3 1/5] bpf: Skip zero-length stream writes

[Kumar Kartikeya Dwivedi]

A stream write whose formatted output is empty still allocates a stream
element and links it into the stream log. Capacity accounting only counts
payload bytes, so such elements never count against the stream limit, and a
program that keeps producing empty output allocates elements without bound
until a reader drains them.

Return success without allocating anything when there is nothing to write,
for both bpf_stream_vprintk() and staged writes. Every element in a stream
log then carries at least one byte, which later changes rely on when they
derive read readiness from published byte counts.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Suggested-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/stream.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index 2b80a0599865..9829d1ebce70 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -75,8 +75,13 @@ static void bpf_stream_release_capacity(struct bpf_stream *stream, int len)
 
 static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int len)
 {
-	int ret = bpf_stream_consume_capacity(stream, len);
+	int ret;
+
+	/* Nothing to publish; do not allocate an element for it. */
+	if (!len)
+		return 0;
 
+	ret = bpf_stream_consume_capacity(stream, len);
 	if (ret)
 		return ret;
 
@@ -333,8 +338,8 @@ int bpf_stream_stage_printk(struct bpf_stream_stage *ss, const char *fmt, ...)
 	va_start(args, fmt);
 	len = vscnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
 	va_end(args);
-	/* Exclude NULL byte during push. */
-	ret = __bpf_stream_push_str(&ss->log, buf->buf, len);
+	/* Exclude NULL byte during push; skip empty output entirely. */
+	ret = len ? __bpf_stream_push_str(&ss->log, buf->buf, len) : 0;
 	if (!ret)
 		ss->len += len;
 	bpf_put_buffers();
-- 
2.53.0

[PATCH bpf-next v3 2/5] bpf: Add file descriptor interface for program streams

[Kumar Kartikeya Dwivedi]

The existing BPF_PROG_STREAM_READ_BY_FD command only supports polling a
program stream through repeated bpf() calls. It cannot block for new data
or integrate with poll-based event loops.

Add BPF_PROG_STREAM_OPEN to return a read-only, close-on-exec file
descriptor for a selected program stream. Reads block by default and
BPF_F_STREAM_NONBLOCK, the only accepted flag, provides non-blocking
behavior. poll reports readable data and reports hangup once the program
has been freed. Like pipes and sockets, the descriptor is not seekable and
lseek fails with ESPIPE.

A stream descriptor deliberately does not retain the program. Move each
stream into a separately refcounted allocation so program teardown can mark
it dead and wake descriptor users while outstanding descriptors drain
buffered data safely. Readers sample the dead flag before looking for data,
so EOF is reported only when the stream was already dead before it was
found empty; data published right before teardown is never skipped.

Only programs loaded through BPF_PROG_LOAD get streams. Classic BPF
filters, JIT subprograms and shim programs never write to one, and
kernel-side writers already resolve a subprogram to its main program, so
those programs no longer carry stream state.

Readiness needs its own counter. Stream capacity is charged before
allocation and before an element is published to the stream log, so using
that reservation as the read and poll condition can report readable data
while no element exists: a blocking reader retries instead of sleeping and
a lone non-blocking reader can see POLLIN followed by EAGAIN. Publish bytes
with release ordering after adding elements to the lockless log, use
acquire loads before consuming them or reporting readiness, limit each read
to its readable snapshot and subtract only bytes actually copied. This
keeps the aggregate count correct even when concurrent publishers update it
out of publication order. With several readers on one stream, readiness
remains advisory, as it is for pipes. The capacity counter is kept solely
for enforcing the stream size limit.

Wakeups are always deferred through irq_work. Stream writers run in
whatever context the program runs in: NMI context for perf_event programs,
sections with interrupts disabled inside bpf_spin_lock or rqspinlock
critical sections since bpf_stream_vprintk() is KF_SPINLOCK_SAFE, and
tracing programs attached anywhere in the kernel, including inside the wait
queue and epoll code itself. Waking waiters directly from there can
deadlock, and no cheap context check covers every case: on PREEMPT_RT,
spinlock_t sections do not disable interrupts, so in_nmi() or
irqs_disabled() cannot tell such a program apart from a benign one. Queue
an irq_work item instead, as bpf_ringbuf does.

Queue it only when a publication turns an empty stream readable. Readers
block and pollers wait only after finding the stream empty, and the
readable count never drops below zero because each read is bounded by its
snapshot, so the first publication after such an observation is the one
that makes the count positive, and it is the one that queues the wakeup.
Publications into a stream that already holds data raise no interrupt, so
a program that prints while nobody drains its stream pays for a single
irq_work until the stream is emptied again. This matches bpf_ringbuf, which
notifies only once the consumer has caught up. Blocking readers and
level-triggered pollers re-check the readable count before waiting, so they
cannot miss data, and edge-triggered epoll consumers drain until EAGAIN
before waiting again, as epoll(7) requires.

Synchronize pending work before releasing the final stream reference so
the callback cannot outlive the stream, but only when the work was ever
queued: irq_work_sync() waits for an RCU grace period on PREEMPT_RT and on
architectures without an irq_work interrupt.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf.h            |  14 ++-
 include/uapi/linux/bpf.h       |  39 ++++++
 kernel/bpf/core.c              |   8 +-
 kernel/bpf/stream.c            | 209 ++++++++++++++++++++++++++++++---
 kernel/bpf/syscall.c           |  29 +++++
 tools/include/uapi/linux/bpf.h |  39 ++++++
 6 files changed, 315 insertions(+), 23 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 8594c8aff745..b2b4a769bcca 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -17,6 +17,7 @@
 #include <linux/numa.h>
 #include <linux/mm_types.h>
 #include <linux/wait.h>
+#include <linux/irq_work_types.h>
 #include <linux/refcount.h>
 #include <linux/mutex.h>
 #include <linux/module.h>
@@ -1771,12 +1772,18 @@ enum {
 };
 
 struct bpf_stream {
-	atomic_t capacity;
+	refcount_t refcnt;
+	atomic_t capacity;	/* bytes reserved against the stream limit */
+	atomic_t readable;	/* published bytes available to readers */
 	struct llist_head log;	/* list of in-flight stream elements in LIFO order */
 
 	struct mutex lock;  /* lock protecting backlog_{head,tail} */
 	struct llist_node *backlog_head; /* list of in-flight stream elements in FIFO order */
 	struct llist_node *backlog_tail; /* tail of the list above */
+	wait_queue_head_t waitq;
+	struct irq_work notify_work;
+	bool notify_used;	/* notify_work was queued at least once */
+	bool dead;
 };
 
 struct bpf_stream_stage {
@@ -1915,7 +1922,7 @@ struct bpf_prog_aux {
 		struct work_struct work;
 		struct rcu_head	rcu;
 	};
-	struct bpf_stream stream[2];
+	struct bpf_stream *stream[2];
 	struct mutex st_ops_assoc_mutex;
 	struct bpf_map __rcu *st_ops_assoc;
 };
@@ -4224,9 +4231,10 @@ void bpf_bprintf_cleanup(struct bpf_bprintf_data *data);
 int bpf_try_get_buffers(struct bpf_bprintf_buffers **bufs);
 void bpf_put_buffers(void);
 
-void bpf_prog_stream_init(struct bpf_prog *prog);
+int bpf_prog_stream_init(struct bpf_prog *prog, gfp_t gfp_extra_flags);
 void bpf_prog_stream_free(struct bpf_prog *prog);
 int bpf_prog_stream_read(struct bpf_prog *prog, enum bpf_stream_id stream_id, void __user *buf, u32 len);
+int bpf_prog_stream_new_fd(struct bpf_prog *prog, enum bpf_stream_id stream_id, u32 flags);
 void bpf_stream_stage_init(struct bpf_stream_stage *ss);
 void bpf_stream_stage_free(struct bpf_stream_stage *ss);
 __printf(2, 3)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 0aaa54359aeb..4687c3310996 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -936,6 +936,33 @@ union bpf_iter_link_info {
  * 		0 on success or -1 if an error occurred (in which case,
  * 		*errno* is set appropriately).
  *
+ * BPF_PROG_STREAM_OPEN
+ *	Description
+ *		Open a file descriptor for one of the BPF streams associated
+ *		with the program identified by *prog_fd*. The stream is selected
+ *		by *stream_id*.
+ *
+ *		The returned file descriptor supports **read**\ (2) and
+ *		**poll**\ (2). Reads block while the stream is empty unless
+ *		**BPF_F_STREAM_NONBLOCK** is specified in *flags*. A non-blocking
+ *		read of an empty stream fails with **EAGAIN**.
+ *
+ *		**poll**\ (2) reports **POLLIN** when data is available and
+ *		**POLLHUP** once the program has been freed, that is, after every
+ *		reference to it, including links and other file descriptors, has
+ *		been dropped. Hangup may lag the final release because program
+ *		teardown is deferred. Buffered data remains readable after
+ *		**POLLHUP** and a read returns zero after all such data has been
+ *		consumed.
+ *
+ *		The file descriptor is read-only and has the close-on-exec flag
+ *		set. It is not seekable and **lseek**\ (2) fails with **ESPIPE**.
+ *		*flags* may only contain **BPF_F_STREAM_NONBLOCK**.
+ *
+ *	Return
+ *		A new file descriptor (a nonnegative integer), or -1 if an
+ *		error occurred (in which case, *errno* is set appropriately).
+ *
  * NOTES
  *	eBPF objects (maps and programs) can be shared between processes.
  *
@@ -993,6 +1020,7 @@ enum bpf_cmd {
 	BPF_TOKEN_CREATE,
 	BPF_PROG_STREAM_READ_BY_FD,
 	BPF_PROG_ASSOC_STRUCT_OPS,
+	BPF_PROG_STREAM_OPEN,
 	__MAX_BPF_CMD,
 	BPF_COMMON_ATTRS = 1 << 16, /* Indicate carrying syscall common attrs. */
 };
@@ -1524,6 +1552,11 @@ enum {
 	BPF_STREAM_STDERR = 2,
 };
 
