[pseudo] [PATCH 0/2] close_range: implement it rather than return ENOSYS

[pseudo] [PATCH 0/2] close_range: implement it rather than return ENOSYS

[Baban]

pseudo's close_range() wrapper has returned ENOSYS unconditionally since
35433e6 ("ports/linux/guts: Add close_range wrapper for glibc 2.34"). That
was a deliberate call, and the commit message says why: "This one is
straight forward as it allows ENOSYS to be returned and the caller has to
handle it so lets do that."

It held for five years. It doesn't any more. systemd's close_all_fds() has
called close_range() since v247, but v260 raised its kernel baseline to
5.10, concluded close_range() (5.9) is always present, deleted its
/proc/self/fd fallback and started treating a failure as fatal.
safe_fork_full() then aborts the child, so under pseudo every fork+exec
dies:

    Failed to close all file descriptors: Function not implemented
    '(mkfs)' failed with exit status 1.

That is [YOCTO #16339], reported by Vincent Haupert against a
systemd-repart image build.

Vincent - Randy asked on the bug whether you wanted to send a patch for
this, and that question was addressed to you, not to the list. It had been
quiet for a few days and this looked worth picking up, but if you already
have it in progress please say so and ignore this series. I'm happy to
defer.

1/2 implements it. The stub was protecting something real: pseudo keeps its
own descriptors, including the socket to its server, inside the range a
caller asks to close, and the kernel does the closing, so pseudo cannot ask
it to skip one. Returning ENOSYS pushed callers onto the /proc loop, which
closes one fd at a time through close() - a path pseudo already wraps and
can defend itself inside. With the fallback gone, that no longer works.

So this follows the shape 21ff2fb ("ports/linux/guts: Add closefrom support
for glibc 2.34") already established for the same problem: a client-side op
works out the first fd above every descriptor pseudo keeps, closes the ones
below it by hand while stepping around its own, and hands that fd back so
the kernel is only turned loose on the tail that is safe. close_range() has
a top end where closefrom() does not, so both the manual loop and the path
table cleanup are bounded by maxfd.

The flags are settled before any of that. CLOSE_RANGE_UNSHARE has to take
effect first, or descriptors get closed for everyone still sharing the
table rather than just for us. CLOSE_RANGE_CLOEXEC closes nothing and
pseudo's own fds are close-on-exec already, so there is nothing to protect
and it goes straight to the kernel. An unknown flag or an inverted range is
refused up front, matching the kernel.

2/2 adds a test. It probes for kernel support with PSEUDO_DISABLED=1 first,
so that a wrapper which goes back to returning ENOSYS fails the test rather
than quietly skipping it.

Tested on x86_64 (Ubuntu 24.04, glibc 2.39). close_range() under pseudo is
identical to a run without pseudo across eight cases: single fd, bounded
range, both flags, an inverted range, an unknown flag, an already-closed
fd, and an unbounded range. pseudo also comes through close_range(3, ~0U, 0)
with its server connection intact and still tracking ownership, which is
the case that actually matters here. run_tests.sh is unchanged from master
apart from the new test passing (two pre-existing flaky parallel-* failures
show up either way).

Comparing against the real kernel is what caught the argument validation:
without it, close_range(3, 3, 0x80) returned success and closed fd 3, on a
call the kernel rejects outright.

Not tested: x86_64 only, no 32-bit/aarch64/riscv build; no end-to-end
systemd-repart reproduction, the ENOSYS-to-working transition is shown
directly instead; and CLOSE_RANGE_UNSHARE is exercised for return value and
closing behaviour but not against a real process sharing the descriptor
table, so its isolating effect rests on unshare(CLONE_FILES) semantics
rather than observation.

One question while I was in here. OP_CLOSEFROM steps around five of pseudo's
descriptors, but there are others it doesn't mention - pseudo_prefix_dir_fd,
pseudo_pwd_lck_fd, pseudo_util_evlog_fd. I kept the new op consistent with
the existing five rather than change closefrom()'s behaviour as a drive-by,
but if that list is short then it's short in both places and I'm happy to
send a follow-up.

