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Subject[PATCH] selftests/x86: clarify that there is no buffer overflow on sscanf usage

Suggested-by: Ingo Molnar <mingo@kernel.org>
CC: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

---

> Yeah, probably - but still, this connection and the sscanf() guarantee is not
> obvious at first sight, so please improve this to derive from the same value
> (define a LINE_MAX size or such), plus maybe add a comment to the sscanf() line
> that this is safe because strlen(name) >= strlen(line).

Sounds reasonable. Patch (which applies on top of the five patches for
selftests/x86 I sent out earlier today) is attached.

Thanks,
Dominik

diff --git a/tools/testing/selftests/x86/test_vdso.c b/tools/testing/selftests/x86/test_vdso.c
index 558c8207e7b9..7ade625f10ed 100644
--- a/tools/testing/selftests/x86/test_vdso.c
+++ b/tools/testing/selftests/x86/test_vdso.c
@@ -26,6 +26,9 @@
# endif
#endif

+/* max length of lines in /proc/self/maps - anything longer is skipped here */
+#define MAPS_LINE_LEN 128
+
int nerrs = 0;

typedef long (*getcpu_t)(unsigned *, unsigned *, void *);
@@ -37,17 +40,19 @@ static void *vsyscall_getcpu(void)
{
#ifdef __x86_64__
FILE *maps;
- char line[128];
+ char line[MAPS_LINE_LEN];
bool found = false;

maps = fopen("/proc/self/maps", "r");
if (!maps) /* might still be present, but ignore it here, as we test vDSO not vsyscall */
return NULL;

- while (fgets(line, sizeof(line), maps)) {
+ while (fgets(line, MAPS_LINE_LEN, maps)) {
char r, x;
void *start, *end;
- char name[128];
+ char name[MAPS_LINE_LEN];
+
+ /* sscanf is safe here as strlen(name) >= strlen(line) */
if (sscanf(line, "%p-%p %c-%cp %*x %*x:%*x %*u %s",
&start, &end, &r, &x, name) != 5)
continue;
diff --git a/tools/testing/selftests/x86/test_vsyscall.c b/tools/testing/selftests/x86/test_vsyscall.c
index 7a744fa7b786..ee92e4727f18 100644
--- a/tools/testing/selftests/x86/test_vsyscall.c
+++ b/tools/testing/selftests/x86/test_vsyscall.c
@@ -33,6 +33,9 @@
# endif
#endif

+/* max length of lines in /proc/self/maps - anything longer is skipped here */
+#define MAPS_LINE_LEN 128
+
static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
int flags)
{
@@ -98,7 +101,7 @@ static int init_vsys(void)
#ifdef __x86_64__
int nerrs = 0;
FILE *maps;
- char line[128];
+ char line[MAPS_LINE_LEN];
bool found = false;

maps = fopen("/proc/self/maps", "r");
@@ -108,10 +111,12 @@ static int init_vsys(void)
return 0;
}

- while (fgets(line, sizeof(line), maps)) {
+ while (fgets(line, MAPS_LINE_LEN, maps)) {
char r, x;
void *start, *end;
- char name[128];
+ char name[MAPS_LINE_LEN];
+
+ /* sscanf is safe here as strlen(name) >= strlen(line) */
if (sscanf(line, "%p-%p %c-%cp %*x %*x:%*x %*u %s",
&start, &end, &r, &x, name) != 5)
continue;
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