[PATCH bpf-next 0/3] bpf: Check tail zero of bpf_map_info and bpf_prog_info

[PATCH bpf-next 0/3] bpf: Check tail zero of bpf_map_info and bpf_prog_info

[Leon Hwang]

Check the tail bytes of bpf_map_info and bpf_prog_info due to padding
when getting map info and prog info via BPF_OBJ_GET_INFO_BY_FD, which
was discussed in the thread
"bpf: Check tail zero of bpf_common_attr using offsetofend" [1].

Links:
[1] https://lore.kernel.org/bpf/20260518145446.6794-2-leon.hwang@linux.dev/

Leon Hwang (3):
  bpf: Check tail zero of bpf_map_info
  bpf: Check tail zero of bpf_prog_info
  selftests/bpf: Add tests to verify checking padding bytes for
    bpf_[map,prog]_info

 kernel/bpf/syscall.c                          | 10 ++--
 .../selftests/bpf/prog_tests/bpf_attr_size.c  | 55 +++++++++++++++++++
 2 files changed, 61 insertions(+), 4 deletions(-)

-- 
2.54.0

[PATCH bpf-next 1/3] bpf: Check tail zero of bpf_map_info

[Leon Hwang]

Since there're 4 bytes padding at the end of struct bpf_map_info, they
won't be checked by bpf_check_uarg_tail_zero().

pahole -C bpf_map_info ./vmlinux
struct bpf_map_info {
	...
	__u64                      hash __attribute__((__aligned__(8))); /*    88     8 */
	__u32                      hash_size;            /*    96     4 */

	/* size: 104, cachelines: 2, members: 18 */
	/* padding: 4 */
	/* forced alignments: 1 */
	/* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

If a future kernel extension adds a new 4-byte field, older userspace
programs allocating this structure on the stack might inadvertently pass
uninitialized stack garbage into the new field, permanently breaking
backward compatibility. -- sashiko [1]

Fix it by changing sizeof(info) to
offsetofend(struct bpf_map_info, hash_size).

[1] https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/

Fixes: ea2e6467ac36 ("bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/syscall.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 625a4366fe6d..1007fb7c87e9 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5400,10 +5400,11 @@ static int bpf_map_get_info_by_fd(struct file *file,
 {
 	struct bpf_map_info __user *uinfo = u64_to_user_ptr(attr->info.info);
 	struct bpf_map_info info;
-	u32 info_len = attr->info.info_len;
+	u32 info_len = attr->info.info_len, len;
 	int err;
 
-	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
+	len = offsetofend(struct bpf_map_info, hash_size);
+	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
 	if (err)
 		return err;
 	info_len = min_t(u32, sizeof(info), info_len);
-- 
2.54.0

[PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[Leon Hwang]

Since there're 4 bytes padding at the end of struct bpf_prog_info, they
won't be checked by bpf_check_uarg_tail_zero().

pahole -C bpf_prog_info ./vmlinux
struct bpf_prog_info {
	...
	__u32                      attach_btf_obj_id;    /*   220     4 */
	__u32                      attach_btf_id;        /*   224     4 */

	/* size: 232, cachelines: 4, members: 38 */
	/* sum members: 224 */
	/* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 31 bits */
	/* padding: 4 */
	/* forced alignments: 9 */
	/* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

If a future kernel extension adds a new 4-byte field, older userspace
programs allocating this structure on the stack might inadvertently pass
uninitialized stack garbage into the new field, permanently breaking
backward compatibility. -- sashiko [1]

Fix it by changing sizeof(info) to
offsetofend(struct bpf_prog_info, attach_btf_id).

[1] https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/

Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/syscall.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 1007fb7c87e9..0c86a70d31c1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5115,10 +5115,11 @@ static int bpf_prog_get_info_by_fd(struct file *file,
 	u32 info_len = attr->info.info_len;
 	struct bpf_prog_kstats stats;
 	char __user *uinsns;
-	u32 ulen;
+	u32 ulen, len;
 	int err;
 
-	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
+	len = offsetofend(struct bpf_prog_info, attach_btf_id);
+	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
 	if (err)
 		return err;
 	info_len = min_t(u32, sizeof(info), info_len);
-- 
2.54.0

[PATCH bpf-next 3/3] selftests/bpf: Add tests to verify checking padding bytes for bpf_[map,prog]_info

[Leon Hwang]

Add two tests to verify that the tail padding 4 bytes of struct
bpf_map_info and bpf_prog_info are checked in syscall.c using
bpf_check_uarg_tail_zero().

