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

[Rong Tao <rtoax@xxxxxxxxxxx>]

From: Rong Tao <rongtao@xxxxxxxx>

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@xxxxxxxx>
---
 kernel/bpf/helpers.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 401b4932cc49..65bd0050c560 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3681,6 +3681,8 @@ __bpf_kfunc int bpf_strnstr(const char *s1__ign, const char *s2__ign, size_t len
 				return -ENOENT;
 			if (c1 != c2)
 				break;
+			if (j == len - 1)
+				return i;
 		}
 		if (j == XATTR_SIZE_MAX)
 			return -E2BIG;
-- 
2.51.0