Fix access-off-end-of-array in clog.c.

Sloppy loop coding in set_status_by_pages() resulted in fetching one array
element more than it should from the subxids[] array.  The odds of this
resulting in SIGSEGV are pretty small, but we've certainly seen that happen
with similar mistakes elsewhere.  While at it, we can get rid of an extra
TransactionIdToPage() calculation per loop.

Per report from David Binderman.  Back-patch to all supported branches,
since this code is quite old.

Discussion: https://postgr.es/m/HE1PR0802MB2331CBA919CBFFF0C465EB429C710@HE1PR0802MB2331.eurprd08.prod.outlook.com

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 0add733984dd2c803cfa5024f762a93fcd276c04..27df4c233f6cbceb012f6419cff20357f004132e 100644 (file)
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -223,21 +223,27 @@ set_status_by_pages(int nsubxids, TransactionId *subxids,
    int         offset = 0;
    int         i = 0;
 
+   Assert(nsubxids > 0);       /* else the pageno fetch above is unsafe */
+
    while (i < nsubxids)
    {
        int         num_on_page = 0;
+       int         nextpageno;
 
-       while (TransactionIdToPage(subxids[i]) == pageno && i < nsubxids)
+       do
        {
+           nextpageno = TransactionIdToPage(subxids[i]);
+           if (nextpageno != pageno)
+               break;
            num_on_page++;
            i++;
-       }
+       } while (i < nsubxids);
 
        TransactionIdSetPageStatus(InvalidTransactionId,
                                   num_on_page, subxids + offset,
                                   status, lsn, pageno);
        offset = i;
-       pageno = TransactionIdToPage(subxids[offset]);
+       pageno = nextpageno;
    }
 }