[PATCH bpf-next v4 1/2] bpf/helpers: bpf_strnstr: Exact match length

[Rong Tao]

From: Rong Tao <rong...@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, indicating a successful match.

Fixes: e91370550f1f ("bpf: Add kfuncs for read-only string operations")
Signed-off-by: Rong Tao <rong...@cestc.cn>
---
 kernel/bpf/helpers.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 401b4932cc49..bf04881f96ec 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3672,10 +3672,18 @@ __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;
+                       /**
+                        * corner case i+j==len to ensure that we matched
+                        * entire s2. for example, param len=3:
+                        *     s1: A B C D E F  -> i==1
+                        *     s2:   B C D      -> j==2
+                        */
+                       if (i + j == len)
+                               break;
                        __get_kernel_nofault(&c1, s1__ign + j, char, err_out);
                        if (c1 == '\0')
                                return -ENOENT;
-- 
2.51.0