+/* flags for BPF_PROG_STREAM_OPEN command */
+enum {
+	BPF_F_STREAM_NONBLOCK = (1U << 0),
+};
+
 union bpf_attr {
 	struct { /* anonymous struct used by BPF_MAP_CREATE command */
 		__u32	map_type;	/* one of enum bpf_map_type */
@@ -1950,6 +1983,12 @@ union bpf_attr {
 		__u32		flags;
 	} prog_assoc_struct_ops;
 
+	struct {
+		__u32		prog_fd;
+		__u32		stream_id;
+		__u32		flags;
+	} prog_stream_open;
+
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index aa72ad3ec689..d3b8b626ec0f 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -142,10 +142,6 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
 	mutex_init(&fp->aux->dst_mutex);
 	mutex_init(&fp->aux->st_ops_assoc_mutex);
 
-#ifdef CONFIG_BPF_SYSCALL
-	bpf_prog_stream_init(fp);
-#endif
-
 	return fp;
 }
 
@@ -289,6 +285,9 @@ struct bpf_prog *bpf_prog_realloc(struct bpf_prog *fp_old, unsigned int size,
 void __bpf_prog_free(struct bpf_prog *fp)
 {
 	if (fp->aux) {
+#ifdef CONFIG_BPF_SYSCALL
+		bpf_prog_stream_free(fp);
+#endif
 		mutex_destroy(&fp->aux->used_maps_mutex);
 		mutex_destroy(&fp->aux->dst_mutex);
 		mutex_destroy(&fp->aux->st_ops_assoc_mutex);
@@ -3086,7 +3085,6 @@ static void bpf_prog_free_deferred(struct work_struct *work)
 	aux = container_of(work, struct bpf_prog_aux, work);
 #ifdef CONFIG_BPF_SYSCALL
 	bpf_free_kfunc_btf_tab(aux->kfunc_btf_tab);
-	bpf_prog_stream_free(aux->prog);
 #endif
 #ifdef CONFIG_CGROUP_BPF
 	if (aux->cgroup_atype != CGROUP_BPF_ATTACH_TYPE_INVALID)
diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index 9829d1ebce70..83e803ea54ef 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -2,11 +2,15 @@
 /* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
 
 #include <linux/bpf.h>
+#include <linux/anon_inodes.h>
 #include <linux/filter.h>
 #include <linux/bpf_mem_alloc.h>
 #include <linux/gfp.h>
+#include <linux/irq_work.h>
 #include <linux/memory.h>
 #include <linux/mutex.h>
+#include <linux/poll.h>
+#include <linux/refcount.h>
 
 static void bpf_stream_elem_init(struct bpf_stream_elem *elem, int len)
 {
@@ -73,6 +77,51 @@ static void bpf_stream_release_capacity(struct bpf_stream *stream, int len)
 	atomic_sub(len, &stream->capacity);
 }
 
+static void bpf_stream_notify(struct irq_work *work)
+{
+	struct bpf_stream *stream = container_of(work, struct bpf_stream, notify_work);
+
+	/*
+	 * Writers run in arbitrary program contexts, including NMI and regions
+	 * that already hold wait queue or epoll locks. Wake waiters from
+	 * irq_work instead, where taking those locks is safe.
+	 */
+	wake_up_interruptible_poll(&stream->waitq, EPOLLIN | EPOLLRDNORM);
+}
+
+static void bpf_stream_queue_notify(struct bpf_stream *stream)
+{
+	/*
+	 * Record that the work has been used so that teardown only pays for
+	 * irq_work_sync(), which may wait for an RCU grace period, when a
+	 * callback could actually be in flight.
+	 */
+	if (!READ_ONCE(stream->notify_used))
+		WRITE_ONCE(stream->notify_used, true);
+	irq_work_queue(&stream->notify_work);
+}
+
+static int bpf_stream_readable_bytes(struct bpf_stream *stream)
+{
+	return atomic_read_acquire(&stream->readable);
+}
+
+static void bpf_stream_publish(struct bpf_stream *stream, int len)
+{
+	/*
+	 * Pairs with atomic_read_acquire() in bpf_stream_readable_bytes().
+	 *
+	 * Notify only when the stream turns from empty to readable. Readers
+	 * block and pollers wait only after finding it empty, and a read never
+	 * takes the count below zero, so the publication that makes it positive
+	 * is the one they wait for. Publishing into a stream that already holds
+	 * data would only raise an interrupt for waiters that are being woken
+	 * already, or for nobody at all.
+	 */
+	if (atomic_add_return_release(len, &stream->readable) == len)
+		bpf_stream_queue_notify(stream);
+}
+
 static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int len)
 {
 	int ret;
@@ -88,6 +137,8 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
 	ret = __bpf_stream_push_str(&stream->log, str, len);
 	if (ret)
 		bpf_stream_release_capacity(stream, len);
+	else
+		bpf_stream_publish(stream, len);
 
 	return ret;
 }
@@ -96,7 +147,7 @@ static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bp
 {
 	if (stream_id != BPF_STDOUT && stream_id != BPF_STDERR)
 		return NULL;
-	return &aux->stream[stream_id - 1];
+	return aux->stream[stream_id - 1];
 }
 
 static void bpf_stream_free_elem(struct bpf_stream_elem *elem)
@@ -164,14 +215,16 @@ static bool bpf_stream_consume_elem(struct bpf_stream_elem *elem, int *len)
 
 static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 {
-	int rem_len = len, cons_len, ret = 0;
+	int read_len, rem_len, cons_len, ret = 0;
 	struct bpf_stream_elem *elem = NULL;
 	struct llist_node *node;
 
 	mutex_lock(&stream->lock);
+	read_len = min(len, bpf_stream_readable_bytes(stream));
+	rem_len = read_len;
 
 	while (rem_len) {
-		int pos = len - rem_len;
+		int pos = read_len - rem_len;
 		int chunk, n;
 		bool cont;
 
@@ -193,7 +246,7 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 			/* Keep any successfully copied bytes; -EFAULT only if none. */
 			elem->consumed_len -= n;
 			rem_len += n;
-			ret = (len == rem_len) ? -EFAULT : 0;
+			ret = (read_len == rem_len) ? -EFAULT : 0;
 			break;
 		}
 
@@ -204,8 +257,9 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 		bpf_stream_free_elem(elem);
 	}
 
+	atomic_sub(read_len - rem_len, &stream->readable);
 	mutex_unlock(&stream->lock);
-	return ret ? ret : len - rem_len;
+	return ret ? ret : read_len - rem_len;
 }
 
 int bpf_prog_stream_read(struct bpf_prog *prog, enum bpf_stream_id stream_id, void __user *buf, u32 len)
@@ -220,6 +274,111 @@ int bpf_prog_stream_read(struct bpf_prog *prog, enum bpf_stream_id stream_id, vo
 	return bpf_stream_read(stream, buf, len);
 }
 
+static bool bpf_stream_has_data(struct bpf_stream *stream)
+{
+	return bpf_stream_readable_bytes(stream) > 0;
+}
+
+static void bpf_stream_put(struct bpf_stream *stream)
+{
+	if (refcount_dec_and_test(&stream->refcnt)) {
+		struct llist_node *list;
+
+		/* Only a stream that ever queued its work can have a callback in flight. */
+		if (READ_ONCE(stream->notify_used))
+			irq_work_sync(&stream->notify_work);
+		list = llist_del_all(&stream->log);
+		bpf_stream_free_list(list);
+		bpf_stream_free_list(stream->backlog_head);
+		mutex_destroy(&stream->lock);
+		kfree(stream);
+	}
+}
+
+static int bpf_stream_release(struct inode *inode, struct file *file)
+{
+	bpf_stream_put(file->private_data);
+	return 0;
+}
+
+static ssize_t bpf_stream_file_read(struct file *file, char __user *buf, size_t len,
+				    loff_t *ppos)
+{
+	struct bpf_stream *stream = file->private_data;
+	bool dead;
+	int ret;
+
+	if (!len)
+		return 0;
+
+	for (;;) {
+		/*
+		 * Sample teardown state before looking for data. Nothing is
+		 * published once the program is gone, so finding the stream
+		 * empty after observing dead means EOF. The opposite order could
+		 * report EOF while data published just before teardown is still
+		 * buffered.
+		 */
+		dead = smp_load_acquire(&stream->dead);
+		ret = bpf_stream_read(stream, buf, len);
+		if (ret)
+			return ret;
+		if (dead)
+			return 0;
+		if (file->f_flags & O_NONBLOCK)
+			return -EAGAIN;
+
+		ret = wait_event_interruptible(stream->waitq,
+					       bpf_stream_has_data(stream) ||
+					       READ_ONCE(stream->dead));
+		if (ret)
+			return ret;
+	}
+}
+
+static __poll_t bpf_stream_poll(struct file *file, struct poll_table_struct *pts)
+{
+	struct bpf_stream *stream = file->private_data;
+	__poll_t events = 0;
+
+	/*
+	 * poll_wait() only registers the wait queue callback. Register before
+	 * checking persistent state so a concurrent publication or teardown is
+	 * observed either by the callback or by the checks below.
+	 */
+	poll_wait(file, &stream->waitq, pts);
+	if (bpf_stream_has_data(stream))
+		events |= EPOLLIN | EPOLLRDNORM;
+	if (READ_ONCE(stream->dead))
+		events |= EPOLLHUP;
+	return events;
+}
+
+static const struct file_operations bpf_stream_fops = {
+	.release = bpf_stream_release,
+	.read = bpf_stream_file_read,
+	.poll = bpf_stream_poll,
+};
+
+int bpf_prog_stream_new_fd(struct bpf_prog *prog, enum bpf_stream_id stream_id, u32 flags)
+{
+	struct bpf_stream *stream;
+	int fd_flags = O_RDONLY | O_CLOEXEC;
+	int fd;
+
+	stream = bpf_stream_get(stream_id, prog->aux);
+	if (!stream)
+		return -ENOENT;
+	if (flags & BPF_F_STREAM_NONBLOCK)
+		fd_flags |= O_NONBLOCK;
+
+	refcount_inc(&stream->refcnt);
+	fd = anon_inode_getfd("bpf-stream", &bpf_stream_fops, stream, fd_flags);
+	if (fd < 0)
+		bpf_stream_put(stream);
+	return fd;
+}
+
 __bpf_kfunc_start_defs();
 
