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lib/zlib: remove outdated and incorrect pre-increment optimization
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The zlib inflate code has an old micro-optimization based on the
assumption that for pre-increment memory accesses, the compiler will
generate code that fits better into the processor's pipeline than what
would be generated for post-increment memory accesses.

This optimization was already removed in upstream zlib in 2016:
madler/zlib@9aaec95e8211

This optimization causes UB according to C99, which says in section 6.5.6
"Additive operators": "If both the pointer operand and the result point to
elements of the same array object, or one past the last element of the
array object, the evaluation shall not produce an overflow; otherwise, the
behavior is undefined".

This UB is not only a theoretical concern, but can also cause trouble for
future work on compiler-based sanitizers.

According to the zlib commit, this optimization also is not optimal
anymore with modern compilers.

Replace uses of OFF, PUP and UP_UNALIGNED with their definitions in the
POSTINC case, and remove the macro definitions, just like in the upstream
patch.

Signed-off-by: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Mikhail Zaslonko <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>
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@thejh @torvalds
thejh authored and torvalds committed Jun 5, 2020
1 parent 02223e3 commit acaab73
Showing 1 changed file with 35 additions and 56 deletions.
91 changes: 35 additions & 56 deletions lib/zlib_inflate/inffast.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -10,17 +10,6 @@

#ifndef ASMINF

/* Allow machine dependent optimization for post-increment or pre-increment.
Based on testing to date,
Pre-increment preferred for:
- PowerPC G3 (Adler)
- MIPS R5000 (Randers-Pehrson)
Post-increment preferred for:
- none
No measurable difference:
- Pentium III (Anderson)
- M68060 (Nikl)
*/
union uu {
unsigned short us;
unsigned char b[2];
Expand All @@ -38,16 +27,6 @@ get_unaligned16(const unsigned short *p)
return mm.us;
}

#ifdef POSTINC
# define OFF 0
# define PUP(a) *(a)++
# define UP_UNALIGNED(a) get_unaligned16((a)++)
#else
# define OFF 1
# define PUP(a) *++(a)
# define UP_UNALIGNED(a) get_unaligned16(++(a))
#endif

/*
Decode literal, length, and distance codes and write out the resulting
literal and match bytes until either not enough input or output is
Expand Down Expand Up @@ -115,9 +94,9 @@ void inflate_fast(z_streamp strm, unsigned start)