Babanpreet Singh (2):
  ports/linux/guts: Implement close_range() instead of returning ENOSYS
  tests: Add close_range() test

 enums/op.in                    |   1 +
 ports/linux/guts/close_range.c |  55 +++++++++--
 ports/linux/portdefs.h         |  16 ++++
 pseudo_client.c                |  60 ++++++++++++
 test/test-close-range.c        | 167 +++++++++++++++++++++++++++++++++
 test/test-close-range.sh       |  16 ++++
 6 files changed, 308 insertions(+), 7 deletions(-)
 create mode 100644 test/test-close-range.c
 create mode 100755 test/test-close-range.sh

-- 
2.43.0

[pseudo] [PATCH 1/2] ports/linux/guts: Implement close_range() instead of returning ENOSYS

[Baban]

close_range() has been wrapped since 35433e6 ("ports/linux/guts: Add
close_range wrapper for glibc 2.34"), but the wrapper only sets ENOSYS,
on the grounds that callers have to handle that anyway. That was true
when it was written. It no longer is: systemd's close_all_fds() has used
close_range() since v247, and v260 raised its kernel baseline to 5.10,
dropped the /proc fallback and began treating a failure as fatal.
safe_fork_full() aborts the child when it fails, so under pseudo every
fork+exec dies, eg for "pseudo systemd-repart" spawning mkfs:

    Failed to close all file descriptors: Function not implemented
    '(mkfs)' failed with exit status 1.

Implement it the way closefrom() already is, per 21ff2fb ("ports/linux/
guts: Add closefrom support for glibc 2.34"): a client side op works out
the first fd above every descriptor pseudo keeps for itself, closes the
ones below it by hand while stepping around its own, and hands that fd
back so the caller can turn the kernel loose on the rest. close_range()
has a top end where closefrom() does not, so both the manual loop and
the path table cleanup are bounded by maxfd.

The flags are dealt with before any of that. CLOSE_RANGE_UNSHARE has to
take effect first, or descriptors would be closed for everyone still
sharing the table rather than just for us. CLOSE_RANGE_CLOEXEC closes
nothing, and pseudo's own fds are close-on-exec already, so there is
nothing to protect and it can go straight to the kernel. An unknown flag
or an inverted range is refused before anything is touched, matching the
kernel: otherwise a call which should have failed cleanly takes
descriptors with it on the way out.

A range starting above INT_MAX cannot hold any of pseudo's own fds and
is passed through, since the client op takes the low end as an int.

[YOCTO #16339]

AI-Generated: Uses Claude (claude-opus-4-8)
Signed-off-by: Babanpreet Singh <bbnpreetsingh@gmail.com>
---
 enums/op.in                    |  1 +
 ports/linux/guts/close_range.c | 55 +++++++++++++++++++++++++++----
 ports/linux/portdefs.h         | 16 +++++++++
 pseudo_client.c                | 60 ++++++++++++++++++++++++++++++++++
 4 files changed, 125 insertions(+), 7 deletions(-)

diff --git a/enums/op.in b/enums/op.in
index 5b5e21b..5013892 100644
--- a/enums/op.in
+++ b/enums/op.in
@@ -28,3 +28,4 @@ set-xattr, 0
 create-xattr, 1
 replace-xattr, 1
 closefrom, 0
+close-range, 0
diff --git a/ports/linux/guts/close_range.c b/ports/linux/guts/close_range.c
index 4bd2fe1..15258be 100644
--- a/ports/linux/guts/close_range.c
+++ b/ports/linux/guts/close_range.c
@@ -6,14 +6,55 @@
  * int close_range(unsigned int lowfd, unsigned int maxfd, int flags)
  *      int rc = -1;
  */
+	pseudo_msg_t *msg;
 
-        (void) lowfd;
-        (void) maxfd;
-        (void) flags;
-        /* for now pretend the kernel doesn't support it regardless 
-           which users are supposed to be able to handle */
-        errno = ENOSYS;
-        rc = -1;
+	/* The kernel rejects both of these outright, so check before doing
+	 * anything: otherwise a call which should have failed cleanly takes
+	 * descriptors with it on the way out.
+	 */
+	if (flags & ~(CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC)) {
+		errno = EINVAL;
+		return -1;
+	}
+	if (lowfd > maxfd) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	/* CLOSE_RANGE_UNSHARE has to take effect before anything is closed:
+	 * while the descriptor table is still shared, closing a descriptor
+	 * would close it for everyone sharing the table, not just for us.
+	 */
+	if (flags & CLOSE_RANGE_UNSHARE) {
+		if (unshare(CLONE_FILES) == -1)
+			return -1;
+		flags &= ~CLOSE_RANGE_UNSHARE;
+	}
+
+	/* CLOSE_RANGE_CLOEXEC closes nothing, it only marks descriptors, and
+	 * pseudo's own are close-on-exec already (pseudo_fd() sets that on
+	 * every one of them), so there is nothing here to protect.
+	 */
+	if (flags & CLOSE_RANGE_CLOEXEC)
+		return real_close_range(lowfd, maxfd, flags);
+
+	/* Descriptors are ints, so a range starting above INT_MAX cannot hold
+	 * any of pseudo's own and there is nothing to step around. Worth its
+	 * own case because pseudo_client_op() takes the low end as an int.
+	 */
+	if (lowfd > INT_MAX)
+		return real_close_range(lowfd, maxfd, flags);
+
+	/* Same shape as closefrom(): the client op closes the descriptors its
+	 * own are mixed in with by hand, stepping around the ones pseudo needs
+	 * to keep, and hands back the first fd the kernel can safely be turned
+	 * loose on.
+	 */
+	msg = pseudo_client_op(OP_CLOSE_RANGE, 0, lowfd, -1, 0, 0, maxfd);
+	if (maxfd >= (unsigned int) msg->fd)
+		rc = real_close_range(msg->fd, maxfd, flags);
+	else
+		rc = 0;
 