 /*
@@ -287,28 +446,47 @@ __bpf_kfunc_end_defs();
 
 /* Added kfunc to common_btf_ids */
 
-void bpf_prog_stream_init(struct bpf_prog *prog)
+int bpf_prog_stream_init(struct bpf_prog *prog, gfp_t gfp_extra_flags)
 {
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(prog->aux->stream); i++) {
-		atomic_set(&prog->aux->stream[i].capacity, 0);
-		init_llist_head(&prog->aux->stream[i].log);
-		mutex_init(&prog->aux->stream[i].lock);
-		prog->aux->stream[i].backlog_head = NULL;
-		prog->aux->stream[i].backlog_tail = NULL;
+		struct bpf_stream *stream;
+
+		/* On failure, bpf_prog_stream_free() releases the streams allocated so far. */
+		stream = kzalloc_obj(*stream,
+				     bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
+		if (!stream)
+			return -ENOMEM;
+
+		refcount_set(&stream->refcnt, 1);
+		init_llist_head(&stream->log);
+		mutex_init(&stream->lock);
+		init_waitqueue_head(&stream->waitq);
+		init_irq_work(&stream->notify_work, bpf_stream_notify);
+		prog->aux->stream[i] = stream;
 	}
+	return 0;
 }
 
 void bpf_prog_stream_free(struct bpf_prog *prog)
 {
-	struct llist_node *list;
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(prog->aux->stream); i++) {
-		list = llist_del_all(&prog->aux->stream[i].log);
-		bpf_stream_free_list(list);
-		bpf_stream_free_list(prog->aux->stream[i].backlog_head);
+		struct bpf_stream *stream = prog->aux->stream[i];
+
+		if (!stream)
+			continue;
+		/*
+		 * Pairs with smp_load_acquire() in bpf_stream_file_read(): every
+		 * publication precedes the dead flag, so a reader that observes
+		 * it also observes all buffered data.
+		 */
+		smp_store_release(&stream->dead, true);
+		wake_up_interruptible_poll(&stream->waitq, EPOLLHUP);
+		bpf_stream_put(stream);
+		prog->aux->stream[i] = NULL;
 	}
 }
 
@@ -371,6 +549,7 @@ int bpf_stream_stage_commit(struct bpf_stream_stage *ss, struct bpf_prog *prog,
 		list = tail;
 	}
 	llist_add_batch(head, tail, &stream->log);
+	bpf_stream_publish(stream, ss->len);
 	return 0;
 }
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index e1bcfd43f54f..ac52f4ae414c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3080,6 +3080,10 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, struct bpf_log_at
 	prog->aux->user = get_current_user();
 	prog->len = attr->insn_cnt;
 
+	err = bpf_prog_stream_init(prog, GFP_USER);
+	if (err)
+		goto free_prog;
+
 	err = -EFAULT;
 	if (copy_from_bpfptr(prog->insns,
 			     make_bpfptr(attr->insns, uattr.is_kernel),
@@ -6316,6 +6320,28 @@ static int prog_assoc_struct_ops(union bpf_attr *attr)
 	return ret;
 }
 
+#define BPF_PROG_STREAM_OPEN_LAST_FIELD prog_stream_open.flags
+
+static int prog_stream_open(union bpf_attr *attr)
+{
+	struct bpf_prog *prog;
+	u32 flags = attr->prog_stream_open.flags;
+	int ret;
+
+	if (CHECK_ATTR(BPF_PROG_STREAM_OPEN))
+		return -EINVAL;
+	if (flags & ~BPF_F_STREAM_NONBLOCK)
+		return -EINVAL;
+
+	prog = bpf_prog_get(attr->prog_stream_open.prog_fd);
+	if (IS_ERR(prog))
+		return PTR_ERR(prog);
+
+	ret = bpf_prog_stream_new_fd(prog, attr->prog_stream_open.stream_id, flags);
+	bpf_prog_put(prog);
+	return ret;
+}
+
 static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size,
 		     bpfptr_t uattr_common, unsigned int size_common)
 {
@@ -6488,6 +6514,9 @@ static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size,
 	case BPF_PROG_ASSOC_STRUCT_OPS:
 		err = prog_assoc_struct_ops(&attr);
 		break;
+	case BPF_PROG_STREAM_OPEN:
+		err = prog_stream_open(&attr);
+		break;
 	default:
 		err = -EINVAL;
 		break;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 0aaa54359aeb..4687c3310996 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -936,6 +936,33 @@ union bpf_iter_link_info {
  * 		0 on success or -1 if an error occurred (in which case,
  * 		*errno* is set appropriately).
  *
+ * BPF_PROG_STREAM_OPEN
+ *	Description
+ *		Open a file descriptor for one of the BPF streams associated
+ *		with the program identified by *prog_fd*. The stream is selected
+ *		by *stream_id*.
+ *
+ *		The returned file descriptor supports **read**\ (2) and
+ *		**poll**\ (2). Reads block while the stream is empty unless
+ *		**BPF_F_STREAM_NONBLOCK** is specified in *flags*. A non-blocking
+ *		read of an empty stream fails with **EAGAIN**.
+ *
+ *		**poll**\ (2) reports **POLLIN** when data is available and
+ *		**POLLHUP** once the program has been freed, that is, after every
+ *		reference to it, including links and other file descriptors, has
+ *		been dropped. Hangup may lag the final release because program
+ *		teardown is deferred. Buffered data remains readable after
+ *		**POLLHUP** and a read returns zero after all such data has been
+ *		consumed.
+ *
+ *		The file descriptor is read-only and has the close-on-exec flag
+ *		set. It is not seekable and **lseek**\ (2) fails with **ESPIPE**.
+ *		*flags* may only contain **BPF_F_STREAM_NONBLOCK**.
+ *
+ *	Return
+ *		A new file descriptor (a nonnegative integer), or -1 if an
+ *		error occurred (in which case, *errno* is set appropriately).
+ *
  * NOTES
  *	eBPF objects (maps and programs) can be shared between processes.
  *
@@ -993,6 +1020,7 @@ enum bpf_cmd {
 	BPF_TOKEN_CREATE,
 	BPF_PROG_STREAM_READ_BY_FD,
 	BPF_PROG_ASSOC_STRUCT_OPS,
+	BPF_PROG_STREAM_OPEN,
 	__MAX_BPF_CMD,
 	BPF_COMMON_ATTRS = 1 << 16, /* Indicate carrying syscall common attrs. */
 };
@@ -1524,6 +1552,11 @@ enum {
 	BPF_STREAM_STDERR = 2,
 };
 
+/* flags for BPF_PROG_STREAM_OPEN command */
+enum {
+	BPF_F_STREAM_NONBLOCK = (1U << 0),
+};
+
 union bpf_attr {
 	struct { /* anonymous struct used by BPF_MAP_CREATE command */
 		__u32	map_type;	/* one of enum bpf_map_type */
@@ -1950,6 +1983,12 @@ union bpf_attr {
 		__u32		flags;
 	} prog_assoc_struct_ops;
 
+	struct {
+		__u32		prog_fd;
+		__u32		stream_id;
+		__u32		flags;
+	} prog_stream_open;
+
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF
-- 
2.53.0

[PATCH bpf-next v3 3/5] libbpf: Add bpf_prog_stream_open()

[Kumar Kartikeya Dwivedi]

Expose the BPF_PROG_STREAM_OPEN command through a low-level libbpf wrapper.
The options structure carries stream open flags, including non-blocking
mode, while leaving room for future extensions.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/bpf.c      | 19 +++++++++++++++++++
 tools/lib/bpf/bpf.h      | 24 ++++++++++++++++++++++++
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 44 insertions(+)

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index a9de7f107cf7..b49822d212ae 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -1466,6 +1466,25 @@ int bpf_prog_stream_read(int prog_fd, __u32 stream_id, void *buf, __u32 buf_len,
 	return libbpf_err_errno(err);
 }
 
