bpf_strncasecmp() function performs same like bpf_strcasecmp() except
limiting the comparison to a specific length.

Signed-off-by: Yuzuki Ishiyama <ishiyama@hpc.is.uec.ac.jp>
---
 kernel/bpf/helpers.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9eaa4185e0a7..753753f039ff 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3406,18 +3406,20 @@ __bpf_kfunc void __bpf_trap(void)
  * __get_kernel_nofault instead of plain dereference to make them safe.
  */
 
-static int __bpf_strcasecmp(const char *s1, const char *s2, bool ignore_case)
+static int __bpf_strncasecmp(const char *s1, const char *s2, bool ignore_case, size_t len)
 {
 	char c1, c2;
-	int i;
+	int i, max_sz;
 
 	if (!copy_from_kernel_nofault_allowed(s1, 1) ||
 	    !copy_from_kernel_nofault_allowed(s2, 1)) {
 		return -ERANGE;
 	}
 
+	max_sz = min_t(int, len, XATTR_SIZE_MAX);
+
 	guard(pagefault)();
-	for (i = 0; i < XATTR_SIZE_MAX; i++) {
+	for (i = 0; i < max_sz; i++) {
 		__get_kernel_nofault(&c1, s1, char, err_out);
 		__get_kernel_nofault(&c2, s2, char, err_out);
 		if (ignore_case) {
@@ -3431,7 +3433,7 @@ static int __bpf_strcasecmp(const char *s1, const char *s2, bool ignore_case)
 		s1++;
 		s2++;
 	}
-	return -E2BIG;
+	return i == XATTR_SIZE_MAX ? -E2BIG : 0;
 err_out:
 	return -EFAULT;
 }
@@ -3451,7 +3453,7 @@ static int __bpf_strcasecmp(const char *s1, const char *s2, bool ignore_case)
  */
 __bpf_kfunc int bpf_strcmp(const char *s1__ign, const char *s2__ign)
 {
-	return __bpf_strcasecmp(s1__ign, s2__ign, false);
+	return __bpf_strncasecmp(s1__ign, s2__ign, false, XATTR_SIZE_MAX);
 }
 
 /**
@@ -3469,7 +3471,26 @@ __bpf_kfunc int bpf_strcmp(const char *s1__ign, const char *s2__ign)
  */
 __bpf_kfunc int bpf_strcasecmp(const char *s1__ign, const char *s2__ign)
 {
-	return __bpf_strcasecmp(s1__ign, s2__ign, true);
+	return __bpf_strncasecmp(s1__ign, s2__ign, true, XATTR_SIZE_MAX);
+}
+
+/*
+ * bpf_strncasecmp - Compare two length-limited strings, ignoring case
+ * @s1__ign: One string
+ * @s2__ign: Another string
+ * @len: The maximum number of characters to compare
+ *
+ * Return:
+ * * %0       - Strings are equal
+ * * %-1      - @s1__ign is smaller
+ * * %1       - @s2__ign is smaller
+ * * %-EFAULT - Cannot read one of the strings
+ * * %-E2BIG  - One of strings is too large
+ * * %-ERANGE - One of strings is outside of kernel address space
+ */
+__bpf_kfunc int bpf_strncasecmp(const char *s1__ign, const char *s2__ign, size_t len)
+{
+	return __bpf_strncasecmp(s1__ign, s2__ign, true, len);
 }
 
 /**
@@ -4521,6 +4542,7 @@ BTF_ID_FLAGS(func, bpf_iter_dmabuf_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)
 BTF_ID_FLAGS(func, __bpf_trap)
 BTF_ID_FLAGS(func, bpf_strcmp);
 BTF_ID_FLAGS(func, bpf_strcasecmp);
+BTF_ID_FLAGS(func, bpf_strncasecmp);
 BTF_ID_FLAGS(func, bpf_strchr);
 BTF_ID_FLAGS(func, bpf_strchrnul);
 BTF_ID_FLAGS(func, bpf_strnchr);
-- 
2.52.0