[PATCH bpf-next v3 1/2] bpf/helpers: bpf_strnstr: Exact match length

[PATCH bpf-next v3 1/2] bpf/helpers: bpf_strnstr: Exact match length

[Rong Tao]

From: Rong Tao <rongtao@cestc.cn>

strnstr should not treat the ending '\0' of s2 as a matching character,
otherwise the parameter 'len' will be meaningless, for example:

    1. bpf_strnstr("openat", "open", 4) = -ENOENT
    2. bpf_strnstr("openat", "open", 5) = 0

This patch makes (1) return 0, indicating a successful match.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
---
 kernel/bpf/helpers.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 401b4932cc49..ced7132980fe 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3672,10 +3672,12 @@ __bpf_kfunc int bpf_strnstr(const char *s1__ign, const char *s2__ign, size_t len
 
 	guard(pagefault)();
 	for (i = 0; i < XATTR_SIZE_MAX; i++) {
-		for (j = 0; i + j < len && j < XATTR_SIZE_MAX; j++) {
+		for (j = 0; i + j <= len && j < XATTR_SIZE_MAX; j++) {
 			__get_kernel_nofault(&c2, s2__ign + j, char, err_out);
 			if (c2 == '\0')
 				return i;
+			if (i + j == len)
+				break;
 			__get_kernel_nofault(&c1, s1__ign + j, char, err_out);
 			if (c1 == '\0')
 				return -ENOENT;
-- 
2.51.0

[PATCH bpf-next v3 2/2] selftests/bpf: Add tests for bpf_strnstr

[Rong Tao]

From: Rong Tao <rongtao@cestc.cn>

Add two tests for bpf_strnstr():

    bpf_strnstr("", "", 0) = 0
    bpf_strnstr("hello world", "hello", 5) = 0

Signed-off-by: Rong Tao <rongtao@cestc.cn>
---
 tools/testing/selftests/bpf/progs/string_kfuncs_success.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
index 46697f381878..1b56bd5860e9 100644
--- a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
+++ b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
@@ -30,7 +30,9 @@ __test(2) int test_strcspn(void *ctx) { return bpf_strcspn(str, "lo"); }
 __test(6) int test_strstr_found(void *ctx) { return bpf_strstr(str, "world"); }
 __test(-ENOENT) int test_strstr_notfound(void *ctx) { return bpf_strstr(str, "hi"); }
 __test(0) int test_strstr_empty(void *ctx) { return bpf_strstr(str, ""); }
-__test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr(str, "hello", 6); }
+__test(0) int test_strnstr_found1(void *ctx) { return bpf_strnstr("", "", 0); }
+__test(0) int test_strnstr_found2(void *ctx) { return bpf_strnstr(str, "hello", 5); }
+__test(0) int test_strnstr_found3(void *ctx) { return bpf_strnstr(str, "hello", 6); }
 __test(-ENOENT) int test_strnstr_notfound(void *ctx) { return bpf_strnstr(str, "hi", 10); }
 __test(0) int test_strnstr_empty(void *ctx) { return bpf_strnstr(str, "", 1); }
 
-- 
2.51.0

Re: [PATCH bpf-next v3 2/2] selftests/bpf: Add tests for bpf_strnstr

[Andrii Nakryiko]

On Thu, Aug 28, 2025 at 4:08 AM Rong Tao <rtoax@foxmail.com> wrote:
>
> From: Rong Tao <rongtao@cestc.cn>
>
> Add two tests for bpf_strnstr():
>
>     bpf_strnstr("", "", 0) = 0
>     bpf_strnstr("hello world", "hello", 5) = 0
>
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
>  tools/testing/selftests/bpf/progs/string_kfuncs_success.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
> index 46697f381878..1b56bd5860e9 100644
> --- a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
> +++ b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
> @@ -30,7 +30,9 @@ __test(2) int test_strcspn(void *ctx) { return bpf_strcspn(str, "lo"); }
>  __test(6) int test_strstr_found(void *ctx) { return bpf_strstr(str, "world"); }
>  __test(-ENOENT) int test_strstr_notfound(void *ctx) { return bpf_strstr(str, "hi"); }
>  __test(0) int test_strstr_empty(void *ctx) { return bpf_strstr(str, ""); }
> -__test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr(str, "hello", 6); }
> +__test(0) int test_strnstr_found1(void *ctx) { return bpf_strnstr("", "", 0); }
> +__test(0) int test_strnstr_found2(void *ctx) { return bpf_strnstr(str, "hello", 5); }

add (str, "hello", 4) == -ENOENT case?

Also let's add negative ("", "a", 0) == -ENOENT case?

pw-bot: cr

> +__test(0) int test_strnstr_found3(void *ctx) { return bpf_strnstr(str, "hello", 6); }
>  __test(-ENOENT) int test_strnstr_notfound(void *ctx) { return bpf_strnstr(str, "hi", 10); }
>  __test(0) int test_strnstr_empty(void *ctx) { return bpf_strnstr(str, "", 1); }
>
> --
> 2.51.0
>

Re: [PATCH bpf-next v3 1/2] bpf/helpers: bpf_strnstr: Exact match length

[Andrii Nakryiko]

On Thu, Aug 28, 2025 at 4:07 AM Rong Tao <rtoax@foxmail.com> wrote:
>
> From: Rong Tao <rongtao@cestc.cn>
>
> strnstr should not treat the ending '\0' of s2 as a matching character,
> otherwise the parameter 'len' will be meaningless, for example:
>
>     1. bpf_strnstr("openat", "open", 4) = -ENOENT
>     2. bpf_strnstr("openat", "open", 5) = 0
>
> This patch makes (1) return 0, indicating a successful match.
>
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
>  kernel/bpf/helpers.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>

Add Fixes: tag?

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 401b4932cc49..ced7132980fe 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3672,10 +3672,12 @@ __bpf_kfunc int bpf_strnstr(const char *s1__ign, const char *s2__ign, size_t len
>
>         guard(pagefault)();
>         for (i = 0; i < XATTR_SIZE_MAX; i++) {
> -               for (j = 0; i + j < len && j < XATTR_SIZE_MAX; j++) {
> +               for (j = 0; i + j <= len && j < XATTR_SIZE_MAX; j++) {


that <= is a bit subtle, and combined with extra i+j==len check below
is a bit confusing, so I think it would be good to add a short comment
explaining this corner case you caught and that we are doing this to
ensure that we matched entire s2 (and thus we need to find NUL to
confirm).

>                         __get_kernel_nofault(&c2, s2__ign + j, char, err_out);
>                         if (c2 == '\0')
>                                 return i;
> +                       if (i + j == len)
> +                               break;
>                         __get_kernel_nofault(&c1, s1__ign + j, char, err_out);
>                         if (c1 == '\0')
>                                 return -ENOENT;
> --
> 2.51.0
>