+int bpf_prog_stream_open(int prog_fd, __u32 stream_id,
+			 const struct bpf_prog_stream_open_opts *opts)
+{
+	const size_t attr_sz = offsetofend(union bpf_attr, prog_stream_open);
+	union bpf_attr attr;
+	int fd;
+
+	if (!OPTS_VALID(opts, bpf_prog_stream_open_opts))
+		return libbpf_err(-EINVAL);
+
+	memset(&attr, 0, attr_sz);
+	attr.prog_stream_open.prog_fd = prog_fd;
+	attr.prog_stream_open.stream_id = stream_id;
+	attr.prog_stream_open.flags = OPTS_GET(opts, flags, 0);
+
+	fd = sys_bpf_fd(BPF_PROG_STREAM_OPEN, &attr, attr_sz);
+	return libbpf_err_errno(fd);
+}
+
 int bpf_prog_assoc_struct_ops(int prog_fd, int map_fd,
 			      struct bpf_prog_assoc_struct_ops_opts *opts)
 {
diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index 490e8cb4ba53..826d9cc9ab65 100644
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -759,10 +759,34 @@ struct bpf_prog_stream_read_opts {
  *
  * @return The number of bytes read, on success; negative error code, otherwise
  * (errno is also set to the error code)
+ *
+ * For blocking reads and polling, prefer **bpf_prog_stream_open**.
  */
 LIBBPF_API int bpf_prog_stream_read(int prog_fd, __u32 stream_id, void *buf, __u32 buf_len,
 				    struct bpf_prog_stream_read_opts *opts);
 
+struct bpf_prog_stream_open_opts {
+	size_t sz;
+	__u32 flags;
+	size_t :0;
+};
+#define bpf_prog_stream_open_opts__last_field flags
+
+/**
+ * @brief **bpf_prog_stream_open** opens a file descriptor for a BPF stream of
+ * a given BPF program.
+ *
+ * @param prog_fd FD for the BPF program whose BPF stream is to be opened.
+ * @param stream_id ID of the BPF stream to be opened.
+ * @param opts optional options, can be NULL. BPF_F_STREAM_NONBLOCK requests a
+ * non-blocking descriptor.
+ *
+ * @return A new stream FD, on success; negative error code, otherwise (errno
+ * is also set to the error code)
+ */
+LIBBPF_API int bpf_prog_stream_open(int prog_fd, __u32 stream_id,
+				    const struct bpf_prog_stream_open_opts *opts);
+
 struct bpf_prog_assoc_struct_ops_opts {
 	size_t sz;
 	__u32 flags;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 18d27d20102e..53b7b591325f 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -459,6 +459,7 @@ LIBBPF_1.7.0 {
 LIBBPF_1.8.0 {
 	global:
 		bpf_map__attach_cgroup_opts;
+		bpf_prog_stream_open;
 		bpf_program__add_flags;
 		bpf_program__attach_tracing_multi;
 		bpf_program__clear_flags;
-- 
2.53.0

[PATCH bpf-next v3 4/5] bpftool: Add option to wait for program stream output

[Kumar Kartikeya Dwivedi]

bpftool prog tracelog { stdout | stderr } PROG dumps the output buffered
in a program stream and exits, which is all BPF_PROG_STREAM_READ_BY_FD
allows. Waiting for further output means rerunning the command.

Add a -w/--wait option that opens the stream with bpf_prog_stream_open()
in its default blocking mode and keeps printing output as the program
produces it. Flush each chunk as it arrives so redirected output is not
held in stdio buffers while the next read blocks. Waiting is opt-in: the
default dump keeps using BPF_PROG_STREAM_READ_BY_FD and exits once the
buffered output is drained, so existing scripts behave the same on old and
new kernels.

A stream descriptor does not keep its program alive, so bpftool drops the
program descriptor once the stream is open. Waiting then ends with EOF
when the program is unloaded and otherwise runs until interrupted, as
bpftool prog tracelog already does for the trace pipe. Exit from the
SIGINT, SIGHUP and SIGTERM handlers like that command does. A flag set by
the handler and checked before each read would miss a signal that lands
between the check and the blocking read(), leaving bpftool asleep until the
next print.

Waiting requires BPF_PROG_STREAM_OPEN. The bpf() syscall fails with EINVAL
for an unknown command, which bpftool reports as missing kernel support
rather than degrading into a dump. Document the option and offer it in the
bash completion.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../bpftool/Documentation/bpftool-prog.rst    | 12 +++-
 tools/bpf/bpftool/bash-completion/bpftool     |  2 +-
 tools/bpf/bpftool/main.c                      |  7 ++-
 tools/bpf/bpftool/main.h                      |  1 +
 tools/bpf/bpftool/prog.c                      | 62 ++++++++++++++++---
 5 files changed, 73 insertions(+), 11 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-prog.rst b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
index 90fe8c61bf42..17aa0d560e32 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-prog.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
@@ -18,7 +18,8 @@ SYNOPSIS
 
 *OPTIONS* := { |COMMON_OPTIONS| |
 { **-f** | **--bpffs** } | { **-m** | **--mapcompat** } | { **-n** | **--nomount** } |
-{ **-L** | **--use-loader** } | [ { **-S** | **--sign** } **-k** <private_key.pem> **-i** <certificate.x509> ] }
+{ **-L** | **--use-loader** } | [ { **-S** | **--sign** } **-k** <private_key.pem> **-i** <certificate.x509> ] |
+{ **-w** | **--wait** } }
 
 *COMMANDS* :=
 { **show** | **list** | **dump xlated** | **dump jited** | **pin** | **load** |
@@ -186,6 +187,11 @@ bpftool prog tracelog { stdout | stderr } *PROG*
     error messages to the standard error stream. This facility should be used
     only for debugging purposes.
 
+    By default, bpftool prints the output buffered so far and exits. With
+    **-w** or **--wait**, it keeps printing new output as the program produces
+    it, until the program is unloaded or <Ctrl+C> is hit. Waiting requires a
+    kernel that supports opening a stream as a file descriptor.
+
 bpftool prog run *PROG* data_in *FILE* [data_out *FILE* [data_size_out *L*]] [ctx_in *FILE* [ctx_out *FILE* [ctx_size_out *M*]]] [repeat *N*]
     Run BPF program *PROG* in the kernel testing infrastructure for BPF,
     meaning that the program works on the data and context provided by the
@@ -267,6 +273,10 @@ OPTIONS
     Path to the X.509 certificate file in PEM or DER format, required when
     signing.
 
+-w, --wait
+    When dumping a program stream with **bpftool prog tracelog**, wait for new
+    output instead of exiting once the buffered output has been printed.
+
 EXAMPLES
 ========
 **# bpftool prog show**
diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
index 9d9ced270685..a28af31937ad 100644
--- a/tools/bpf/bpftool/bash-completion/bpftool
+++ b/tools/bpf/bpftool/bash-completion/bpftool
@@ -269,7 +269,7 @@ _bpftool()
     # Deal with options
     if [[ ${words[cword]} == -* ]]; then
         local c='--version --json --pretty --bpffs --mapcompat --debug \
-            --use-loader --base-btf --sign -i -k'
+            --use-loader --base-btf --sign -i -k --wait'
         COMPREPLY=( $( compgen -W "$c" -- "$cur" ) )
         return 0
     fi
diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
index 83884b21e708..d15fec71b64e 100644
--- a/tools/bpf/bpftool/main.c
+++ b/tools/bpf/bpftool/main.c
@@ -31,6 +31,7 @@ bool block_mount;
 bool verifier_logs;
 bool relaxed_maps;
 bool use_loader;
+bool wait_output;
 struct btf *base_btf;
 struct hashmap *refs_table;
 bool sign_progs;
@@ -462,6 +463,7 @@ int main(int argc, char **argv)
 		{ "nomount",	no_argument,	NULL,	'n' },
 		{ "debug",	no_argument,	NULL,	'd' },
 		{ "use-loader",	no_argument,	NULL,	'L' },
+		{ "wait",	no_argument,	NULL,	'w' },
 		{ "sign",	no_argument,	NULL,	'S' },
 		{ "base-btf",	required_argument, NULL, 'B' },
 		{ 0 }
@@ -480,7 +482,7 @@ int main(int argc, char **argv)
 	bin_name = "bpftool";
 
 	opterr = 0;
-	while ((opt = getopt_long(argc, argv, "VhpjfLmndSi:k:B:l",
+	while ((opt = getopt_long(argc, argv, "VhpjfLmndSi:k:B:lw",
 				  options, NULL)) >= 0) {
 		switch (opt) {
 		case 'V':
@@ -530,6 +532,9 @@ int main(int argc, char **argv)
 		case 'L':
 			use_loader = true;
 			break;
+		case 'w':
+			wait_output = true;
+			break;
 		case 'S':
 			sign_progs = true;
 			use_loader = true;
diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
index 48eedcb9f6c0..fa2c877a3cbf 100644
--- a/tools/bpf/bpftool/main.h
+++ b/tools/bpf/bpftool/main.h
@@ -89,6 +89,7 @@ extern bool block_mount;
 extern bool verifier_logs;
 extern bool relaxed_maps;
 extern bool use_loader;
+extern bool wait_output;
 extern struct btf *base_btf;
 extern struct hashmap *refs_table;
 extern bool sign_progs;
diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
index 24e40dfab469..5e27b22444f2 100644
--- a/tools/bpf/bpftool/prog.c
+++ b/tools/bpf/bpftool/prog.c
@@ -1119,21 +1119,66 @@ enum prog_tracelog_mode {
 	TRACE_STDERR,
 };
 
