[PATCH bpf-next v5 1/2] bpf/helpers: bpf_strnstr: Exact match length

[PATCH bpf-next v5 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
if the parameter 'len' equal to s2 string length, for example:

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

This patch makes (1) return 0, fix just the `len == strlen(s2)` case.

And fix a more general case when s2 is a suffix of the first len
characters of s1.

Fixes: e91370550f1f ("bpf: Add kfuncs for read-only string operations")
Signed-off-by: Rong Tao <rongtao@cestc.cn>
---
 kernel/bpf/helpers.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 401b4932cc49..91ad124844ae 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3672,10 +3672,17 @@ __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;
+			/**
+			 * We allow reading an extra byte from s2 (note the
+			 * `i + j <= len` above) to cover the case when s2 is
+			 * a suffix of the first len chars of s1.
+			 */
+			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 v5 2/2] selftests/bpf: Add tests for bpf_strnstr

[Rong Tao]

From: Rong Tao <rongtao@cestc.cn>

Add tests for bpf_strnstr():

    bpf_strnstr("", "", 0) = 0
    bpf_strnstr("hello world", "hello", 5) = 0
    bpf_strnstr(str, "hello", 4) = -ENOENT
    bpf_strnstr("", "a", 0) = -ENOENT

Signed-off-by: Rong Tao <rongtao@cestc.cn>
---
 tools/testing/selftests/bpf/progs/string_kfuncs_success.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
index 46697f381878..a47690174e0e 100644
--- a/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
+++ b/tools/testing/selftests/bpf/progs/string_kfuncs_success.c
@@ -30,8 +30,12 @@ __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(-ENOENT) int test_strnstr_notfound(void *ctx) { return bpf_strnstr(str, "hi", 10); }
+__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_notfound1(void *ctx) { return bpf_strnstr(str, "hi", 10); }
+__test(-ENOENT) int test_strnstr_notfound2(void *ctx) { return bpf_strnstr(str, "hello", 4); }
+__test(-ENOENT) int test_strnstr_notfound3(void *ctx) { return bpf_strnstr("", "a", 0); }
 __test(0) int test_strnstr_empty(void *ctx) { return bpf_strnstr(str, "", 1); }
 
 char _license[] SEC("license") = "GPL";
-- 
2.51.0

Re: [PATCH bpf-next v5 1/2] bpf/helpers: bpf_strnstr: Exact match length

[Andrii Nakryiko]

On Fri, Aug 29, 2025 at 9:32 AM Rong Tao <rtoax@foxmail.com> wrote:
>
> From: Rong Tao <rongtao@cestc.cn>
>

I reworded patch subject a bit, please check the final version

> strnstr should not treat the ending '\0' of s2 as a matching character
> if the parameter 'len' equal to s2 string length, for example:
>
>     1. bpf_strnstr("openat", "open", 4) = -ENOENT
>     2. bpf_strnstr("openat", "open", 5) = 0
>
> This patch makes (1) return 0, fix just the `len == strlen(s2)` case.
>
> And fix a more general case when s2 is a suffix of the first len
> characters of s1.
>
> Fixes: e91370550f1f ("bpf: Add kfuncs for read-only string operations")
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
>  kernel/bpf/helpers.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 401b4932cc49..91ad124844ae 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3672,10 +3672,17 @@ __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;
> +                       /**

fixed up comment style (extra *) when applying to bpf tree, thanks.


> +                        * We allow reading an extra byte from s2 (note the
> +                        * `i + j <= len` above) to cover the case when s2 is
> +                        * a suffix of the first len chars of s1.
> +                        */
> +                       if (i + j == len)
> +                               break;
>                         __get_kernel_nofault(&c1, s1__ign + j, char, err_out);
>                         if (c1 == '\0')
>                                 return -ENOENT;
> --
> 2.51.0
>