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replace jbig2_arith_get_offset with jbig2_arith_has_reached_marker

The DOS check introduced with 6e1f2259115efac14cd6c7ad9d119b43013a32a1
tries to determine whether arithmetic coding has run out of data and
has entered an infinite loop. It does so by checking how many bytes
have been read and compares with the number of bytes available. The fix
however fails to take into account bytes which have been cached in
_Jbig2ArithState::next_word which causes valid images to be rejected.

This patch corrects that bug by replacing that check with a generic
check as to whether the end of the data stream has been reached (which
is the case if the only two remaining cached bytes are 0xFF and a value
larger than 0x8F).

Signed-off-by: Henry Stiles <[email protected]>

--- a/jbig2_arith.c

+++ b/jbig2_arith.c

@@ -379,10 +379,10 @@

     }

 }

-int

-jbig2_arith_get_offset(Jbig2ArithState *as)

+bool

+jbig2_arith_has_reached_marker(Jbig2ArithState *as)

 {

-  return as->offset;

+  return as->next_word_bytes == 2 && (as->next_word >> 16) > 0xFF8F;

 }

 #ifdef TEST

--- a/jbig2_arith.h

+++ b/jbig2_arith.h

@@ -33,6 +33,6 @@

 bool

 jbig2_arith_decode (Jbig2ArithState *as, Jbig2ArithCx *pcx);

-/* return the state's offset (for sanity checks) */

-int

-jbig2_arith_get_offset(Jbig2ArithState *as);

+/* returns true if the end of the data stream has been reached (for sanity checks) */

+bool

+jbig2_arith_has_reached_marker(Jbig2ArithState *as);

--- a/jbig2_symbol_dict.c

+++ b/jbig2_symbol_dict.c

@@ -361,7 +361,7 @@

 	  "error or OOB decoding height class delta (%d)\n", code);

       }

-      if (!params->SDHUFF && jbig2_arith_get_offset(as) >= size) {

+      if (!params->SDHUFF && jbig2_arith_has_reached_marker(as)) {

           code = jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number,

               "prevent DOS while decoding height classes");

           goto cleanup2;