[PATCH xlsatoms] Count % signs, require at least 2 for a format string (#39614)

[Peter Hutterer]

xlsatoms -format "%s" sounds like a good idea, the resulting crash isn't.
I'm too lazy to check for all possible combinations that we allow here (it
is a printf-compatible string), so let's just check that we have two
specifiers %.

X.Org Bug 39614 <http://bugs.freedesktop.org/show_bug.cgi?id=39614>

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
This isn't meant as a security fix, just as a mere sanity fix for a
simple-to-detect case.

 xlsatoms.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/xlsatoms.c b/xlsatoms.c
index eb4e21d..f21fadd 100644
--- a/xlsatoms.c
+++ b/xlsatoms.c
@@ -196,6 +196,20 @@ say_batch(xcb_connection_t *c, const char *format, xcb_get_atom_name_cookie_t *c
     char atom_name[1024];
     long i;
     int done = 0;
+    int format_specifiers = 0;
+
+    i = 0;
+
+    while(i < strlen(format) - 1) {
+        if (format[i] == '%' && format[++i] != '%')
+            format_specifiers++;
+        i++;
+    }
+
+    if (format_specifiers != 2) {
+        fprintf(stderr, "Invalid format specifier: '%s'. Need %%d and %%s.\n", format);
+        return 1;
+    }
 
     for (i = 0; i < count; i++)
 	cookie[i] = xcb_get_atom_name(c, i + low);
-- 
1.8.2.1