+static void exit_stream(int signo)
+{
+	exit(0);
+}
+
+/* Consumes prog_fd. */
 static int
 prog_tracelog_stream(int prog_fd, enum prog_tracelog_mode mode)
 {
+	const struct sigaction act = { .sa_handler = exit_stream };
+	const int signals[] = { SIGHUP, SIGINT, SIGTERM };
 	FILE *file = mode == TRACE_STDOUT ? stdout : stderr;
 	int stream_id = mode == TRACE_STDOUT ? 1 : 2;
 	char buf[512];
-	int ret;
+	unsigned int i;
+	int fd, ret;
+
+	if (!wait_output) {
+		do {
+			ret = bpf_prog_stream_read(prog_fd, stream_id, buf, sizeof(buf), NULL);
+			if (ret > 0)
+				fwrite(buf, sizeof(buf[0]), ret, file);
+		} while (ret > 0);
+		if (ret < 0)
+			p_err("failed to read stream: %s", strerror(-ret));
+		close(prog_fd);
+		goto out;
+	}
 
-	ret = 0;
-	do {
-		ret = bpf_prog_stream_read(prog_fd, stream_id, buf, sizeof(buf), NULL);
-		if (ret > 0)
-			fwrite(buf, sizeof(buf[0]), ret, file);
-	} while (ret > 0);
+	fd = bpf_prog_stream_open(prog_fd, stream_id, NULL);
+	/*
+	 * The stream descriptor does not keep the program alive. Drop the
+	 * program reference so that reads return EOF once the program is gone.
+	 */
+	close(prog_fd);
+	if (fd == -EINVAL) {
+		/* The bpf() syscall rejects unknown commands with EINVAL. */
+		p_err("waiting for stream output is not supported by this kernel");
+		return -1;
+	}
+	if (fd < 0) {
+		p_err("failed to open stream: %s", strerror(-fd));
+		return -1;
+	}
 
+	/*
+	 * Exit from the handler like the trace pipe variant does. A flag checked
+	 * between reads would miss a signal that lands before read() blocks and
+	 * only end the loop at the next print.
+	 */
+	for (i = 0; i < ARRAY_SIZE(signals); i++)
+		sigaction(signals[i], &act, NULL);
+	while ((ret = read(fd, buf, sizeof(buf))) > 0) {
+		fwrite(buf, sizeof(buf[0]), ret, file);
+		fflush(file);
+	}
+	if (ret < 0)
+		p_err("failed to read stream: %s", strerror(errno));
+	close(fd);
+out:
 	fflush(file);
 	return ret ? -1 : 0;
 }
@@ -2659,7 +2704,8 @@ static int do_help(int argc, char **argv)
 		"       METRIC := { cycles | instructions | l1d_loads | llc_misses | itlb_misses | dtlb_misses }\n"
 		"       " HELP_SPEC_OPTIONS " |\n"
 		"                    {-f|--bpffs} | {-m|--mapcompat} | {-n|--nomount} |\n"
-		"                    {-L|--use-loader} | [ {-S|--sign } {-k} <private_key.pem> {-i} <certificate.x509> ] \n"
+		"                    {-L|--use-loader} | [ {-S|--sign } {-k} <private_key.pem> {-i} <certificate.x509> ] |\n"
+		"                    {-w|--wait} }\n"
 		"",
 		bin_name, argv[-2]);
 
-- 
2.53.0

Re: [PATCH bpf-next v3 4/5] bpftool: Add option to wait for program stream output

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpf: bpftool deadlock in exit_stream signal handler
--

commit 482ed48b5d6910c6aec52e854a7926f6f16ce58a
Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>

bpftool: Add option to wait for program stream output

bpftool prog tracelog { stdout | stderr } PROG dumps the output buffered
in a program stream and exits, which is all BPF_PROG_STREAM_READ_BY_FD
allows. Waiting for further output means rerunning the command.

> diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> index 24e40dfab4690..5e27b22444f25 100644
> --- a/tools/bpf/bpftool/prog.c
> +++ b/tools/bpf/bpftool/prog.c
> @@ -1119,21 +1119,66 @@ enum prog_tracelog_mode {
>  	TRACE_STDERR,
>  };
>  
> +static void exit_stream(int signo)
> +{
> +	exit(0);
> +}
> +

[Severity: Medium]
Since exit_stream() is used as a signal handler for SIGINT, SIGHUP, and
SIGTERM, is it safe to call exit(0) here?

exit() is not async-signal-safe and attempts to flush open stdio streams.
If the signal interrupts the main loop in prog_tracelog_stream() while it
is executing fwrite() or fflush(), it could try to acquire the same
non-reentrant lock, resulting in a deadlock.

Could _exit(0) be used instead to avoid this deadlock?

[ ... ]

> +	/*
> +	 * Exit from the handler like the trace pipe variant does. A flag checked
> +	 * between reads would miss a signal that lands before read() blocks and
> +	 * only end the loop at the next print.
> +	 */
> +	for (i = 0; i < ARRAY_SIZE(signals); i++)
> +		sigaction(signals[i], &act, NULL);
> +	while ((ret = read(fd, buf, sizeof(buf))) > 0) {
> +		fwrite(buf, sizeof(buf[0]), ret, file);
> +		fflush(file);
> +	}
> +	if (ret < 0)
> +		p_err("failed to read stream: %s", strerror(errno));
> +	close(fd);
> +out:
>  	fflush(file);
>  	return ret ? -1 : 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260925045536.1480933-1-memxor@gmail.com?part=4

Re: [PATCH bpf-next v3 4/5] bpftool: Add option to wait for program stream output

[Kumar Kartikeya Dwivedi]

On Fri, 25 Sept 2026 at 07:15, <sashiko-bot@kernel.org> wrote:
>
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] bpf: bpftool deadlock in exit_stream signal handler
> --
>
> commit 482ed48b5d6910c6aec52e854a7926f6f16ce58a
> Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>
> bpftool: Add option to wait for program stream output
>
> bpftool prog tracelog { stdout | stderr } PROG dumps the output buffered
> in a program stream and exits, which is all BPF_PROG_STREAM_READ_BY_FD
> allows. Waiting for further output means rerunning the command.
>
> > diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> > index 24e40dfab4690..5e27b22444f25 100644
> > --- a/tools/bpf/bpftool/prog.c
> > +++ b/tools/bpf/bpftool/prog.c
> > @@ -1119,21 +1119,66 @@ enum prog_tracelog_mode {
> >       TRACE_STDERR,
> >  };
> >
> > +static void exit_stream(int signo)
> > +{
> > +     exit(0);
> > +}
> > +
>
> [Severity: Medium]
> Since exit_stream() is used as a signal handler for SIGINT, SIGHUP, and
> SIGTERM, is it safe to call exit(0) here?
>
> exit() is not async-signal-safe and attempts to flush open stdio streams.
> If the signal interrupts the main loop in prog_tracelog_stream() while it
> is executing fwrite() or fflush(), it could try to acquire the same
> non-reentrant lock, resulting in a deadlock.
>
> Could _exit(0) be used instead to avoid this deadlock?
>

I can follow up separately, otherwise include in the next respin if
that becomes necessary.

> [ ... ]
>
> > +     /*
> > +      * Exit from the handler like the trace pipe variant does. A flag checked
> > +      * between reads would miss a signal that lands before read() blocks and
> > +      * only end the loop at the next print.
> > +      */
> > +     for (i = 0; i < ARRAY_SIZE(signals); i++)
> > +             sigaction(signals[i], &act, NULL);
> > +     while ((ret = read(fd, buf, sizeof(buf))) > 0) {
> > +             fwrite(buf, sizeof(buf[0]), ret, file);
> > +             fflush(file);
> > +     }
> > +     if (ret < 0)
> > +             p_err("failed to read stream: %s", strerror(errno));
> > +     close(fd);
> > +out:
> >       fflush(file);
> >       return ret ? -1 : 0;
> >  }
>
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20260925045536.1480933-1-memxor@gmail.com?part=4

Re: [PATCH bpf-next v3 4/5] bpftool: Add option to wait for program stream output

[Quentin Monnet]

2026-09-25 06:55 UTC+0200 ~ Kumar Kartikeya Dwivedi <memxor@gmail.com>
> bpftool prog tracelog { stdout | stderr } PROG dumps the output buffered
> in a program stream and exits, which is all BPF_PROG_STREAM_READ_BY_FD
> allows. Waiting for further output means rerunning the command.
> 
> Add a -w/--wait option that opens the stream with bpf_prog_stream_open()
> in its default blocking mode and keeps printing output as the program
> produces it. Flush each chunk as it arrives so redirected output is not
> held in stdio buffers while the next read blocks. Waiting is opt-in: the
> default dump keeps using BPF_PROG_STREAM_READ_BY_FD and exits once the
> buffered output is drained, so existing scripts behave the same on old and
> new kernels.
> 
> A stream descriptor does not keep its program alive, so bpftool drops the
> program descriptor once the stream is open. Waiting then ends with EOF
> when the program is unloaded and otherwise runs until interrupted, as
> bpftool prog tracelog already does for the trace pipe. Exit from the
> SIGINT, SIGHUP and SIGTERM handlers like that command does. A flag set by
> the handler and checked before each read would miss a signal that lands
> between the check and the blocking read(), leaving bpftool asleep until the
> next print.
> 
> Waiting requires BPF_PROG_STREAM_OPEN. The bpf() syscall fails with EINVAL
> for an unknown command, which bpftool reports as missing kernel support
> rather than degrading into a dump. Document the option and offer it in the
> bash completion.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  .../bpftool/Documentation/bpftool-prog.rst    | 12 +++-
>  tools/bpf/bpftool/bash-completion/bpftool     |  2 +-
>  tools/bpf/bpftool/main.c                      |  7 ++-
>  tools/bpf/bpftool/main.h                      |  1 +
>  tools/bpf/bpftool/prog.c                      | 62 ++++++++++++++++---
>  5 files changed, 73 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/Documentation/bpftool-prog.rst b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> index 90fe8c61bf42..17aa0d560e32 100644
> --- a/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> +++ b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> @@ -18,7 +18,8 @@ SYNOPSIS
>  
>  *OPTIONS* := { |COMMON_OPTIONS| |
>  { **-f** | **--bpffs** } | { **-m** | **--mapcompat** } | { **-n** | **--nomount** } |
> -{ **-L** | **--use-loader** } | [ { **-S** | **--sign** } **-k** <private_key.pem> **-i** <certificate.x509> ] }
> +{ **-L** | **--use-loader** } | [ { **-S** | **--sign** } **-k** <private_key.pem> **-i** <certificate.x509> ] |
> +{ **-w** | **--wait** } }
>  
>  *COMMANDS* :=
>  { **show** | **list** | **dump xlated** | **dump jited** | **pin** | **load** |
> @@ -186,6 +187,11 @@ bpftool prog tracelog { stdout | stderr } *PROG*
>      error messages to the standard error stream. This facility should be used
>      only for debugging purposes.
>  
> +    By default, bpftool prints the output buffered so far and exits. With
> +    **-w** or **--wait**, it keeps printing new output as the program produces
> +    it, until the program is unloaded or <Ctrl+C> is hit. Waiting requires a
> +    kernel that supports opening a stream as a file descriptor.


