[PATCH bpf-next v5 0/2] bpf: Add a generic bits iterator

[PATCH bpf-next v5 0/2] bpf: Add a generic bits iterator

[Yafang Shao]

Three new kfuncs, namely bpf_iter_bits_{new,next,destroy}, have been
added for the new bpf_iter_bits functionality. These kfuncs enable the
iteration of the bits from a given address and a given number of bits.

- bpf_iter_bits_new
  Initialize a new bits iterator for a given memory area. Due to the
  limitation of bpf memalloc, the max number of bits to be iterated
  over is (4096 * 8).
- bpf_iter_bits_next
  Get the next bit in a bpf_iter_bits
- bpf_iter_bits_destroy
  Destroy a bpf_iter_bits

The bits iterator can be used in any context and on any address.

Changes:
- v4->v5:
  - Simplify test cases (Andrii)
- v3->v4:
  - Fix endianness error on s390x (Andrii)
  - zero-initialize kit->bits_copy and zero out nr_bits (Andrii)
- v2->v3:
  - Optimization for u64/u32 mask (Andrii)
- v1->v2:
  - Simplify the CPU number verification code to avoid the failure on s390x
    (Eduard)
- bpf: Add bpf_iter_cpumask
  https://lwn.net/Articles/961104/
- bpf: Add new bpf helper bpf_for_each_cpu
  https://lwn.net/Articles/939939/

Yafang Shao (2):
  bpf: Add bits iterator
  selftests/bpf: Add selftest for bits iter

 kernel/bpf/helpers.c                          | 120 ++++++++++++++++++
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../selftests/bpf/progs/verifier_bits_iter.c  |  57 +++++++++
 3 files changed, 179 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_bits_iter.c

-- 
2.39.1

[PATCH bpf-next v5 1/2] bpf: Add bits iterator

[Yafang Shao]

Add three new kfuncs for the bits iterator:
- bpf_iter_bits_new
  Initialize a new bits iterator for a given memory area. Due to the
  limitation of bpf memalloc, the max number of bits that can be iterated
  over is limited to (4096 * 8).
- bpf_iter_bits_next
  Get the next bit in a bpf_iter_bits
- bpf_iter_bits_destroy
  Destroy a bpf_iter_bits

The bits iterator facilitates the iteration of the bits of a memory area,
such as cpumask. It can be used in any context and on any address.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/helpers.c | 120 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 120 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index d9e7aca8ae9e..dec5d3ccd25b 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2549,6 +2549,123 @@ __bpf_kfunc void bpf_throw(u64 cookie)
 	WARN(1, "A call to BPF exception callback should never return\n");
 }
 
+struct bpf_iter_bits {
+	__u64 __opaque[2];
+} __aligned(8);
+
+struct bpf_iter_bits_kern {
+	union {
+		unsigned long *bits;
+		unsigned long bits_copy;
+	};
+	u32 nr_bits;
+	int bit;
+} __aligned(8);
+
+/**
+ * bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
+ * @it: The new bpf_iter_bits to be created
+ * @unsafe_ptr__ign: A ponter pointing to a memory area to be iterated over
+ * @nr_bits: The number of bits to be iterated over. Due to the limitation of
+ * memalloc, it can't greater than (4096 * 8).
+ *
+ * This function initializes a new bpf_iter_bits structure for iterating over
+ * a memory area which is specified by the @unsafe_ptr__ign and @nr_bits. It
+ * copy the data of the memory area to the newly created bpf_iter_bits @it for
+ * subsequent iteration operations.
+ *
+ * On success, 0 is returned. On failure, ERR is returned.
+ */
+__bpf_kfunc int
+bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign, u32 nr_bits)
+{
+	u32 size = BITS_TO_LONGS(nr_bits) * sizeof(unsigned long);
+	struct bpf_iter_bits_kern *kit = (void *)it;
+	int err;
+
+	BUILD_BUG_ON(sizeof(struct bpf_iter_bits_kern) != sizeof(struct bpf_iter_bits));
+	BUILD_BUG_ON(__alignof__(struct bpf_iter_bits_kern) !=
+		     __alignof__(struct bpf_iter_bits));
+
+	if (!unsafe_ptr__ign || !nr_bits) {
+		kit->bits = NULL;
+		return -EINVAL;
+	}
+
+	kit->nr_bits = 0;
+	kit->bits_copy = 0;
+	/* Optimization for u64/u32 mask */
+	if (nr_bits <= 64) {
+		err = bpf_probe_read_kernel_common(&kit->bits_copy, size, unsafe_ptr__ign);
+		if (err)
+			return -EFAULT;
+
+		kit->nr_bits = nr_bits;
+		kit->bit = -1;
+		return 0;
+	}
+
+	/* Fallback to memalloc */
+	kit->bits = bpf_mem_alloc(&bpf_global_ma, size);
+	if (!kit->bits)
+		return -ENOMEM;
+
+	err = bpf_probe_read_kernel_common(kit->bits, size, unsafe_ptr__ign);
+	if (err) {
+		bpf_mem_free(&bpf_global_ma, kit->bits);
+		return err;
+	}
+
+	kit->nr_bits = nr_bits;
+	kit->bit = -1;
+	return 0;
+}
+
+/**
+ * bpf_iter_bits_next() - Get the next bit in a bpf_iter_bits
+ * @it: The bpf_iter_bits to be checked
+ *
+ * This function returns a pointer to a number representing the value of the
+ * next bit in the bits.
+ *
+ * If there are no further bit available, it returns NULL.
+ */
+__bpf_kfunc int *bpf_iter_bits_next(struct bpf_iter_bits *it)
+{
+	struct bpf_iter_bits_kern *kit = (void *)it;
+	u32 nr_bits = kit->nr_bits;
+	const unsigned long *bits;
+	int bit;
+
+	if (nr_bits == 0)
+		return NULL;
+
+	bits = nr_bits <= 64 ? &kit->bits_copy : kit->bits;
+	bit = find_next_bit(bits, nr_bits, kit->bit + 1);
+	if (bit >= nr_bits) {
+		kit->nr_bits = 0;
+		return NULL;
+	}
+
+	kit->bit = bit;
+	return &kit->bit;
+}
+
+/**
+ * bpf_iter_bits_destroy() - Destroy a bpf_iter_bits
+ * @it: The bpf_iter_bits to be destroyed
+ *
+ * Destroy the resource associated with the bpf_iter_bits.
+ */
+__bpf_kfunc void bpf_iter_bits_destroy(struct bpf_iter_bits *it)
+{
+	struct bpf_iter_bits_kern *kit = (void *)it;
+
+	if (kit->nr_bits <= 64)
+		return;
+	bpf_mem_free(&bpf_global_ma, kit->bits);
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(generic_btf_ids)
@@ -2626,6 +2743,9 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
 BTF_ID_FLAGS(func, bpf_dynptr_size)
 BTF_ID_FLAGS(func, bpf_dynptr_clone)
 BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
+BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
+BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
-- 
2.30.1 (Apple Git-130)

