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Remove offset pointer optimization in inftrees.c.

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inftrees.c was subtracting an offset from a pointer to an array,
in order to provide a pointer that allowed indexing starting at
the offset. This is not compliant with the C standard, for which
the behavior of a pointer decremented before its allocated memory
is undefined. Per the recommendation of a security audit of the
zlib code by Trail of Bits and TrustInSoft, in support of the
Mozilla Foundation, this tiny optimization was removed, in order
to avoid the possibility of undefined behavior.
This commit is contained in:
Mark Adler 2016-09-21 23:35:50 -07:00
parent 9aaec95e82
commit 6a043145ca
1 changed files with 8 additions and 10 deletions
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18
inftrees.c
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@ -54,7 +54,7 @@ unsigned short FAR *work;
code FAR *next; /* next available space in table */ code FAR *next; /* next available space in table */
const unsigned short FAR *base; /* base value table to use */ const unsigned short FAR *base; /* base value table to use */
const unsigned short FAR *extra; /* extra bits table to use */ const unsigned short FAR *extra; /* extra bits table to use */
int end; /* use base and extra for symbol > end */ unsigned match; /* use base and extra for symbol >= match */
unsigned short count[MAXBITS+1]; /* number of codes of each length */ unsigned short count[MAXBITS+1]; /* number of codes of each length */
unsigned short offs[MAXBITS+1]; /* offsets in table for each length */ unsigned short offs[MAXBITS+1]; /* offsets in table for each length */
static const unsigned short lbase[31] = { /* Length codes 257..285 base */ static const unsigned short lbase[31] = { /* Length codes 257..285 base */
@ -181,19 +181,17 @@ unsigned short FAR *work;
switch (type) { switch (type) {
case CODES: case CODES:
base = extra = work; /* dummy value--not used */ base = extra = work; /* dummy value--not used */
end = 19; match = 20;
break; break;
case LENS: case LENS:
base = lbase; base = lbase;
base -= 257;
extra = lext; extra = lext;
extra -= 257; match = 257;
end = 256;
break; break;
default: /* DISTS */ default: /* DISTS */
base = dbase; base = dbase;
extra = dext; extra = dext;
end = -1; match = 0;
} }
/* initialize state for loop */ /* initialize state for loop */
@ -216,13 +214,13 @@ unsigned short FAR *work;
for (;;) { for (;;) {
/* create table entry */ /* create table entry */
here.bits = (unsigned char)(len - drop); here.bits = (unsigned char)(len - drop);
if ((int)(work[sym]) < end) { if (work[sym] + 1 < match) {
here.op = (unsigned char)0; here.op = (unsigned char)0;
here.val = work[sym]; here.val = work[sym];
} }
else if ((int)(work[sym]) > end) { else if (work[sym] >= match) {
here.op = (unsigned char)(extra[work[sym]]); here.op = (unsigned char)(extra[work[sym] - match]);
here.val = base[work[sym]]; here.val = base[work[sym] - match];
} }
else { else {
here.op = (unsigned char)(32 + 64); /* end of block */ here.op = (unsigned char)(32 + 64); /* end of block */