[PATCH bpf-next v6 6/9] selftests/bpf: make cmdline_contains stricter

["Alexis Lothoré (eBPF Foundation)" <alexis.lothore@bootlin.com>]

cmdline_contains is used by BPF selftests to check the presence of
specific kernel commandline parameters, but it currently suffers from
two issues:
- the read commandline isn't NULL terminated right after the read data
  but only at the end of the buffer, leaving uninitialized bytes that
  are then possibly tokenized
- the comparison of found tokens is done based on the size of found
  token. This could lead to too-short-but-matching tokens to wrongly
  match the search pattern.

Enforce stricter checks in cmdline_contains to avoid accidental matches.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
---
 tools/testing/selftests/bpf/unpriv_helpers.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/unpriv_helpers.c b/tools/testing/selftests/bpf/unpriv_helpers.c
index f997d7ec8fd0..c99d81df2aa2 100644
--- a/tools/testing/selftests/bpf/unpriv_helpers.c
+++ b/tools/testing/selftests/bpf/unpriv_helpers.c
@@ -72,8 +72,9 @@ static int config_contains(const char *pat)
 
 static bool cmdline_contains(const char *pat)
 {
+	int fd, cnt, ret = false;
 	char cmdline[4096], *c;
-	int fd, ret = false;
+	size_t pat_len;
 
 	fd = open("/proc/cmdline", O_RDONLY);
 	if (fd < 0) {
@@ -81,14 +82,16 @@ static bool cmdline_contains(const char *pat)
 		return false;
 	}
 
-	if (read(fd, cmdline, sizeof(cmdline) - 1) < 0) {
+	cnt = read(fd, cmdline, sizeof(cmdline) - 1);
+	if (cnt < 0) {
 		perror("read /proc/cmdline");
 		goto out;
 	}
 
-	cmdline[sizeof(cmdline) - 1] = '\0';
+	cmdline[cnt] = '\0';
+	pat_len = strlen(pat);
 	for (c = strtok(cmdline, " \n"); c; c = strtok(NULL, " \n")) {
-		if (strncmp(c, pat, strlen(c)))
+		if (strlen(c) != pat_len || strcmp(c, pat))
 			continue;
 		ret = true;
 		break;

-- 
2.55.0