[FFmpeg-cvslog] avutil/avstring: Fix bug and undefined behavior in av_strncasecmp()

[Michael Niedermayer]

ffmpeg | branch: release/4.1 | Michael Niedermayer <michael at niedermayer.cc> | Tue Apr 16 00:09:38 2019 +0200| [d8bdc954579b0bb2e2b34d19e8cf3ff3d6201601] | committer: Michael Niedermayer

avutil/avstring: Fix bug and undefined behavior in av_strncasecmp()

The function in case of n=0 would read more bytes than 0.
The end pointer could be beyond the allocated space, which
is undefined.

Reviewed-by: Paul B Mahol <onemda at gmail.com>
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
(cherry picked from commit 6f0e9a863466bfcbd75ee15d4d8a6aad2a5126a4)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=d8bdc954579b0bb2e2b34d19e8cf3ff3d6201601
---

 libavutil/avstring.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libavutil/avstring.c b/libavutil/avstring.c
index f03dd25141..4c068f5bc5 100644
--- a/libavutil/avstring.c
+++ b/libavutil/avstring.c
@@ -222,12 +222,13 @@ int av_strcasecmp(const char *a, const char *b)
 
 int av_strncasecmp(const char *a, const char *b, size_t n)
 {
-    const char *end = a + n;
     uint8_t c1, c2;
+    if (n <= 0)
+        return 0;
     do {
         c1 = av_tolower(*a++);
         c2 = av_tolower(*b++);
-    } while (a < end && c1 && c1 == c2);
+    } while (--n && c1 && c1 == c2);
     return c1 - c2;
 }