[PATCH v3 0/2] bpf: introduce helper for populating bpf_cpumask

[PATCH v3 0/2] bpf: introduce helper for populating bpf_cpumask

[Emil Tsalapatis]

Some BPF programs like scx schedulers have their own internal CPU mask types, 
mask types, which they must transform into struct bpf_cpumask instances
before passing them to scheduling-related kfuncs. There is currently no
way to efficiently populate the bitfield of a bpf_cpumask from BPF memory, 
and programs must use multiple bpf_cpumask_[set, clear] calls to do so. 
Introduce a kfunc helper to populate the bitfield of a bpf_cpumask from valid 
BPF memory with a single call.

Changelog :
-----------
v2->v3
v2: https://lore.kernel.org/bpf/20250305021020.1004858-1-emil@etsalapatis.com/

Addressed feedback by Alexei Starovoitov:
	* Added back patch descriptions dropped from v1->v2
	* Elide the alignment check for archs with efficient
	  unaligned accesses

v1->v2
v1: https://lore.kernel.org/bpf/20250228003321.1409285-1-emil@etsalapatis.com/

Addressed feedback by Hou Tao:
	* Add check that the input buffer is aligned to sizeof(long)
	* Adjust input buffer size check to use bitmap_size()
	* Add selftest for checking the bit pattern of the bpf_cpumask
	* Moved all selftests into existing files

Signed-off-by: Emil Tsalapatis (Meta) <emil@etsalapatis.com>

Emil Tsalapatis (2):
  bpf: add kfunc for populating cpumask bits
  selftests: bpf: add bpf_cpumask_fill selftests

 kernel/bpf/cpumask.c                          |  28 +++++
 .../selftests/bpf/prog_tests/cpumask.c        |   3 +
 .../selftests/bpf/progs/cpumask_failure.c     |  38 ++++++
 .../selftests/bpf/progs/cpumask_success.c     | 114 ++++++++++++++++++
 4 files changed, 183 insertions(+)

-- 
2.47.1

[PATCH v3 1/2] bpf: add kfunc for populating cpumask bits

[Emil Tsalapatis]

Add a helper kfunc that sets the bitmap of a bpf_cpumask from BPF memory.

Signed-off-by: Emil Tsalapatis (Meta) <emil@etsalapatis.com>
---
 kernel/bpf/cpumask.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index cfa1c18e3a48..2a0770544fc3 100644
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -420,6 +420,33 @@ __bpf_kfunc u32 bpf_cpumask_weight(const struct cpumask *cpumask)
 	return cpumask_weight(cpumask);
 }
 
+/**
+ * bpf_cpumask_fill() - Populate the CPU mask from the contents of
+ * a BPF memory region.
+ *
+ * @cpumask: The cpumask being populated.
+ * @src: The BPF memory holding the bit pattern.
+ * @src__sz: Length of the BPF memory region in bytes.
+ *
+ */
+__bpf_kfunc int bpf_cpumask_fill(struct cpumask *cpumask, void *src, size_t src__sz)
+{
+	unsigned long source = (unsigned long)src;
+
+	/* The memory region must be large enough to populate the entire CPU mask. */
+	if (src__sz < bitmap_size(nr_cpu_ids))
+		return -EACCES;
+
+	/* If avoiding unaligned accesses, the input region must be aligned to the nearest long. */
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
+		!IS_ALIGNED(source, sizeof(long)))
+		return -EINVAL;
+
+	bitmap_copy(cpumask_bits(cpumask), src, nr_cpu_ids);
+
+	return 0;
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(cpumask_kfunc_btf_ids)
@@ -448,6 +475,7 @@ BTF_ID_FLAGS(func, bpf_cpumask_copy, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_any_distribute, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_any_and_distribute, KF_RCU)
 BTF_ID_FLAGS(func, bpf_cpumask_weight, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_fill, KF_RCU)
 BTF_KFUNCS_END(cpumask_kfunc_btf_ids)
 
 static const struct btf_kfunc_id_set cpumask_kfunc_set = {
-- 
2.47.1

Re: [PATCH v3 1/2] bpf: add kfunc for populating cpumask bits

[Alexei Starovoitov]