 /*      return rc;
  * }
diff --git a/ports/linux/portdefs.h b/ports/linux/portdefs.h
index 19bb232..1f1a41a 100644
--- a/ports/linux/portdefs.h
+++ b/ports/linux/portdefs.h
@@ -35,6 +35,22 @@ GLIBC_COMPAT_SYMBOL(memcpy,2.0);
 #include <sys/prctl.h>
 #include <linux/seccomp.h>
 
+/* close_range()'s flags, and unshare(), are only declared by glibc under
+ * _GNU_SOURCE, which pseudo does not build with. <linux/close_range.h> is
+ * not an option either: it is absent on hosts with pre-5.9 kernel headers,
+ * the same problem SYS_openat2 has below. Both values are kernel ABI.
+ */
+#ifndef CLOSE_RANGE_UNSHARE
+#define CLOSE_RANGE_UNSHARE (1U << 1)
+#endif
+#ifndef CLOSE_RANGE_CLOEXEC
+#define CLOSE_RANGE_CLOEXEC (1U << 2)
+#endif
+#ifndef CLONE_FILES
+#define CLONE_FILES 0x00000400
+#endif
+extern int unshare(int flags);
+
 #ifndef _STAT_VER
 #if defined (__aarch64__) || defined (__riscv)
 #define _STAT_VER 0
diff --git a/pseudo_client.c b/pseudo_client.c
index 7041366..1acd948 100644
--- a/pseudo_client.c
+++ b/pseudo_client.c
@@ -993,6 +993,28 @@ pseudo_client_closefrom(int fd) {
 	}
 }
 
+/* Like pseudo_client_closefrom(), but bounded: close_range() has a top end,
+ * so entries above it have to be left alone.
+ */
+static void
+pseudo_client_close_range(int lowfd, unsigned int maxfd) {
+	int i, top;
+
+	if (lowfd < 0 || lowfd >= nfds)
+		return;
+
+	top = (maxfd >= (unsigned int) nfds) ? nfds - 1 : (int) maxfd;
+	for (i = lowfd; i <= top; ++i) {
+		free(fd_paths[i]);
+		fd_paths[i] = 0;
+
+		if (i < linked_nfds) {
+			free(linked_fd_paths[i]);
+			linked_fd_paths[i] = 0;
+		}
+	}
+}
+
 /* spawn server */
 static int
 client_spawn_server(void) {
@@ -1633,6 +1655,7 @@ pseudo_client_op(pseudo_op_t op, int access, int fd, int dirfd, const char *path
 	static size_t alloced_len = 0;
 	int strip_slash;
 	int startfd, i;
+	unsigned int close_range_maxfd = 0;
 
 #ifdef PSEUDO_PROFILING
 	struct timeval tv1_op, tv2_op;
@@ -1753,6 +1776,13 @@ pseudo_client_op(pseudo_op_t op, int access, int fd, int dirfd, const char *path
 	}
 #endif
 