If you do come back for a follow-up (looking at your reply to Sashiko,
regarding _exit(0)): it would be good to mention the mainline kernel
version that brings support for the feature, here.


> +
>  bpftool prog run *PROG* data_in *FILE* [data_out *FILE* [data_size_out *L*]] [ctx_in *FILE* [ctx_out *FILE* [ctx_size_out *M*]]] [repeat *N*]
>      Run BPF program *PROG* in the kernel testing infrastructure for BPF,
>      meaning that the program works on the data and context provided by the
> @@ -267,6 +273,10 @@ OPTIONS
>      Path to the X.509 certificate file in PEM or DER format, required when
>      signing.
>  
> +-w, --wait
> +    When dumping a program stream with **bpftool prog tracelog**, wait for new


Nit: **bpftool prog tracelog { stderr | stdout }**

I think it doesn't apply otherwise?


> +    output instead of exiting once the buffered output has been printed.
> +
>  EXAMPLES
>  ========
>  **# bpftool prog show**
> diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
> index 9d9ced270685..a28af31937ad 100644
> --- a/tools/bpf/bpftool/bash-completion/bpftool
> +++ b/tools/bpf/bpftool/bash-completion/bpftool
> @@ -269,7 +269,7 @@ _bpftool()
>      # Deal with options
>      if [[ ${words[cword]} == -* ]]; then
>          local c='--version --json --pretty --bpffs --mapcompat --debug \
> -            --use-loader --base-btf --sign -i -k'
> +            --use-loader --base-btf --sign -i -k --wait'
>          COMPREPLY=( $( compgen -W "$c" -- "$cur" ) )
>          return 0
>      fi
> diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
> index 83884b21e708..d15fec71b64e 100644
> --- a/tools/bpf/bpftool/main.c
> +++ b/tools/bpf/bpftool/main.c
> @@ -31,6 +31,7 @@ bool block_mount;
>  bool verifier_logs;
>  bool relaxed_maps;
>  bool use_loader;
> +bool wait_output;
>  struct btf *base_btf;
>  struct hashmap *refs_table;
>  bool sign_progs;
> @@ -462,6 +463,7 @@ int main(int argc, char **argv)
>  		{ "nomount",	no_argument,	NULL,	'n' },
>  		{ "debug",	no_argument,	NULL,	'd' },
>  		{ "use-loader",	no_argument,	NULL,	'L' },
> +		{ "wait",	no_argument,	NULL,	'w' },
>  		{ "sign",	no_argument,	NULL,	'S' },
>  		{ "base-btf",	required_argument, NULL, 'B' },
>  		{ 0 }
> @@ -480,7 +482,7 @@ int main(int argc, char **argv)
>  	bin_name = "bpftool";
>  
>  	opterr = 0;
> -	while ((opt = getopt_long(argc, argv, "VhpjfLmndSi:k:B:l",
> +	while ((opt = getopt_long(argc, argv, "VhpjfLmndSi:k:B:lw",
>  				  options, NULL)) >= 0) {
>  		switch (opt) {
>  		case 'V':
> @@ -530,6 +532,9 @@ int main(int argc, char **argv)
>  		case 'L':
>  			use_loader = true;
>  			break;
> +		case 'w':
> +			wait_output = true;
> +			break;
>  		case 'S':
>  			sign_progs = true;
>  			use_loader = true;
> diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
> index 48eedcb9f6c0..fa2c877a3cbf 100644
> --- a/tools/bpf/bpftool/main.h
> +++ b/tools/bpf/bpftool/main.h
> @@ -89,6 +89,7 @@ extern bool block_mount;
>  extern bool verifier_logs;
>  extern bool relaxed_maps;
>  extern bool use_loader;
> +extern bool wait_output;
>  extern struct btf *base_btf;
>  extern struct hashmap *refs_table;
>  extern bool sign_progs;
> diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> index 24e40dfab469..5e27b22444f2 100644
> --- a/tools/bpf/bpftool/prog.c
> +++ b/tools/bpf/bpftool/prog.c
> @@ -1119,21 +1119,66 @@ enum prog_tracelog_mode {
>  	TRACE_STDERR,
>  };
>  
> +static void exit_stream(int signo)
> +{
> +	exit(0);


I'm not sure the deadlock Sashiko mentions can really happen, but given
that the loop already flushes, the suggestion to use _exit(0) here
instead makes sense. Fine by me as a follow-up.

Acked-by: Quentin Monnet <qmo@kernel.org>

Thanks,
Quentin

[PATCH bpf-next v3 5/5] selftests/bpf: Test program stream file descriptors

[Kumar Kartikeya Dwivedi]

Exercise validation and normal file semantics of BPF_PROG_STREAM_OPEN.
Cover read-only and close-on-exec state, write and lseek rejection,
non-blocking reads, partial reads, poll readiness, and a blocking reader
woken by new stream data. Also check that an empty stream write produces
neither readable data nor poll readiness.

Register the descriptor with EPOLLET, consume the initial data event
without draining buffered output, then release the program. Require a
second edge carrying POLLIN and POLLHUP, drain the buffered bytes, and
verify that poll continues to report HUP before read returns EOF. This
ensures program teardown notifies edge-triggered loops even when
readability never transitions.

Notifications are only sent when an empty stream turns readable, so also
consume the first edge of an edge-triggered descriptor, leave part of the
output behind while more arrives, drain the stream and require a new edge
for output published afterwards. This covers the transition that re-arms
notifications once readers, including partial ones, bring the count back
to zero.

Also block a reader on an empty live stream, release the program, and
verify that it wakes and returns EOF. This directly covers the teardown
wait condition and its no-lost-wakeup ordering.

Finally, attach a stream-writing program to a hardware perf event and
busy-poll its epoll registration until output arrives. This both drives the
per-thread event into NMI context and directly verifies that the deferred
irq_work notification reaches epoll, without a separate BSS completion
handshake.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../testing/selftests/bpf/prog_tests/stream.c | 486 ++++++++++++++++++
 tools/testing/selftests/bpf/progs/stream.c    |  20 +
 2 files changed, 506 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
index 74bd15c4bfba..ae400dcfbf24 100644
--- a/tools/testing/selftests/bpf/prog_tests/stream.c
+++ b/tools/testing/selftests/bpf/prog_tests/stream.c
@@ -1,11 +1,17 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
 #include <test_progs.h>
+#include <linux/perf_event.h>
+#include <poll.h>
+#include <sys/epoll.h>
 #include <sys/mman.h>
+#include <sys/syscall.h>
 
 #include "stream.skel.h"
 #include "stream_fail.skel.h"
 
+#define NMI_TIMEOUT_NS (5ULL * 1000 * 1000 * 1000)
+
 void test_stream_failure(void)
 {
 	RUN_TESTS(stream_fail);
@@ -62,6 +68,486 @@ void test_stream_syscall(void)
 	stream__destroy(skel);
 }
 
