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

[Dominik Brodowski]

Suggested-by: Ingo Molnar <mi...@kernel.org>
CC: Andy Lutomirski <l...@kernel.org>
Signed-off-by: Dominik Brodowski <li...@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;