+	if (op == OP_CLOSE_RANGE) {
+		va_list ap;
+		va_start(ap, buf);
+		close_range_maxfd = va_arg(ap, unsigned int);
+		va_end(ap);
+	}
+
 	if (op == OP_RENAME) {
 		va_list ap;
 		if (!path) {
@@ -1959,6 +1989,36 @@ pseudo_client_op(pseudo_op_t op, int access, int fd, int dirfd, const char *path
 		msg.fd = startfd;
 		do_request = 0;
 		break;
+	case OP_CLOSE_RANGE:
+		/* no request needed */
+		startfd = fd;
+		if (pseudo_util_debug_fd > startfd)
+			startfd = pseudo_util_debug_fd + 1;
+		if (pseudo_localstate_dir_fd > startfd)
+			startfd = pseudo_localstate_dir_fd + 1;
+		if (pseudo_pwd_fd > startfd)
+			startfd = pseudo_pwd_fd + 1;
+		if (pseudo_grp_fd > startfd)
+			startfd = pseudo_grp_fd + 1;
+		if (connect_fd > startfd)
+			startfd = connect_fd + 1;
+		/* the fds below startfd are the ones our own are mixed in
+		 * with, so close those by hand and skip the ones we need
+		 */
+		for (i = fd; i < startfd && (unsigned int) i <= close_range_maxfd; ++i) {
+			if (i == pseudo_util_debug_fd || i == pseudo_localstate_dir_fd || i == pseudo_pwd_fd ||
+					i == pseudo_grp_fd || i == connect_fd)
+				continue;
+			pseudo_client_close(i);
+			close(i);
+		}
+		if (close_range_maxfd >= (unsigned int) startfd)
+			pseudo_client_close_range(startfd, close_range_maxfd);
+		/* tell the caller to start at startfd instead of fd */
+		result = &msg;
+		msg.fd = startfd;
+		do_request = 0;
+		break;
 	case OP_CLOSE:
 		/* no request needed */
 		if (fd >= 0) {
-- 
2.43.0

[pseudo] [PATCH 2/2] tests: Add close_range() test

[Baban]

Covers the ENOSYS regression itself, that the wrapper agrees with the
kernel about bad arguments, and the part that actually needs the care:
pseudo keeps its own descriptors in the range, including the connection
to its server, and has to come out of a close_range(3, ~0U, 0) still
tracking ownership.

close_range() needs a 5.9 kernel, so the shell wrapper probes for it
with PSEUDO_DISABLED=1 before running anything. Asking with pseudo out
of the way keeps a wrapper which has gone back to returning ENOSYS from
passing itself off as an old kernel and quietly skipping the test it is
supposed to fail.

[YOCTO #16339]

AI-Generated: Uses Claude (claude-opus-4-8)
Signed-off-by: Babanpreet Singh <bbnpreetsingh@gmail.com>
---
 test/test-close-range.c  | 167 +++++++++++++++++++++++++++++++++++++++
 test/test-close-range.sh |  16 ++++
 2 files changed, 183 insertions(+)
 create mode 100644 test/test-close-range.c
 create mode 100755 test/test-close-range.sh