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 .../selftests/bpf/prog_tests/bpf_attr_size.c  | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c b/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c
index 32159dc64da8..87842c4347a6 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c
@@ -62,8 +62,63 @@ static void test_query_size_boundaries(void)
 	cgroup_skb_direct_packet_access__destroy(skel);
 }
 
+static void test_map_info_tail_zero(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, map_opts);
+	struct bpf_map_info_fake {
+		__u8 info[offsetofend(struct bpf_map_info, hash_size)];
+		__u32 pad;
+	} info = {
+		.pad = 1,
+	};
+	int map_fd, err;
+	__u32 info_len;
+
+	map_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "arr", sizeof(int), 1, 1, &map_opts);
+	if (!ASSERT_GE(map_fd, 0, "bpf_map_create"))
+		return;
+
+	info_len = sizeof(info);
+	err = bpf_obj_get_info_by_fd(map_fd, &info, &info_len);
+	ASSERT_EQ(err, -E2BIG, "bpf_obj_get_info_by_fd");
+
+	close(map_fd);
+}
+
+static void test_prog_info_tail_zero(void)
+{
+	LIBBPF_OPTS(bpf_prog_load_opts, prog_opts);
+	struct bpf_insn insns[] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_EXIT_INSN(),
+	};
+	struct bpf_prog_info_fake {
+		__u8 info[offsetofend(struct bpf_prog_info, attach_btf_id)];
+		__u32 pad;
+	} info = {
+		.pad = 1,
+	};
+	int prog_fd, err;
+	__u32 info_len;
+
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, "test_prog", "GPL", insns,
+				ARRAY_SIZE(insns), &prog_opts);
+	if (!ASSERT_GE(prog_fd, 0, "bpf_prog_load"))
+		return;
+
+	info_len = sizeof(info);
+	err = bpf_obj_get_info_by_fd(prog_fd, &info, &info_len);
+	ASSERT_EQ(err, -E2BIG, "bpf_obj_get_info_by_fd");
+
+	close(prog_fd);
+}
+
 void test_bpf_attr_size(void)
 {
 	if (test__start_subtest("query_size_boundaries"))
 		test_query_size_boundaries();
+	if (test__start_subtest("map_info_tail_zero"))
+		test_map_info_tail_zero();
+	if (test__start_subtest("prog_info_tail_zero"))
+		test_prog_info_tail_zero();
 }
-- 
2.54.0

Re: [PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] UAPI regression and fundamentally flawed forward-compatibility logic due to strict zero-checks on previously ignored struct padding.
--

commit 7758939eea6a099c4db0364bf334e0034c2864fc
Author: Leon Hwang <leon.hwang@linux.dev>

bpf: Check tail zero of bpf_prog_info

This commit changes the size passed to bpf_check_uarg_tail_zero() to exclude
the trailing padding of struct bpf_prog_info. The goal is to prevent older
userspace programs from passing uninitialized stack garbage into a future
4-byte field.

Link: https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 1007fb7c87e91..0c86a70d31c17 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5115,10 +5115,11 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>  	u32 info_len = attr->info.info_len;
>  	struct bpf_prog_kstats stats;
>  	char __user *uinsns;
> -	u32 ulen;
> +	u32 ulen, len;
>  	int err;
>  
> -	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
> +	len = offsetofend(struct bpf_prog_info, attach_btf_id);
> +	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);

[Severity: Critical]
Does this change break the ABI for existing deployed binaries?

Previously, expected_size was 232 (sizeof(info)). If an existing binary
passes info_len = 232 with uninitialized stack garbage in the 4-byte
padding, bpf_check_uarg_tail_zero() would see actual_size <= expected_size
(232 <= 232) and immediately return 0.

