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client: Fix integer wrap around in ring_peek()

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Peeking beyond either end of the ring buffer must return EOF.  We
first compute the index, then check whether it's in range.

Unfortunately, the index computation r->prod - -n can wrap around
while r->prod is still <= RING_SIZE.  If it happens, ring_peek()
returns r->buf[(r->prod - -n) % RING_SIZE] instead of EOF.

Currently harmless, because no caller peeks out of range.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
This commit is contained in:
Markus Armbruster 2015-12-27 15:32:00 +01:00
parent 93dfd60c4b
commit 63a6288435
1 changed files with 12 additions and 4 deletions
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16
src/client/ringbuf.c
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@ -27,7 +27,7 @@
* ringbuf.c: Simple ring buffer
*
* Known contributors to this file:
* Markus Armbruster, 2007-2009
* Markus Armbruster, 2007-2015
*/
#include <config.h>
@ -82,9 +82,17 @@ ring_peek(struct ring *r, int n)
assert(-RING_SIZE - 1 <= n && n <= RING_SIZE);
idx = n >= 0 ? r->cons + n : r->prod - -n;
if (idx < r->cons && idx >= r->prod)
return EOF;
if (n >= 0) {
idx = r->cons + n;
if (idx >= r->prod)
return EOF;
} else {
/* Beware, r->prod - -n can wrap around, avoid that */
if (r->prod < r->cons + -n)
return EOF;
idx = r->prod - -n;
}
return r->buf[idx % RING_SIZE];
}