diff --git a/test/test-close-range.c b/test/test-close-range.c
new file mode 100644
index 0000000..61f729b
--- /dev/null
+++ b/test/test-close-range.c
@@ -0,0 +1,167 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-only
+ *
+ * close_range() was stubbed out to return ENOSYS unconditionally, on the
+ * assumption that callers all cope with that. Current systemd does not: it
+ * dropped its /proc fallback once its kernel baseline reached 5.10, so every
+ * fork+exec under pseudo failed. Check the wrapper works, that it agrees
+ * with the kernel about bad arguments, and that pseudo still functions after
+ * a caller closes every descriptor from 3 up.
+ */
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+/* <linux/close_range.h> is missing on hosts with pre-5.9 kernel headers */
+#ifndef CLOSE_RANGE_CLOEXEC
+#define CLOSE_RANGE_CLOEXEC (1U << 2)
+#endif
+
+/* matches test-openat2-syscall: the shell wrapper turns 77 into "skipped" */
+#define SKIP 77
+
+#define TESTFILE "test-close-range-file"
+
+static int open_null(void) {
+	int fd = open("/dev/null", O_RDONLY);
+	if (fd < 0)
+		perror("open /dev/null");
+	return fd;
+}
+
+static int is_closed(int fd) {
+	return fcntl(fd, F_GETFD) == -1 && errno == EBADF;
+}
+
+/* Is close_range() there at all? The wrapper runs this with PSEUDO_DISABLED
+ * set, so the answer is the kernel's and a stubbed wrapper cannot pass
+ * itself off as an old kernel.
+ */
+static int probe(void) {
+	int fd = open_null();
+
+	if (fd < 0)
+		return 1;
+	if (close_range(fd, fd, 0) == -1) {
+		int err = errno;
+		close(fd);
+		return (err == ENOSYS) ? SKIP : 1;
+	}
+	return 0;
+}
+
+int main(int argc, char **argv) {
+	struct stat st;
+	int a, b, c;
+
+	if (argc > 1 && !strcmp(argv[1], "--probe"))
+		return probe();
+
+	/* the reported bug: this used to fail with ENOSYS every time */
+	a = open_null();
+	if (a < 0)
+		return 1;
+	if (close_range(a, a, 0) == -1) {
+		fprintf(stderr, "close_range(%d, %d, 0): %s\n", a, a, strerror(errno));
+		return 1;
+	}
+	if (!is_closed(a)) {
+		fprintf(stderr, "close_range() left fd %d open\n", a);
+		return 1;
+	}
+
+	/* a bounded range closes all of itself */
+	a = open_null();
+	b = open_null();
+	c = open_null();
+	if (a < 0 || b < 0 || c < 0)
+		return 1;
+	if (close_range(a, c, 0) == -1) {
+		perror("close_range(a, c, 0)");
+		return 1;
+	}
+	if (!is_closed(a) || !is_closed(b) || !is_closed(c)) {
+		fprintf(stderr, "close_range(%d, %d, 0) left descriptors open\n", a, c);
+		return 1;
+	}
+
+	/* CLOSE_RANGE_CLOEXEC marks descriptors, it does not close them */
+	a = open_null();
+	if (a < 0)
+		return 1;
+	if (close_range(a, a, CLOSE_RANGE_CLOEXEC) == -1) {
+		perror("close_range(a, a, CLOSE_RANGE_CLOEXEC)");
+		return 1;
+	}
+	if (is_closed(a)) {
+		fprintf(stderr, "CLOSE_RANGE_CLOEXEC closed fd %d\n", a);
+		return 1;
+	}
+	if (!(fcntl(a, F_GETFD) & FD_CLOEXEC)) {
+		fprintf(stderr, "CLOSE_RANGE_CLOEXEC did not set FD_CLOEXEC on fd %d\n", a);
+		return 1;
+	}
+	close(a);
+
+	/* bad arguments are refused, and nothing is closed on the way out */
+	a = open_null();
+	if (a < 0)
+		return 1;
+	if (close_range(a, a, 0x80) != -1 || errno != EINVAL) {
+		fprintf(stderr, "close_range() with an unknown flag should fail EINVAL\n");
+		return 1;
+	}
+	if (is_closed(a)) {
+		fprintf(stderr, "a rejected close_range() closed fd %d anyway\n", a);
+		return 1;
+	}
+	if (close_range(a + 1, a, 0) != -1 || errno != EINVAL) {
+		fprintf(stderr, "close_range() with lowfd > maxfd should fail EINVAL\n");
+		return 1;
+	}
+	close(a);
+
+	/* a range entirely above INT_MAX holds no descriptors at all, but it
+	 * still has to be accepted rather than mangled on the way through
+	 */
+	if (close_range((unsigned int) INT_MAX + 1, ~0U, 0) == -1) {
+		fprintf(stderr, "close_range(INT_MAX+1, ~0U, 0): %s\n", strerror(errno));
+		return 1;
+	}
+
+	/* And the point of the care in the wrapper: pseudo keeps descriptors
+	 * of its own in this range and has to come out of it still working.
+	 */
+	a = open(TESTFILE, O_CREAT | O_WRONLY, 0644);
+	if (a < 0) {
+		perror("open " TESTFILE);
+		return 1;
+	}
+	close(a);
+	if (chown(TESTFILE, 0, 0) == -1) {
+		perror("chown " TESTFILE);
+		return 1;
+	}
+	if (close_range(3, ~0U, 0) == -1) {
+		fprintf(stderr, "close_range(3, ~0U, 0): %s\n", strerror(errno));
+		return 1;
+	}
+	if (stat(TESTFILE, &st) == -1) {
+		perror("stat after close_range");
+		return 1;
+	}
+	if (st.st_uid != 0 || st.st_gid != 0) {
+		fprintf(stderr, "pseudo lost track after close_range: uid %d, gid %d\n",
+			(int) st.st_uid, (int) st.st_gid);
+		return 1;
+	}
+	unlink(TESTFILE);
+
+	return 0;
+}
diff --git a/test/test-close-range.sh b/test/test-close-range.sh
new file mode 100755
index 0000000..65be016
--- /dev/null
+++ b/test/test-close-range.sh
@@ -0,0 +1,16 @@
+#!/bin/bash
+#
+# SPDX-License-Identifier: LGPL-2.1-only
+#
+
+# close_range() needs a 5.9 kernel. Ask with pseudo out of the way, so that a
+# wrapper which has gone back to returning ENOSYS cannot pass itself off as an
+# old kernel and quietly skip the test it is supposed to fail.
+PSEUDO_DISABLED=1 ./test/test-close-range --probe
+case $? in
+0)  ;;
+77) exit 255 ;;
+*)  exit 1 ;;
+esac
+
+./test/test-close-range
-- 
2.43.0