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Fix tcache count maximum (BZ #24531)

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The tcache counts[] array is a char, which has a very small range and thus
may overflow.  When setting tcache_count tunable, there is no overflow check.
However the tunable must not be larger than the maximum value of the tcache
counts[] array, otherwise it can overflow when filling the tcache.

	[BZ #24531]
	* malloc/malloc.c (MAX_TCACHE_COUNT): New define.
	(do_set_tcache_count): Only update if count is small enough.
	* manual/tunables.texi (glibc.malloc.tcache_count): Document max value.

(cherry picked from commit 5ad533e8e6)
This commit is contained in:
Wilco Dijkstra 2019-05-10 16:38:21 +01:00
parent f9c3c12f33
commit 54ba8bcd42
3 changed files with 16 additions and 4 deletions
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7
ChangeLog
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@ -1,3 +1,10 @@
2019-05-22 Wilco Dijkstra <wdijkstr@arm.com>
[BZ #24531]
* malloc/malloc.c (MAX_TCACHE_COUNT): New define.
(do_set_tcache_count): Only update if count is small enough.
* manual/tunables.texi (glibc.malloc.tcache_count): Document max value.
2019-05-15 Andreas Schwab <schwab@suse.de>
[BZ #20568]

9
malloc/malloc.c
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@ -2919,6 +2919,8 @@ typedef struct tcache_perthread_struct
tcache_entry *entries[TCACHE_MAX_BINS];
} tcache_perthread_struct;
#define MAX_TCACHE_COUNT 127 /* Maximum value of counts[] entries. */
static __thread bool tcache_shutting_down = false;
static __thread tcache_perthread_struct *tcache = NULL;
@ -5105,8 +5107,11 @@ static inline int
__always_inline
do_set_tcache_count (size_t value)
{
LIBC_PROBE (memory_tunable_tcache_count, 2, value, mp_.tcache_count);
mp_.tcache_count = value;
if (value <= MAX_TCACHE_COUNT)
{
LIBC_PROBE (memory_tunable_tcache_count, 2, value, mp_.tcache_count);
mp_.tcache_count = value;
}
return 1;
}

4
manual/tunables.texi
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@ -188,8 +188,8 @@ per-thread cache. The default (and maximum) value is 1032 bytes on
@deftp Tunable glibc.malloc.tcache_count
The maximum number of chunks of each size to cache. The default is 7.
There is no upper limit, other than available system memory. If set
to zero, the per-thread cache is effectively disabled.
The upper limit is 127. If set to zero, the per-thread cache is effectively
disabled.
The approximate maximum overhead of the per-thread cache is thus equal
to the number of bins times the chunk count in each bin times the size