/* copy state to local variables */
state = (struct inflate_state *)strm->state;
in = strm->next_in - OFF;
in = strm->next_in;
last = in + (strm->avail_in - 5);
out = strm->next_out - OFF;
out = strm->next_out;
beg = out - (start - strm->avail_out);
end = out + (strm->avail_out - 257);
#ifdef INFLATE_STRICT
Expand All @@ -138,9 +117,9 @@ void inflate_fast(z_streamp strm, unsigned start)
input data or output space */
do {
if (bits < 15) {
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
}
this = lcode[hold & lmask];
Expand All @@ -150,24 +129,24 @@ void inflate_fast(z_streamp strm, unsigned start)
bits -= op;
op = (unsigned)(this.op);
if (op == 0) { /* literal */
PUP(out) = (unsigned char)(this.val);
*out++ = (unsigned char)(this.val);
}
else if (op & 16) { /* length base */
len = (unsigned)(this.val);
op &= 15; /* number of extra bits */
if (op) {
if (bits < op) {
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
}
len += (unsigned)hold & ((1U << op) - 1);
hold >>= op;
bits -= op;
}
if (bits < 15) {
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
}
this = dcode[hold & dmask];
Expand All @@ -180,10 +159,10 @@ void inflate_fast(z_streamp strm, unsigned start)
dist = (unsigned)(this.val);
op &= 15; /* number of extra bits */
if (bits < op) {
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
if (bits < op) {
hold += (unsigned long)(PUP(in)) << bits;
hold += (unsigned long)(*in++) << bits;
bits += 8;
}
}
Expand All @@ -205,13 +184,13 @@ void inflate_fast(z_streamp strm, unsigned start)
state->mode = BAD;
break;
}
from = window - OFF;
from = window;
if (write == 0) { /* very common case */
from += wsize - op;
if (op < len) { /* some from window */
len -= op;
do {
PUP(out) = PUP(from);
*out++ = *from++;
} while (--op);
from = out - dist; /* rest from output */
}
Expand All @@ -222,14 +201,14 @@ void inflate_fast(z_streamp strm, unsigned start)
if (op < len) { /* some from end of window */
len -= op;
do {
PUP(out) = PUP(from);
*out++ = *from++;
} while (--op);
from = window - OFF;
from = window;
if (write < len) { /* some from start of window */
op = write;
len -= op;
do {
PUP(out) = PUP(from);
*out++ = *from++;
} while (--op);
from = out - dist; /* rest from output */
}
Expand All @@ -240,21 +219,21 @@ void inflate_fast(z_streamp strm, unsigned start)
if (op < len) { /* some from window */
len -= op;
do {
PUP(out) = PUP(from);
*out++ = *from++;
} while (--op);
from = out - dist; /* rest from output */
}
}
while (len > 2) {
PUP(out) = PUP(from);
PUP(out) = PUP(from);
PUP(out) = PUP(from);
*out++ = *from++;
*out++ = *from++;
*out++ = *from++;
len -= 3;
}
if (len) {
PUP(out) = PUP(from);
*out++ = *from++;
if (len > 1)
PUP(out) = PUP(from);
*out++ = *from++;
}
}
else {
Expand All @@ -264,29 +243,29 @@ void inflate_fast(z_streamp strm, unsigned start)
from = out - dist; /* copy direct from output */
/* minimum length is three */
/* Align out addr */
if (!((long)(out - 1 + OFF) & 1)) {
PUP(out) = PUP(from);
if (!((long)(out - 1) & 1)) {
*out++ = *from++;
len--;
}
sout = (unsigned short *)(out - OFF);
sout = (unsigned short *)(out);
if (dist > 2) {
unsigned short *sfrom;

sfrom = (unsigned short *)(from - OFF);
sfrom = (unsigned short *)(from);
loops = len >> 1;
do
#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
PUP(sout) = PUP(sfrom);
*sout++ = *sfrom++;
#else
PUP(sout) = UP_UNALIGNED(sfrom);
*sout++ = get_unaligned16(sfrom++);
#endif
while (--loops);
out = (unsigned char *)sout + OFF;
from = (unsigned char *)sfrom + OFF;
out = (unsigned char *)sout;
from = (unsigned char *)sfrom;
} else { /* dist == 1 or dist == 2 */
unsigned short pat16;

pat16 = *(sout-1+OFF);
pat16 = *(sout-1);
if (dist == 1) {
union uu mm;
/* copy one char pattern to both bytes */
Expand All @@ -296,12 +275,12 @@ void inflate_fast(z_streamp strm, unsigned start)
}
loops = len >> 1;
do
PUP(sout) = pat16;
*sout++ = pat16;
while (--loops);
out = (unsigned char *)sout + OFF;
out = (unsigned char *)sout;
}
if (len & 1)
PUP(out) = PUP(from);
*out++ = *from++;
}
}
else if ((op & 64) == 0) { /* 2nd level distance code */
Expand Down Expand Up @@ -336,8 +315,8 @@ void inflate_fast(z_streamp strm, unsigned start)
hold &= (1U << bits) - 1;

/* update state and return */
strm->next_in = in + OFF;
strm->next_out = out + OFF;
strm->next_in = in;
strm->next_out = out;
strm->avail_in = (unsigned)(in < last ? 5 + (last - in) : 5 - (in - last));
strm->avail_out = (unsigned)(out < end ?
257 + (end - out) : 257 - (out - end));
Expand Down

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