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Add Asserts to validate prevbit values in bms_prev_member

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bms_prev_member() could attempt to access memory outside of the words[]
array in cases where the prevbit was a number < -1 or > a->nwords *
BITS_PER_BITMAPWORD + 1.

Here we add the Asserts to help draw attention to bogus callers so we're
more likely to catch them during development.

In passing, fix wording of bms_prev_member's header comment which talks
about how we expect the callers to ensure only valid prevbit values are
used.

Author: Greg Burd <greg@burd.me>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2000A717-1FFE-4031-827B-9330FB2E9065%40getmailspring.com
This commit is contained in:
David Rowley
2025-08-15 16:33:07 +12:00
parent 69f75d6714
commit b4632883d4
1 changed files with 5 additions and 1 deletions
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6
src/backend/nodes/bitmapset.c
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@@ -1343,7 +1343,7 @@ bms_next_member(const Bitmapset *a, int prevbit)
*
* Returns largest member less than "prevbit", or -2 if there is none.
* "prevbit" must NOT be more than one above the highest possible bit that can
* be set at the Bitmapset at its current size.
* be set in the Bitmapset at its current size.
*
* To ease finding the highest set bit for the initial loop, the special
* prevbit value of -1 can be passed to have the function find the highest
@@ -1379,6 +1379,10 @@ bms_prev_member(const Bitmapset *a, int prevbit)
if (a == NULL || prevbit == 0)
return -2;
/* Validate callers didn't give us something out of range */
Assert(prevbit <= a->nwords * BITS_PER_BITMAPWORD);
Assert(prevbit >= -1);
/* transform -1 to the highest possible bit we could have set */
if (prevbit == -1)
prevbit = a->nwords * BITS_PER_BITMAPWORD - 1;