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bpf: Fix bpf_strnstr() to handle suffix match cases better

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bpf_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: e91370550f ("bpf: Add kfuncs for read-only string operations")
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/tencent_17DC57B9D16BC443837021BEACE84B7C1507@qq.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Rong Tao 2025-08-30 00:31:58 +08:00 committed by Alexei Starovoitov
parent 387be23a95
commit 7edfc02470
1 changed files with 8 additions and 1 deletions
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9
kernel/bpf/helpers.c
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@ -3664,10 +3664,17 @@ __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;