[PATCH] lib/zlib: Remove outdated and incorrect pre-increment optimization

From: Jann Horn
Date: Thu May 07 2020 - 08:32:08 EST

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:
https://github.com/madler/zlib/commit/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.

(This patch lights up checkpatch like a christmas tree because spaces are
used for indentation; but almost the entire file uses four spaces to
indent, so I don't think there's anything I can reasonably do about that.)

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
lib/zlib_inflate/inffast.c | 91 +++++++++++++++-----------------------
1 file changed, 35 insertions(+), 56 deletions(-)

diff --git a/lib/zlib_inflate/inffast.c b/lib/zlib_inflate/inffast.c
index 2c13ecc5bb2c7..ed1f3df272602 100644
--- a/lib/zlib_inflate/inffast.c
+++ b/lib/zlib_inflate/inffast.c
@@ -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];
@@ -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
@@ -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
@@ -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];
@@ -150,14 +129,14 @@ 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);
@@ -165,9 +144,9 @@ void inflate_fast(z_streamp strm, unsigned start)
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];
@@ -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;
}
}
@@ -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 */
}
@@ -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 */
}
@@ -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 {
@@ -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 */
@@ -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 */
@@ -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));

base-commit: a811c1fa0a02c062555b54651065899437bacdbe
--
2.26.2.526.g744177e7f7-goog