[2.28 COMMITTED] Fix tcache count maximum (BZ #24531)

[2.28 COMMITTED] Fix tcache count maximum (BZ #24531)

[Wilco Dijkstra]

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 5ad533e8e65092be962e414e0417112c65d154fb)

--
diff --git a/malloc/malloc.c b/malloc/malloc.c
index ce77137..0abd653 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -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;
 
@@ -5124,8 +5126,11 @@ 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;
 }
 
diff --git a/manual/tunables.texi b/manual/tunables.texi
index af82082..8edfea4 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -189,8 +189,8 @@ 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