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Commit7f69ed4

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pg_trgm: fix crash in 2-item picksplit
Whether from size overflow in gistSplit or from secondary splits,picksplit is (rarely) called with exactly two items to split.Formerly, due to special-case handling of the last item, this wouldlead to access to an uninitialized cache entry; prior to PG 13 thismight have been harmless or at worst led to an incorrect union datum,but in 13 onwards it can cause a backend crash from using anuninitialized pointer.Repair by removing the special case, which was deemed not to have beenappropriate anyway. Backpatch all the way, because this bug hasexisted since pg_trgm was added.Per report on IRC from user "ftzdomino". Analysis and testing by me,patch from Alexander Korotkov.Discussion:https://postgr.es/m/87k0usfdxg.fsf@news-spur.riddles.org.uk
1 parent0d0626e commit7f69ed4

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‎contrib/pg_trgm/trgm_gist.c‎

Lines changed: 4 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -760,7 +760,7 @@ Datum
760760
gtrgm_picksplit(PG_FUNCTION_ARGS)
761761
{
762762
GistEntryVector*entryvec= (GistEntryVector*)PG_GETARG_POINTER(0);
763-
OffsetNumbermaxoff=entryvec->n-2;
763+
OffsetNumbermaxoff=entryvec->n-1;
764764
GIST_SPLITVEC*v= (GIST_SPLITVEC*)PG_GETARG_POINTER(1);
765765
OffsetNumberk,
766766
j;
@@ -783,7 +783,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
783783
SPLITCOST*costvector;
784784

785785
/* cache the sign data for each existing item */
786-
cache= (CACHESIGN*)palloc(sizeof(CACHESIGN)* (maxoff+2));
786+
cache= (CACHESIGN*)palloc(sizeof(CACHESIGN)* (maxoff+1));
787787
for (k=FirstOffsetNumber;k <=maxoff;k=OffsetNumberNext(k))
788788
fillcache(&cache[k],GETENTRY(entryvec,k));
789789

@@ -810,7 +810,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
810810
}
811811

812812
/* initialize the result vectors */
813-
nbytes=(maxoff+2)*sizeof(OffsetNumber);
813+
nbytes=maxoff*sizeof(OffsetNumber);
814814
v->spl_left=left= (OffsetNumber*)palloc(nbytes);
815815
v->spl_right=right= (OffsetNumber*)palloc(nbytes);
816816
v->spl_nleft=0;
@@ -846,8 +846,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
846846

847847
union_l=GETSIGN(datum_l);
848848
union_r=GETSIGN(datum_r);
849-
maxoff=OffsetNumberNext(maxoff);
850-
fillcache(&cache[maxoff],GETENTRY(entryvec,maxoff));
849+
851850
/* sort before ... */
852851
costvector= (SPLITCOST*)palloc(sizeof(SPLITCOST)*maxoff);
853852
for (j=FirstOffsetNumber;j <=maxoff;j=OffsetNumberNext(j))
@@ -933,7 +932,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
933932
}
934933
}
935934

936-
*right=*left=FirstOffsetNumber;
937935
v->spl_ldatum=PointerGetDatum(datum_l);
938936
v->spl_rdatum=PointerGetDatum(datum_r);
939937

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