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Be more wary about 32-bit integer overflow in pg_stat_statements.

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We've heard a couple of reports of people having trouble with
multi-gigabyte-sized query-texts files.  It occurred to me that on
32-bit platforms, there could be an issue with integer overflow
of calculations associated with the total query text size.
Address that with several changes:

1. Limit pg_stat_statements.max to INT_MAX / 2 not INT_MAX.
The hashtable code will bound it to that anyway unless "long"
is 64 bits.  We still need overflow guards on its use, but
this helps.

2. Add a check to prevent extending the query-texts file to
more than MaxAllocHugeSize.  If it got that big, qtext_load_file
would certainly fail, so there's not much point in allowing it.
Without this, we'd need to consider whether extent, query_offset,
and related variables shouldn't be off_t not size_t.

3. Adjust the comparisons in need_gc_qtexts() to be done in 64-bit
arithmetic on all platforms.  It appears possible that under duress
those multiplications could overflow 32 bits, yielding a false
conclusion that we need to garbage-collect the texts file, which
could lead to repeatedly garbage-collecting after every hash table
insertion.

Per report from Bruno da Silva.  I'm not convinced that these
issues fully explain his problem; there may be some other bug that's
contributing to the query-texts file becoming so large in the first
place.  But it did get that big, so #2 is a reasonable defense,
and #3 could explain the reported performance difficulties.

(See also commit 8bbe4cbd9, which addressed some related bugs.
The second Discussion: link is the thread that led up to that.)

This issue is old, and is primarily a problem for old platforms,
so back-patch.

Discussion: https://postgr.es/m/CAB+Nuk93fL1Q9eLOCotvLP07g7RAv4vbdrkm0cVQohDVMpAb9A@mail.gmail.com
Discussion: https://postgr.es/m/5601D354.5000703@BlueTreble.com
This commit is contained in:
Tom Lane 2022-08-02 18:05:34 -04:00
parent 51d8b52fcf
commit 06f6a07ba4
1 changed files with 22 additions and 4 deletions
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26
contrib/pg_stat_statements/pg_stat_statements.c
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@ -367,7 +367,7 @@ _PG_init(void)
&pgss_max,
5000,
100,
INT_MAX,
INT_MAX / 2,
PGC_POSTMASTER,
0,
NULL,
@ -1842,6 +1842,18 @@ qtext_store(const char *query, int query_len,
*query_offset = off;
/*
* Don't allow the file to grow larger than what qtext_load_file can
* (theoretically) handle. This has been seen to be reachable on 32-bit
* platforms.
*/
if (unlikely(query_len >= MaxAllocHugeSize - off))
{
errno = EFBIG; /* not quite right, but it'll do */
fd = -1;
goto error;
}
/* Now write the data into the successfully-reserved part of the file */
fd = OpenTransientFile(PGSS_TEXT_FILE, O_RDWR | O_CREAT | PG_BINARY);
if (fd < 0)
@ -2027,8 +2039,14 @@ need_gc_qtexts(void)
SpinLockRelease(&s->mutex);
}
/* Don't proceed if file does not exceed 512 bytes per possible entry */
if (extent < 512 * pgss_max)
/*
* Don't proceed if file does not exceed 512 bytes per possible entry.
*
* Here and in the next test, 32-bit machines have overflow hazards if
* pgss_max and/or mean_query_len are large. Force the multiplications
* and comparisons to be done in uint64 arithmetic to forestall trouble.
*/
if ((uint64) extent < (uint64) 512 * pgss_max)
return false;
/*
@ -2038,7 +2056,7 @@ need_gc_qtexts(void)
* query length in order to prevent garbage collection from thrashing
* uselessly.
*/
if (extent < pgss->mean_query_len * pgss_max * 2)
if ((uint64) extent < (uint64) pgss->mean_query_len * pgss_max * 2)
return false;
return true;