+static bool stream_fd_trigger(struct bpf_program *prog)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	int ret;
+
+	ret = bpf_prog_test_run_opts(bpf_program__fd(prog), &opts);
+	return ASSERT_OK(ret, "test_run") && ASSERT_OK(opts.retval, "retval");
+}
+
+static void test_stream_fd_open(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts);
+	struct stream *skel;
+	int fd, prog_fd;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	prog_fd = bpf_program__fd(skel->progs.stream_syscall);
+	fd = bpf_prog_stream_open(0, BPF_STREAM_STDOUT, NULL);
+	ASSERT_EQ(fd, -EINVAL, "bad_prog_fd");
+
+	fd = bpf_prog_stream_open(prog_fd, 0, NULL);
+	ASSERT_EQ(fd, -ENOENT, "bad_stream_id");
+
+	opts.flags = BPF_F_RDONLY;
+	fd = bpf_prog_stream_open(prog_fd, BPF_STREAM_STDOUT, &opts);
+	ASSERT_EQ(fd, -EINVAL, "access_flag");
+
+	opts.flags = 1U << 31;
+	fd = bpf_prog_stream_open(prog_fd, BPF_STREAM_STDOUT, &opts);
+	ASSERT_EQ(fd, -EINVAL, "unknown_flag");
+
+	fd = bpf_prog_stream_open(prog_fd, BPF_STREAM_STDERR, NULL);
+	if (ASSERT_OK_FD(fd, "stderr"))
+		close(fd);
+
+	stream__destroy(skel);
+}
+
+static void test_stream_fd_nonblock(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts,
+		.flags = BPF_F_STREAM_NONBLOCK,
+	);
+	struct pollfd pfd = { .events = POLLIN | POLLHUP };
+	struct stream *skel;
+	char buf[4] = {};
+	int fd, flags, ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_syscall),
+				  BPF_STREAM_STDOUT, &opts);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out_destroy;
+	pfd.fd = fd;
+
+	flags = fcntl(fd, F_GETFD);
+	ASSERT_GE(flags, 0, "getfd");
+	ASSERT_NEQ(flags & FD_CLOEXEC, 0, "cloexec");
+	flags = fcntl(fd, F_GETFL);
+	ASSERT_GE(flags, 0, "getfl");
+	ASSERT_EQ(flags & O_ACCMODE, O_RDONLY, "readonly");
+	ASSERT_NEQ(flags & O_NONBLOCK, 0, "nonblock");
+
+	ret = write(fd, "x", 1);
+	ASSERT_EQ(ret, -1, "write");
+	ASSERT_EQ(errno, EBADF, "write_errno");
+	ret = lseek(fd, 0, SEEK_SET);
+	ASSERT_EQ(ret, -1, "lseek");
+	ASSERT_EQ(errno, ESPIPE, "lseek_errno");
+
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 0, "poll_empty");
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, -1, "read_empty");
+	ASSERT_EQ(errno, EAGAIN, "read_empty_errno");
+
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_close;
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 1, "poll_data");
+	ASSERT_NEQ(pfd.revents & POLLIN, 0, "pollin");
+	ASSERT_EQ(pfd.revents & POLLHUP, 0, "no_pollhup");
+
+	ret = read(fd, buf, 2);
+	ASSERT_EQ(ret, 2, "read_first");
+	ASSERT_OK(memcmp(buf, "fo", 2), "read_first_data");
+	pfd.revents = 0;
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 1, "poll_partial");
+	ASSERT_NEQ(pfd.revents & POLLIN, 0, "pollin_partial");
+
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, 1, "read_rest");
+	ASSERT_EQ(buf[0], 'o', "read_rest_data");
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, -1, "read_drained");
+	ASSERT_EQ(errno, EAGAIN, "read_drained_errno");
+
+out_close:
+	close(fd);
+out_destroy:
+	stream__destroy(skel);
+}
+
+static void test_stream_fd_empty(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts,
+		.flags = BPF_F_STREAM_NONBLOCK,
+	);
+	struct pollfd pfd = { .events = POLLIN | POLLHUP };
+	struct stream *skel;
+	char buf[4];
+	int fd, ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_empty),
+				  BPF_STREAM_STDOUT, &opts);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out_destroy;
+	pfd.fd = fd;
+
+	/* Empty output produces neither data nor readiness. */
+	if (!stream_fd_trigger(skel->progs.stream_empty))
+		goto out_close;
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 0, "poll_empty_write");
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, -1, "read_empty_write");
+	ASSERT_EQ(errno, EAGAIN, "read_empty_write_errno");
+
+out_close:
+	close(fd);
+out_destroy:
+	stream__destroy(skel);
+}
+
+struct stream_fd_read_ctx {
+	int fd;
+	ssize_t ret;
+	char buf[4];
+};
+
+static void *stream_fd_read_thread(void *arg)
+{
+	struct stream_fd_read_ctx *ctx = arg;
+
+	ctx->ret = read(ctx->fd, ctx->buf, sizeof(ctx->buf));
+	return NULL;
+}
+
+static int stream_fd_timed_join(pthread_t thread)
+{
+	struct timespec timeout;
+
+	clock_gettime(CLOCK_REALTIME, &timeout);
+	timeout.tv_sec += 5;
+	return pthread_timedjoin_np(thread, NULL, &timeout);
+}
+
+static void test_stream_fd_blocking(void)
+{
+	struct stream_fd_read_ctx ctx = {};
+	struct stream *skel;
+	pthread_t thread;
+	bool thread_live = false;
+	int fd, flags, ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_syscall),
+				  BPF_STREAM_STDOUT, NULL);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out_destroy;
+	ctx.fd = fd;
+
+	flags = fcntl(fd, F_GETFL);
+	ASSERT_GE(flags, 0, "getfl");
+	ASSERT_EQ(flags & O_NONBLOCK, 0, "blocking");
+
+	ret = pthread_create(&thread, NULL, stream_fd_read_thread, &ctx);
+	if (!ASSERT_OK(ret, "pthread_create"))
+		goto out_close;
+	thread_live = true;
+
+	usleep(50000);
+	ret = pthread_tryjoin_np(thread, NULL);
+	if (!ASSERT_EQ(ret, EBUSY, "read_blocks")) {
+		thread_live = ret != 0;
+		goto out_thread;
+	}
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_thread;
+
+	ret = stream_fd_timed_join(thread);
+	if (!ASSERT_OK(ret, "pthread_join"))
+		goto out_thread;
+	thread_live = false;
+	ASSERT_EQ(ctx.ret, 3, "read_len");
+	ASSERT_OK(memcmp(ctx.buf, "foo", 3), "read_data");
+
+	memset(&ctx, 0, sizeof(ctx));
+	ctx.fd = fd;
+	ret = pthread_create(&thread, NULL, stream_fd_read_thread, &ctx);
+	if (!ASSERT_OK(ret, "pthread_create_eof"))
+		goto out_close;
+	thread_live = true;
+
+	usleep(50000);
+	ret = pthread_tryjoin_np(thread, NULL);
+	if (!ASSERT_EQ(ret, EBUSY, "read_eof_blocks")) {
+		thread_live = ret != 0;
+		goto out_thread;
+	}
+
+	stream__destroy(skel);
+	skel = NULL;
+	ret = stream_fd_timed_join(thread);
+	if (!ASSERT_OK(ret, "pthread_join_eof"))
+		goto out_thread;
+	thread_live = false;
+	ASSERT_EQ(ctx.ret, 0, "read_eof");
+
+out_thread:
+	if (thread_live) {
+		stream__destroy(skel);
+		skel = NULL;
+		ret = stream_fd_timed_join(thread);
+		if (ret) {
+			pthread_cancel(thread);
+			pthread_join(thread, NULL);
+		}
+	}
+out_close:
+	close(fd);
+out_destroy:
+	stream__destroy(skel);
+}
+
+static void test_stream_fd_hup(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts,
+		.flags = BPF_F_STREAM_NONBLOCK,
+	);
+	struct epoll_event event = {
+		.events = EPOLLIN | EPOLLET,
+	};
+	struct pollfd pfd = { .events = POLLIN | POLLHUP };
+	struct stream *skel;
+	char buf[4] = {};
+	int epfd = -1, fd, ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_syscall),
+				  BPF_STREAM_STDOUT, &opts);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out_destroy;
+	pfd.fd = fd;
+	epfd = epoll_create1(EPOLL_CLOEXEC);
+	if (!ASSERT_OK_FD(epfd, "epoll_create"))
+		goto out_close;
+	event.data.fd = fd;
+	ret = epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);
+	if (!ASSERT_OK(ret, "epoll_ctl"))
+		goto out_close;
+
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_close;
+	ret = epoll_wait(epfd, &event, 1, 5000);
+	if (!ASSERT_EQ(ret, 1, "epoll_wait_data"))
+		goto out_close;
+	ASSERT_NEQ(event.events & EPOLLIN, 0, "epollin");
+	ASSERT_EQ(event.events & EPOLLHUP, 0, "no_epollhup");
+
+	stream__destroy(skel);
+	skel = NULL;
+	event.events = 0;
+	ret = epoll_wait(epfd, &event, 1, 5000);
+	if (!ASSERT_EQ(ret, 1, "epoll_wait_hup"))
+		goto out_close;
+	ASSERT_NEQ(event.events & EPOLLIN, 0, "epollin_with_hup");
+	ASSERT_NEQ(event.events & EPOLLHUP, 0, "epollhup");
+
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, 3, "read_buffered");
+	ASSERT_OK(memcmp(buf, "foo", 3), "read_buffered_data");
+	pfd.revents = 0;
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 1, "poll_drained_hup");
+	ASSERT_EQ(pfd.revents & POLLIN, 0, "no_pollin_after_drain");
+	ASSERT_NEQ(pfd.revents & POLLHUP, 0, "pollhup_after_drain");
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, 0, "read_eof");
+
+out_close:
+	if (epfd >= 0)
+		close(epfd);
+	close(fd);
+out_destroy:
+	stream__destroy(skel);
+}
+
+static void test_stream_fd_edge(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts,
+		.flags = BPF_F_STREAM_NONBLOCK,
+	);
+	struct epoll_event event = {
+		.events = EPOLLIN | EPOLLET,
+	};
+	struct stream *skel;
+	char buf[8] = {};
+	int epfd = -1, fd, ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_syscall),
+				  BPF_STREAM_STDOUT, &opts);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out_destroy;
+	epfd = epoll_create1(EPOLL_CLOEXEC);
+	if (!ASSERT_OK_FD(epfd, "epoll_create"))
+		goto out_close;
+	event.data.fd = fd;
+	ret = epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);
+	if (!ASSERT_OK(ret, "epoll_ctl"))
+		goto out_close;
+
+	/* Output into an empty stream raises an edge. */
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_close;
+	ret = epoll_wait(epfd, &event, 1, 5000);
+	if (!ASSERT_EQ(ret, 1, "epoll_wait_first"))
+		goto out_close;
+	ASSERT_NEQ(event.events & EPOLLIN, 0, "epollin_first");
+
+	/*
+	 * Leave data behind, add more, then drain everything. Whether output
+	 * into a non-empty stream raises an edge of its own is not part of the
+	 * contract, so discard any pending event before re-arming the stream.
+	 */
+	ret = read(fd, buf, 2);
+	ASSERT_EQ(ret, 2, "read_partial");
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_close;
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, 4, "read_rest");
+	ASSERT_OK(memcmp(buf, "ofoo", 4), "read_rest_data");
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, -1, "read_drained");
+	ASSERT_EQ(errno, EAGAIN, "read_drained_errno");
+	epoll_wait(epfd, &event, 1, 0);
+
+	/* Output into the drained stream raises an edge again. */
+	if (!stream_fd_trigger(skel->progs.stream_syscall))
+		goto out_close;
+	ret = epoll_wait(epfd, &event, 1, 5000);
+	if (!ASSERT_EQ(ret, 1, "epoll_wait_after_drain"))
+		goto out_close;
+	ASSERT_NEQ(event.events & EPOLLIN, 0, "epollin_after_drain");
+
+out_close:
+	if (epfd >= 0)
+		close(epfd);
+	close(fd);
+out_destroy:
+	stream__destroy(skel);
+}
+
+static void test_stream_fd_nmi_epoll(void)
+{
+	LIBBPF_OPTS(bpf_prog_stream_open_opts, opts,
+		.flags = BPF_F_STREAM_NONBLOCK,
+	);
+	struct perf_event_attr attr = {
+		.size = sizeof(attr),
+		.type = PERF_TYPE_HARDWARE,
+		.config = PERF_COUNT_HW_CPU_CYCLES,
+		.freq = 1,
+		.sample_freq = 10,
+	};
+	struct epoll_event event = {
+		.events = EPOLLIN,
+	};
+	struct bpf_link *link = NULL;
+	struct stream *skel;
+	__u64 deadline;
+	char buf[4] = {};
+	int epfd = -1, fd = -1, pmu_fd = -1;
+	int ret;
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+	fd = bpf_prog_stream_open(bpf_program__fd(skel->progs.stream_nmi),
+				  BPF_STREAM_STDOUT, &opts);
+	if (!ASSERT_OK_FD(fd, "stream_open"))
+		goto out;
+
+	epfd = epoll_create1(EPOLL_CLOEXEC);
+	if (!ASSERT_OK_FD(epfd, "epoll_create"))
+		goto out;
+	event.data.fd = fd;
+	ret = epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);
+	if (!ASSERT_OK(ret, "epoll_ctl"))
+		goto out;
+
+	pmu_fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1,
+			 PERF_FLAG_FD_CLOEXEC);
+	if (pmu_fd < 0 && (errno == ENOENT || errno == EOPNOTSUPP)) {
+		printf("%s:SKIP:no PERF_COUNT_HW_CPU_CYCLES\n", __func__);
+		test__skip();
+		goto out;
+	}
+	if (!ASSERT_GE(pmu_fd, 0, "perf_event_open"))
+		goto out;
+
+	link = bpf_program__attach_perf_event(skel->progs.stream_nmi, pmu_fd);
+	if (!ASSERT_OK_PTR(link, "attach_perf_event")) {
+		link = NULL;
+		goto out;
+	}
+	pmu_fd = -1;
+
+	deadline = get_time_ns() + NMI_TIMEOUT_NS;
+	do {
+		ret = epoll_wait(epfd, &event, 1, 0);
+	} while (ret == 0 && get_time_ns() < deadline);
+	if (!ASSERT_EQ(ret, 1, "epoll_wait"))
+		goto out;
+	ASSERT_NEQ(event.events & EPOLLIN, 0, "epollin");
+
+	ret = read(fd, buf, sizeof(buf));
+	ASSERT_EQ(ret, 3, "read_len");
+	ASSERT_OK(memcmp(buf, "nmi", 3), "read_data");
+
+out:
+	bpf_link__destroy(link);
+	if (pmu_fd >= 0)
+		close(pmu_fd);
+	if (epfd >= 0)
+		close(epfd);
+	if (fd >= 0)
+		close(fd);
+	stream__destroy(skel);
+}
+
+void test_stream_fd(void)
+{
+	if (test__start_subtest("open"))
+		test_stream_fd_open();
+	if (test__start_subtest("nonblock"))
+		test_stream_fd_nonblock();
+	if (test__start_subtest("empty"))
+		test_stream_fd_empty();
+	if (test__start_subtest("blocking"))
+		test_stream_fd_blocking();
+	if (test__start_subtest("hup"))
+		test_stream_fd_hup();
+	if (test__start_subtest("edge"))
+		test_stream_fd_edge();
+	if (test__start_subtest("nmi_epoll"))
+		test_stream_fd_nmi_epoll();
+}
+
 void test_stream_oversize(void)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
index 12fc29e45487..1ae015c4e6b4 100644
--- a/tools/testing/selftests/bpf/progs/stream.c
+++ b/tools/testing/selftests/bpf/progs/stream.c
@@ -44,6 +44,7 @@ struct {
 		  _X64 _X64 _X64 _X64 _X64 _X64 _X64 _X64)
 