On Wed, Mar 5, 2025 at 8:13 AM Emil Tsalapatis <emil@etsalapatis.com> wrote:
>
> Add a helper kfunc that sets the bitmap of a bpf_cpumask from BPF memory.
>
> Signed-off-by: Emil Tsalapatis (Meta) <emil@etsalapatis.com>
> ---
>  kernel/bpf/cpumask.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> index cfa1c18e3a48..2a0770544fc3 100644
> --- a/kernel/bpf/cpumask.c
> +++ b/kernel/bpf/cpumask.c
> @@ -420,6 +420,33 @@ __bpf_kfunc u32 bpf_cpumask_weight(const struct cpumask *cpumask)
>         return cpumask_weight(cpumask);
>  }
>
> +/**
> + * bpf_cpumask_fill() - Populate the CPU mask from the contents of
> + * a BPF memory region.
> + *
> + * @cpumask: The cpumask being populated.
> + * @src: The BPF memory holding the bit pattern.
> + * @src__sz: Length of the BPF memory region in bytes.
> + *
> + */

Since you're adding kdoc, make it complete.
Otherwise there is a warn during the build:

> kernel/bpf/cpumask.c:433: warning: No description found for return value of 'bpf_cpumask_fill'

and while at it, could you fix it for other kfuncs?

kernel/bpf/cpumask.c:50: warning: No description found for return
value of 'bpf_cpumask_create'
kernel/bpf/cpumask.c:76: warning: No description found for return
value of 'bpf_cpumask_acquire'

Thanks!

Re: [PATCH v3 1/2] bpf: add kfunc for populating cpumask bits

[Tejun Heo]

Hello,

On Wed, Mar 05, 2025 at 11:13:26AM -0500, Emil Tsalapatis wrote:
...
> +/**
> + * bpf_cpumask_fill() - Populate the CPU mask from the contents of
> + * a BPF memory region.

bitmap_fill() fills the bitmap with all 1's, so this name can be pretty
confusing. Maybe bpf_cpumask_populate_from() or something like that?

Thanks.

-- 
tejun

[PATCH v3 2/2] selftests: bpf: add bpf_cpumask_fill selftests

[Emil Tsalapatis]

Add selftests for the bpf_cpumask_fill helper that sets a bpf_cpumask to
a bit pattern provided by a BPF program.

Signed-off-by: Emil Tsalapatis (Meta) <emil@etsalapatis.com>
---
 .../selftests/bpf/prog_tests/cpumask.c        |   3 +
 .../selftests/bpf/progs/cpumask_failure.c     |  38 ++++++
 .../selftests/bpf/progs/cpumask_success.c     | 114 ++++++++++++++++++
 3 files changed, 155 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/cpumask.c b/tools/testing/selftests/bpf/prog_tests/cpumask.c
index e58a04654238..6185b63b931b 100644
--- a/tools/testing/selftests/bpf/prog_tests/cpumask.c
+++ b/tools/testing/selftests/bpf/prog_tests/cpumask.c
@@ -25,6 +25,9 @@ static const char * const cpumask_success_testcases[] = {
 	"test_global_mask_nested_deep_rcu",
 	"test_global_mask_nested_deep_array_rcu",
 	"test_cpumask_weight",
+	"test_fill_reject_unaligned",
+	"test_fill_reject_small_mask",
+	"test_fill",
 };
 
 static void verify_success(const char *prog_name)
diff --git a/tools/testing/selftests/bpf/progs/cpumask_failure.c b/tools/testing/selftests/bpf/progs/cpumask_failure.c
index b40b52548ffb..acecded8b155 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_failure.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_failure.c
@@ -222,3 +222,41 @@ int BPF_PROG(test_invalid_nested_array, struct task_struct *task, u64 clone_flag
 