[PATCH bpf-next v5 2/2] selftests/bpf: Add selftest for bits iter

[Yafang Shao]

Add test cases for the bits iter:
  - bpf_iter_bits_destroy() is required after calling
    bpf_iter_bits_new()
  - bpf_iter_bits_destroy() can only destroy an initialized iter
  - bpf_iter_bits_next() must use an initialized iter

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../selftests/bpf/progs/verifier_bits_iter.c  | 57 +++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_bits_iter.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index c4f9f306646e..7e04ecaaa20a 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -84,6 +84,7 @@
 #include "verifier_xadd.skel.h"
 #include "verifier_xdp.skel.h"
 #include "verifier_xdp_direct_packet_access.skel.h"
+#include "verifier_bits_iter.skel.h"
 
 #define MAX_ENTRIES 11
 
@@ -198,6 +199,7 @@ void test_verifier_var_off(void)              { RUN(verifier_var_off); }
 void test_verifier_xadd(void)                 { RUN(verifier_xadd); }
 void test_verifier_xdp(void)                  { RUN(verifier_xdp); }
 void test_verifier_xdp_direct_packet_access(void) { RUN(verifier_xdp_direct_packet_access); }
+void test_verifier_bits_iter(void) { RUN(verifier_bits_iter); }
 
 static int init_test_val_map(struct bpf_object *obj, char *map_name)
 {
diff --git a/tools/testing/selftests/bpf/progs/verifier_bits_iter.c b/tools/testing/selftests/bpf/progs/verifier_bits_iter.c
new file mode 100644
index 000000000000..5401b0944ca7
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_bits_iter.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2024 Yafang Shao <laoar.shao@gmail.com> */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#include "bpf_misc.h"
+#include "task_kfunc_common.h"
+
+char _license[] SEC("license") = "GPL";
+
+int bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign,
+		      u32 nr_bits) __ksym __weak;
+int *bpf_iter_bits_next(struct bpf_iter_bits *it) __ksym __weak;
+void bpf_iter_bits_destroy(struct bpf_iter_bits *it) __ksym __weak;
+
+SEC("iter.s/cgroup")
+__description("bits iter without destroy")
+__failure __msg("Unreleased reference")
+int BPF_PROG(no_destroy, struct bpf_iter_meta *meta, struct cgroup *cgrp)
+{
+	struct bpf_iter_bits it;
+	struct task_struct *p;
+
+	p = bpf_task_from_pid(1);
+	if (!p)
+		return 1;
+
+	bpf_iter_bits_new(&it, p->cpus_ptr, 8192);
+
+	bpf_iter_bits_next(&it);
+	bpf_task_release(p);
+	return 0;
+}
+
+SEC("iter/cgroup")
+__description("bits iter with uninitialized iter in ->next()")
+__failure __msg("expected an initialized iter_bits as arg #1")
+int BPF_PROG(next_uninit, struct bpf_iter_meta *meta, struct cgroup *cgrp)
+{
+	struct bpf_iter_bits *it = NULL;
+
+	bpf_iter_bits_next(it);
+	return 0;
+}
+
+SEC("iter/cgroup")
+__description("bits iter with uninitialized iter in ->destroy()")
+__failure __msg("expected an initialized iter_bits as arg #1")
+int BPF_PROG(destroy_uninit, struct bpf_iter_meta *meta, struct cgroup *cgrp)
+{
+	struct bpf_iter_bits it = {};
+
+	bpf_iter_bits_destroy(&it);
+	return 0;
+}
-- 
2.30.1 (Apple Git-130)

