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Commit48ab1fa

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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
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‎contrib/pg_trgm/trgm_gist.c

Lines changed: 4 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -786,7 +786,7 @@ Datum
786786
gtrgm_picksplit(PG_FUNCTION_ARGS)
787787
{
788788
GistEntryVector*entryvec= (GistEntryVector*)PG_GETARG_POINTER(0);
789-
OffsetNumbermaxoff=entryvec->n-2;
789+
OffsetNumbermaxoff=entryvec->n-1;
790790
GIST_SPLITVEC*v= (GIST_SPLITVEC*)PG_GETARG_POINTER(1);
791791
intsiglen=GET_SIGLEN();
792792
OffsetNumberk,
@@ -811,8 +811,8 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
811811
SPLITCOST*costvector;
812812

813813
/* cache the sign data for each existing item */
814-
cache= (CACHESIGN*)palloc(sizeof(CACHESIGN)* (maxoff+2));
815-
cache_sign=palloc(siglen* (maxoff+2));
814+
cache= (CACHESIGN*)palloc(sizeof(CACHESIGN)* (maxoff+1));
815+
cache_sign=palloc(siglen* (maxoff+1));
816816

817817
for (k=FirstOffsetNumber;k <=maxoff;k=OffsetNumberNext(k))
818818
fillcache(&cache[k],GETENTRY(entryvec,k),&cache_sign[siglen*k],
@@ -841,7 +841,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
841841
}
842842

843843
/* initialize the result vectors */
844-
nbytes=(maxoff+2)*sizeof(OffsetNumber);
844+
nbytes=maxoff*sizeof(OffsetNumber);
845845
v->spl_left=left= (OffsetNumber*)palloc(nbytes);
846846
v->spl_right=right= (OffsetNumber*)palloc(nbytes);
847847
v->spl_nleft=0;
@@ -853,9 +853,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
853853

854854
union_l=GETSIGN(datum_l);
855855
union_r=GETSIGN(datum_r);
856-
maxoff=OffsetNumberNext(maxoff);
857-
fillcache(&cache[maxoff],GETENTRY(entryvec,maxoff),
858-
&cache_sign[siglen*maxoff],siglen);
859856

860857
/* sort before ... */
861858
costvector= (SPLITCOST*)palloc(sizeof(SPLITCOST)*maxoff);
@@ -944,7 +941,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
944941
}
945942
}
946943

947-
*right=*left=FirstOffsetNumber;
948944
v->spl_ldatum=PointerGetDatum(datum_l);
949945
v->spl_rdatum=PointerGetDatum(datum_r);
950946

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