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 9003b221939a080223219e4647d892cd028ae96a..a3e2b12435135fc52410c8e65389e43f58b29f3d 100644 (file)
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -241,21 +241,27 @@ 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, false);
                offset = i;
-               pageno = TransactionIdToPage(subxids[offset]);
+               pageno = nextpageno;
        }
 }