Re: [PATCH bpf-next v5 0/2] bpf: Add a generic bits iterator

[Andrii Nakryiko]

On Sat, Mar 30, 2024 at 8:44 PM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> Three new kfuncs, namely bpf_iter_bits_{new,next,destroy}, have been
> added for the new bpf_iter_bits functionality. These kfuncs enable the
> iteration of the bits from a given address and a given number of bits.
>
> - bpf_iter_bits_new
>   Initialize a new bits iterator for a given memory area. Due to the
>   limitation of bpf memalloc, the max number of bits to be iterated
>   over is (4096 * 8).
> - bpf_iter_bits_next
>   Get the next bit in a bpf_iter_bits
> - bpf_iter_bits_destroy
>   Destroy a bpf_iter_bits
>
> The bits iterator can be used in any context and on any address.
>
> Changes:
> - v4->v5:
>   - Simplify test cases (Andrii)

hm... I think you oversimplified them :) Your negative tests are good,
but there is now no "positive" test that shows that iterator functions
properly. I'd suggest to add few positive tests (still within
RUN_TESTS framework) using __retval (grep for other use cases using
it) to check actual values.

I think you need to validate that iterator works for:

a) bit mask  smaller than 8 bytes (and also maybe number of bits that
are not multiple of 8, e.g., have 20 bit bitmask, but set entire
32-bit value to 1s, check that you get only 20 iterator next()
returns)

b) a typical case of having 8-byte bit mask (you can have some
irregular pattern of bits, like 0b1111000111001101, and calculate a
sum of bit positions, returned for __retval check. This will catch
endianness issues, if there are still any.

c) another typical case where bit mask is > 8 bytes (same idea with
position sum would work, but probably just counting number of bits
would be enough)

pw-bot: cr

> - v3->v4:
>   - Fix endianness error on s390x (Andrii)
>   - zero-initialize kit->bits_copy and zero out nr_bits (Andrii)
> - v2->v3:
>   - Optimization for u64/u32 mask (Andrii)
> - v1->v2:
>   - Simplify the CPU number verification code to avoid the failure on s390x
>     (Eduard)
> - bpf: Add bpf_iter_cpumask
>   https://lwn.net/Articles/961104/
> - bpf: Add new bpf helper bpf_for_each_cpu
>   https://lwn.net/Articles/939939/
>
> Yafang Shao (2):
>   bpf: Add bits iterator
>   selftests/bpf: Add selftest for bits iter
>
>  kernel/bpf/helpers.c                          | 120 ++++++++++++++++++
>  .../selftests/bpf/prog_tests/verifier.c       |   2 +
>  .../selftests/bpf/progs/verifier_bits_iter.c  |  57 +++++++++
>  3 files changed, 179 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/progs/verifier_bits_iter.c
>
> --
> 2.39.1
>

Re: [PATCH bpf-next v5 0/2] bpf: Add a generic bits iterator

[Yafang Shao]

On Wed, Apr 3, 2024 at 1:22 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Mar 30, 2024 at 8:44 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > Three new kfuncs, namely bpf_iter_bits_{new,next,destroy}, have been
> > added for the new bpf_iter_bits functionality. These kfuncs enable the
> > iteration of the bits from a given address and a given number of bits.
> >
> > - bpf_iter_bits_new
> >   Initialize a new bits iterator for a given memory area. Due to the
> >   limitation of bpf memalloc, the max number of bits to be iterated
> >   over is (4096 * 8).
> > - bpf_iter_bits_next
> >   Get the next bit in a bpf_iter_bits
> > - bpf_iter_bits_destroy
> >   Destroy a bpf_iter_bits
> >
> > The bits iterator can be used in any context and on any address.
> >
> > Changes:
> > - v4->v5:
> >   - Simplify test cases (Andrii)
>
> hm... I think you oversimplified them :) Your negative tests are good,

I misinterpreted your earlier comment :(

> but there is now no "positive" test that shows that iterator functions
> properly. I'd suggest to add few positive tests (still within
> RUN_TESTS framework) using __retval (grep for other use cases using
> it) to check actual values.
>
> I think you need to validate that iterator works for:
>
> a) bit mask  smaller than 8 bytes (and also maybe number of bits that
> are not multiple of 8, e.g., have 20 bit bitmask, but set entire
> 32-bit value to 1s, check that you get only 20 iterator next()
> returns)
>
> b) a typical case of having 8-byte bit mask (you can have some
> irregular pattern of bits, like 0b1111000111001101, and calculate a
> sum of bit positions, returned for __retval check. This will catch
> endianness issues, if there are still any.
>
> c) another typical case where bit mask is > 8 bytes (same idea with
> position sum would work, but probably just counting number of bits
> would be enough)

Thank you for providing such a detailed explanation. I will take some
time to carefully consider it.

-- 
Regards
Yafang