By reducing expected_size to 228, actual_size > expected_size (232 > 228),
so bpf_check_uarg_tail_zero() will actively check the 4 padding bytes and
return -E2BIG if they contain garbage. This rejects previously working
applications.

Additionally, does this actually protect against the stated future issue?

If a future kernel adds a new 4-byte field, the struct size stays 232, and
the new kernel's expected_size will be 232. At that point, those old buggy
binaries passing info_len = 232 will once again hit the fast path
(actual_size <= expected_size), skipping the check and allowing the
uninitialized padding to be consumed as the new field.

>  	if (err)
>  		return err;
>  	info_len = min_t(u32, sizeof(info), info_len);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260603144518.67065-1-leon.hwang@linux.dev?part=2

Re: [PATCH bpf-next 1/3] bpf: Check tail zero of bpf_map_info

[Mykyta Yatsenko]

On 6/3/26 3:45 PM, Leon Hwang wrote:
> Since there're 4 bytes padding at the end of struct bpf_map_info, they
> won't be checked by bpf_check_uarg_tail_zero().
> 
> pahole -C bpf_map_info ./vmlinux
> struct bpf_map_info {
> 	...
> 	__u64                      hash __attribute__((__aligned__(8))); /*    88     8 */
> 	__u32                      hash_size;            /*    96     4 */
> 
> 	/* size: 104, cachelines: 2, members: 18 */
> 	/* padding: 4 */
> 	/* forced alignments: 1 */
> 	/* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));
> 
> If a future kernel extension adds a new 4-byte field, older userspace
> programs allocating this structure on the stack might inadvertently pass
> uninitialized stack garbage into the new field, permanently breaking
> backward compatibility. -- sashiko [1]
> 
> Fix it by changing sizeof(info) to
> offsetofend(struct bpf_map_info, hash_size).
> 
> [1] https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/
> 
> Fixes: ea2e6467ac36 ("bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD")
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

>  kernel/bpf/syscall.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 625a4366fe6d..1007fb7c87e9 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5400,10 +5400,11 @@ static int bpf_map_get_info_by_fd(struct file *file,
>  {
>  	struct bpf_map_info __user *uinfo = u64_to_user_ptr(attr->info.info);
>  	struct bpf_map_info info;
> -	u32 info_len = attr->info.info_len;
> +	u32 info_len = attr->info.info_len, len;
>  	int err;
>  
> -	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
> +	len = offsetofend(struct bpf_map_info, hash_size);
> +	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
>  	if (err)
>  		return err;
>  	info_len = min_t(u32, sizeof(info), info_len);

Re: [PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[Mykyta Yatsenko]

On 6/3/26 3:45 PM, Leon Hwang wrote:
> Since there're 4 bytes padding at the end of struct bpf_prog_info, they
> won't be checked by bpf_check_uarg_tail_zero().
> 
> pahole -C bpf_prog_info ./vmlinux
> struct bpf_prog_info {
> 	...
> 	__u32                      attach_btf_obj_id;    /*   220     4 */
> 	__u32                      attach_btf_id;        /*   224     4 */
> 
> 	/* size: 232, cachelines: 4, members: 38 */
> 	/* sum members: 224 */
> 	/* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 31 bits */
> 	/* padding: 4 */
> 	/* forced alignments: 9 */
> 	/* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));
> 
> If a future kernel extension adds a new 4-byte field, older userspace
> programs allocating this structure on the stack might inadvertently pass
> uninitialized stack garbage into the new field, permanently breaking
> backward compatibility. -- sashiko [1]
> 
> Fix it by changing sizeof(info) to
> offsetofend(struct bpf_prog_info, attach_btf_id).
> 
> [1] https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/
> 
> Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

>  kernel/bpf/syscall.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 1007fb7c87e9..0c86a70d31c1 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5115,10 +5115,11 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>  	u32 info_len = attr->info.info_len;
>  	struct bpf_prog_kstats stats;
>  	char __user *uinsns;
> -	u32 ulen;
> +	u32 ulen, len;
>  	int err;
>  
> -	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
> +	len = offsetofend(struct bpf_prog_info, attach_btf_id);
> +	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
>  	if (err)
>  		return err;
>  	info_len = min_t(u32, sizeof(info), info_len);

Re: [PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1140 bytes --]

> Fix it by changing sizeof(info) to
> offsetofend(struct bpf_prog_info, attach_btf_id).
>
> Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")

Is this the right commit for the Fixes: tag?