 int size;
+int nmi_stream_prints;
 u64 fault_addr;
 void *arena_ptr;
 
@@ -124,6 +125,25 @@ int stream_syscall(void *ctx)
 	return 0;
 }
 
+SEC("syscall")
+__success __retval(0)
+int stream_empty(void *ctx)
+{
+	return bpf_stream_printk(BPF_STDOUT, "");
+}
+
+SEC("perf_event")
+int stream_nmi(void *ctx)
+{
+	if (nmi_stream_prints)
+		return 0;
+	/* Retry on a later sample if the write failed, e.g. with -EBUSY. */
+	if (bpf_stream_printk(BPF_STDOUT, "nmi"))
+		return 0;
+	nmi_stream_prints = 1;
+	return 0;
+}
+
 SEC("syscall")
 __success __retval(0)
 int stream_oversize(void *ctx)
-- 
2.53.0

Re: [PATCH bpf-next v3 0/5] File descriptor interface for BPF streams

[Kumar Kartikeya Dwivedi]

On Fri, 25 Sept 2026 at 06:55, Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
>
> BPF program stdout and stderr streams can currently be consumed only
> through BPF_PROG_STREAM_READ_BY_FD. This requires repeated bpf() calls and
> provides no way to block for output or integrate with poll-based event
> loops.
>
> Add BPF_PROG_STREAM_OPEN to return a read-only, close-on-exec descriptor
> for a program stream. Reads block by default, with an option for
> non-blocking mode. poll and epoll report readable data and report hangup
> when the program is freed, while allowing buffered data to be drained
> before EOF. Stream descriptors retain the stream storage without retaining
> the program itself. Waiters are notified through irq_work only when a
> publication turns an empty stream readable, as bpf_ringbuf does, so a
> program whose stream nobody drains pays for a single notification.
>
> Expose the command through libbpf and add a -w/--wait option to bpftool
> prog tracelog that blocks for new output until the program is unloaded or
> bpftool is interrupted. The default dump keeps using
> BPF_PROG_STREAM_READ_BY_FD and exits once buffered output is drained, so
> existing scripts behave the same on old and new kernels. The legacy
> command remains supported.
>
> The first patch fixes a pre-existing issue where empty stream writes
> allocate elements that escape capacity accounting; the readiness tracking
> added later relies on every element carrying data.
>
> See commit logs for details.
>

CI failures are unrelated, once is GCC unable to handle code pattern
(needs to be silenced in CI, like other existing stream tests), the
other is existing callx failure, which I sent a fix for here:
https://lore.kernel.org/bpf/20260925060945.1993547-1-memxor@gmail.com

Re: [PATCH bpf-next v3 0/5] File descriptor interface for BPF streams

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Fri, 25 Sep 2026 06:55:27 +0200 you wrote:
> BPF program stdout and stderr streams can currently be consumed only
> through BPF_PROG_STREAM_READ_BY_FD. This requires repeated bpf() calls and
> provides no way to block for output or integrate with poll-based event
> loops.
> 
> Add BPF_PROG_STREAM_OPEN to return a read-only, close-on-exec descriptor
> for a program stream. Reads block by default, with an option for
> non-blocking mode. poll and epoll report readable data and report hangup
> when the program is freed, while allowing buffered data to be drained
> before EOF. Stream descriptors retain the stream storage without retaining
> the program itself. Waiters are notified through irq_work only when a
> publication turns an empty stream readable, as bpf_ringbuf does, so a
> program whose stream nobody drains pays for a single notification.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v3,1/5] bpf: Skip zero-length stream writes
    https://git.kernel.org/bpf/bpf-next/c/f17bedcd2951
  - [bpf-next,v3,2/5] bpf: Add file descriptor interface for program streams
    https://git.kernel.org/bpf/bpf-next/c/e831ff570908
  - [bpf-next,v3,3/5] libbpf: Add bpf_prog_stream_open()
    https://git.kernel.org/bpf/bpf-next/c/9054903ab51b
  - [bpf-next,v3,4/5] bpftool: Add option to wait for program stream output
    https://git.kernel.org/bpf/bpf-next/c/0abecacb4bfd
  - [bpf-next,v3,5/5] selftests/bpf: Test program stream file descriptors
    https://git.kernel.org/bpf/bpf-next/c/4e1d734651e5

You are awesome, thank you!
-- 
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