[PATCH 1/4] xprop: prevent accessing not available fields in conditionals

[Bert Wesarg]

Having a format of 32c with two elemtents, and a dformat of '?$3=0(foo)'
would result in an invalid read past the thunks array. Handle references
to invalid fields as error while parsing the dformat.

Signed-off-by: Bert Wesarg <bert.wesarg at googlemail.com>
---
 xprop.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/xprop.c b/xprop.c
index 4c4c702..04b4301 100644
--- a/xprop.c
+++ b/xprop.c
@@ -1220,9 +1220,10 @@ Scan_Term (const char *string, thunk *thunks, const char *format, long *value)
     if (isdigit((unsigned char) *string))
 	string = Scan_Long(string, value);
     else if (*string == '$') {
-	string = Scan_Long(++string, &i);
+	const char *bad_string = string++;
+	string = Scan_Long(string, &i);
 	if (i >= thunks->thunk_count)
-	    i = thunks->thunk_count;
+	    Fatal_Error("Bad field reference %d in conditional %s", (int) i, bad_string);
 	*value = thunks[i].value;
     } else if (*string == 'm') {
 	string = Scan_Long(++string, &i);
-- 
1.7.7.759.gfc8c6