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__failure __msg("type=scalar expected=fp")
+int BPF_PROG(test_fill_invalid_destination, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *invalid = (struct bpf_cpumask *)0x123456;
+	u64 bits;
+	int ret;
+
+	ret = bpf_cpumask_fill((struct cpumask *)invalid, &bits, sizeof(bits));
+	if (!ret)
+		err = 2;
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+__failure __msg("leads to invalid memory access")
+int BPF_PROG(test_fill_invalid_source, struct task_struct *task, u64 clone_flags)
+{
+	void *garbage = (void *)0x123456;
+	struct bpf_cpumask *local;
+	int ret;
+
+	local = create_cpumask();
+	if (!local) {
+		err = 1;
+		return 0;
+	}
+
+	ret = bpf_cpumask_fill((struct cpumask *)local, garbage, 8);
+	if (!ret)
+		err = 2;
+
+	bpf_cpumask_release(local);
+
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
index 80ee469b0b60..ba22878028c1 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_success.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
@@ -757,6 +757,7 @@ int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_fl
 	mask1 = bpf_cpumask_create();
 	mask2 = bpf_cpumask_create();
 
+
 	if (!mask1 || !mask2)
 		goto free_masks_return;
 
@@ -770,3 +771,116 @@ int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_fl
 		bpf_cpumask_release(mask2);
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_fill_reject_small_mask, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *local;
+	u8 toofewbits;
+	int ret;
+
+	local = create_cpumask();
+	if (!local)
+		return 0;
+
+	/* The kfunc should prevent this operation */
+	ret = bpf_cpumask_fill((struct cpumask *)local, &toofewbits, sizeof(toofewbits));
+	if (ret != -EACCES)
+		err = 2;
+
+	bpf_cpumask_release(local);
+
+	return 0;
+}
+
+/* Mask is guaranteed to be large enough for bpf_cpumask_t. */
+#define CPUMASK_TEST_MASKLEN (sizeof(cpumask_t))
+
+/* Add an extra word for the test_fill_reject_unaligned test. */
+u64 bits[CPUMASK_TEST_MASKLEN / 8 + 1];
+extern bool CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS __kconfig __weak;
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_fill_reject_unaligned, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *mask;
+	char *src;
+	int ret;
+
+	/* Skip if unaligned accesses are fine for this arch.  */
+	if (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+		return 0;
+
+	mask = bpf_cpumask_create();
+	if (!mask) {
+		err = 1;
+		return 0;
+	}
+
+	/* Misalign the source array by a byte. */
+	src = &((char *)bits)[1];
+
+	ret = bpf_cpumask_fill((struct cpumask *)mask, src, CPUMASK_TEST_MASKLEN);
+	if (ret != -EINVAL)
+		err = 2;
+
+	bpf_cpumask_release(mask);
+
+	return 0;
+}
+
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_fill, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *mask;
+	bool bit;
+	int ret;
+	int i;
+
+	/* Set only odd bits. */
+	__builtin_memset(bits, 0xaa, CPUMASK_TEST_MASKLEN);
+
+	mask = bpf_cpumask_create();
+	if (!mask) {
+		err = 1;
+		return 0;
+	}
+
+	/* Pass the entire bits array, the kfunc will only copy the valid bits. */
+	ret = bpf_cpumask_fill((struct cpumask *)mask, bits, CPUMASK_TEST_MASKLEN);
+	if (ret) {
+		err = 2;
+		goto out;
+	}
+
+	/*
+	 * Test is there to appease the verifier. We cannot directly
+	 * access NR_CPUS, the upper bound for nr_cpus, so we infer
+	 * it from the size of cpumask_t.
+	 */
+	if (nr_cpus < 0 || nr_cpus >= CPUMASK_TEST_MASKLEN * 8) {
+		err = 3;
+		goto out;
+	}
+
+	bpf_for(i, 0, nr_cpus) {
+		/* Odd-numbered bits should be set, even ones unset. */
+		bit = bpf_cpumask_test_cpu(i, (const struct cpumask *)mask);
+		if (bit == (i % 2 != 0))
+			continue;
+
+		err = 4;
+		break;
+	}
+
+out:
+	bpf_cpumask_release(mask);
+
+	return 0;
+}
+
+#undef CPUMASK_TEST_MASKLEN
-- 
2.47.1