The change avoids the trailing padding that exists because struct
bpf_prog_info has 4 bytes of padding at its end, so sizeof(info) (232)
differs from offsetofend(struct bpf_prog_info, attach_btf_id) (228).

That trailing padding first appeared when __u32 verified_insns was
appended after the last __u64 member (recursion_misses).  Before that,
the struct ended on an 8-byte-aligned member and had no trailing
padding.

b79c9fc9551b only appended two further __u32 fields (attach_btf_obj_id,
attach_btf_id), while the 4-byte trailing padding already existed and
was unchanged.

Would this be more accurate?

  Fixes: aba64c7da983 ("bpf: Add verified_insns to bpf_prog_info and fdinfo")


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/26893539599

Re: [PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[Leon Hwang]

On 3/6/26 23:14, sashiko-bot@kernel.org wrote:
[...]
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 1007fb7c87e91..0c86a70d31c17 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -5115,10 +5115,11 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>>  	u32 info_len = attr->info.info_len;
>>  	struct bpf_prog_kstats stats;
>>  	char __user *uinsns;
>> -	u32 ulen;
>> +	u32 ulen, len;
>>  	int err;
>>  
>> -	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info), info_len);
>> +	len = offsetofend(struct bpf_prog_info, attach_btf_id);
>> +	err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
> 
> [Severity: Critical]
> Does this change break the ABI for existing deployed binaries?
> 
> Previously, expected_size was 232 (sizeof(info)). If an existing binary
> passes info_len = 232 with uninitialized stack garbage in the 4-byte
> padding, bpf_check_uarg_tail_zero() would see actual_size <= expected_size
> (232 <= 232) and immediately return 0.
> 
> By reducing expected_size to 228, actual_size > expected_size (232 > 228),
> so bpf_check_uarg_tail_zero() will actively check the 4 padding bytes and
> return -E2BIG if they contain garbage. This rejects previously working
> applications.
> 
> Additionally, does this actually protect against the stated future issue?
> 
> If a future kernel adds a new 4-byte field, the struct size stays 232, and
> the new kernel's expected_size will be 232. At that point, those old buggy
> binaries passing info_len = 232 will once again hit the fast path
> (actual_size <= expected_size), skipping the check and allowing the
> uninitialized padding to be consumed as the new field.
> 
For this case, forward compatibility is more important than backward
compatibility. Thereafter, those old buggy binaries should fix
themselves instead.

Thanks,
Leon

Re: [PATCH bpf-next 2/3] bpf: Check tail zero of bpf_prog_info

[Leon Hwang]

On 3/6/26 23:37, bot+bpf-ci@kernel.org wrote:
>> Fix it by changing sizeof(info) to
>> offsetofend(struct bpf_prog_info, attach_btf_id).
>>
>> Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
> 
> Is this the right commit for the Fixes: tag?
> 
> The change avoids the trailing padding that exists because struct
> bpf_prog_info has 4 bytes of padding at its end, so sizeof(info) (232)
> differs from offsetofend(struct bpf_prog_info, attach_btf_id) (228).
> 
> That trailing padding first appeared when __u32 verified_insns was
> appended after the last __u64 member (recursion_misses).  Before that,
> the struct ended on an 8-byte-aligned member and had no trailing
> padding.
> 
> b79c9fc9551b only appended two further __u32 fields (attach_btf_obj_id,
> attach_btf_id), while the 4-byte trailing padding already existed and
> was unchanged.
> 
> Would this be more accurate?
> 
>   Fixes: aba64c7da983 ("bpf: Add verified_insns to bpf_prog_info and fdinfo")
> 
> 
Agreed.

Will update the Fixes tag in the next revision.

Thanks,
Leon