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liblzma: Disable external SHA-256 by default.

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This is the sane thing to do. The conflict with OpenSSL
on some OSes and especially that the OS-provided versions
can be significantly slower makes it clear that it was
a mistake to have the external SHA-256 support enabled by
default.

Those who want it can now pass --enable-external-sha256 to
configure. INSTALL was updated with notes about OSes where
this can be a bad idea.

The SHA-256 detection code in configure.ac had some bugs that
could lead to a build failure in some situations. These were
fixed, although it doesn't matter that much now that the
external SHA-256 is disabled by default.

MINIX >= 3.2.0 uses NetBSD's libc and thus has SHA256_Init
in libc instead of libutil. Support for the libutil version
was removed.
This commit is contained in:
Lasse Collin 2016-03-13 20:21:49 +02:00
parent 6fd5ecb589
commit ac398c3baf
3 changed files with 79 additions and 49 deletions
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36
INSTALL
View file

@ -246,6 +246,42 @@ XZ Utils Installation
the liblzma ABI, so this option should be used only when
it is known to not cause problems.
--enable-external-sha256
Try to use SHA-256 code from the operating system libc
or similar base system libraries. This doesn't try to
use OpenSSL or libgcrypt or such libraries.
The reasons to use this option:
- It makes liblzma slightly smaller.
- It might improve SHA-256 speed if the implementation
in the operating is very good (but see below).
External SHA-256 is disabled by default for two reasons:
- On some operating systems the symbol names of the
SHA-256 functions conflict with OpenSSL's libcrypto.
This causes weird problems such as decompression
errors if an application is linked against both
liblzma and libcrypto. This problem affects at least
FreeBSD 10 and older and MINIX 3.3.0 and older, but
other OSes that provide a function "SHA256_Init" might
also be affected. FreeBSD 11 has the problem fixed.
NetBSD had the problem but it was fixed it in 2009
already. OpenBSD uses "SHA256Init" and thus never had
a conflict with libcrypto.
- The SHA-256 code in liblzma is faster than the SHA-256
code provided by some operating systems. If you are
curious, build two copies of xz (internal and external
SHA-256) and compare the decompression (xz --test)
times:
dd if=/dev/zero bs=1024k count=1024 \
| xz -v -0 -Csha256 > foo.xz
time xz --test foo.xz
--disable-xz
--disable-xzdec
--disable-lzmadec

76
configure.ac
View file

@ -275,6 +275,20 @@ m4_foreach([NAME], [SUPPORTED_CHECKS],
[AM_CONDITIONAL(COND_CHECK_[]m4_toupper(NAME), test "x$enable_check_[]NAME" = xyes)
])dnl
AC_MSG_CHECKING([if external SHA-256 should be used])
AC_ARG_ENABLE([external-sha256], AS_HELP_STRING([--enable-external-sha256],
[Use SHA-256 code from the operating system.
See INSTALL for possible subtle problems.]),
[], [enable_external_sha256=no])
if test "x$enable_check_sha256" != "xyes"; then
enable_external_sha256=no
fi
if test "x$enable_external_sha256" = xyes; then
AC_MSG_RESULT([yes])
else
AC_MSG_RESULT([no])
fi
###########################
# Assembler optimizations #
@ -669,48 +683,34 @@ TUKLIB_PHYSMEM
TUKLIB_CPUCORES
TUKLIB_MBSTR
# Check for system-provided SHA-256. The supported implementations are listed
# below. The detection for the ones marked with [*] has been intentionally
# disabled because they have symbol name conflicts with OpenSSL's libcrypto
# which can cause weird problems (clean namespaces would make things too
# boring, I guess).
# If requsted, check for system-provided SHA-256. At least the following
# implementations are supported:
#
# OS Headers Library Type Function
# FreeBSD sys/types.h + sha256.h libmd SHA256_CTX SHA256_Init [*]
# FreeBSD sys/types.h + sha256.h libmd SHA256_CTX SHA256_Init
# NetBSD sys/types.h + sha2.h SHA256_CTX SHA256_Init
# OpenBSD sys/types.h + sha2.h SHA2_CTX SHA256Init
# Solaris sys/types.h + sha2.h libmd SHA256_CTX SHA256Init
# MINIX 3 sys/types.h + minix/sha2.h libutil SHA256_CTX SHA256_Init [*]
# MINIX 3 sys/types.h + sha2.h SHA256_CTX SHA256_Init
# Darwin CommonCrypto/CommonDigest.h CC_SHA256_CTX CC_SHA256_Init
#
# Notes:
# Note that Darwin's CC_SHA256_Update takes buffer size as uint32_t instead
# of size_t.
#
# - NetBSD's SHA256_Init doesn't conflict with libcrypto because
# libcrypto on NetBSD was made to use the libc implementation to avoid
# this exact symbol conflict problem:
# http://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2009-012.txt.asc
#
# - As of 2016-03-10, FreeBSD seems to have the issue fixed in SVN head
# but not in the FreeBSD 10 branch.
#
# - Darwin's CC_SHA256_Update takes buffer size as uint32_t instead
# of size_t.
#
# We don't check for e.g. OpenSSL or libgcrypt because we don't want
# to introduce dependencies to other packages by default. Maybe such
# libraries could be supported via additional configure options though.
#
if test "x$enable_check_sha256" = "xyes"; then
sha256_header_found=no
sha256_type_found=no
sha256_func_found=no
if test "x$enable_external_sha256" = "xyes"; then
# Test for Common Crypto before others, because Darwin has sha256.h
# too and we don't want to use that, because on older versions it
# uses OpenSSL functions, whose SHA256_Init is not guaranteed to
# succeed.
sha256_header_found=no
AC_CHECK_HEADERS(
[CommonCrypto/CommonDigest.h sha256.h sha2.h minix/sha2.h],
[CommonCrypto/CommonDigest.h sha256.h sha2.h],
[sha256_header_found=yes ; break])
if test "x$sha256_header_found" = xyes; then
AC_CHECK_TYPES([CC_SHA256_CTX, SHA256_CTX, SHA2_CTX], [], [],
AC_CHECK_TYPES([CC_SHA256_CTX, SHA256_CTX, SHA2_CTX],
[sha256_type_found=yes], [],
[[#ifdef HAVE_SYS_TYPES_H
# include <sys/types.h>
#endif
@ -722,21 +722,19 @@ if test "x$enable_check_sha256" = "xyes"; then
#endif
#ifdef HAVE_SHA2_H
# include <sha2.h>
#endif
#ifdef HAVE_MINIX_SHA2_H
# include <minix/sha2.h>
#endif]])
dnl Omit detection of the FreeBSD and MINIX 3 versions:
dnl AC_SEARCH_LIBS([SHA256_Init], [md util])
AC_SEARCH_LIBS([SHA256Init], [md])
AC_CHECK_FUNCS([CC_SHA256_Init SHA256_Init SHA256Init],
[break])
if test "x$sha256_type_found" = xyes ; then
AC_SEARCH_LIBS([SHA256Init], [md])
AC_SEARCH_LIBS([SHA256_Init], [md])
AC_CHECK_FUNCS([CC_SHA256_Init SHA256Init SHA256_Init],
[sha256_func_found=yes ; break])
fi
fi
fi
AM_CONDITIONAL([COND_INTERNAL_SHA256],
[test "x$ac_cv_func_SHA256_Init" != xyes \
&& test "x$ac_cv_func_SHA256Init" != xyes \
&& test "x$ac_cv_func_CC_SHA256_Init" != xyes])
AM_CONDITIONAL([COND_INTERNAL_SHA256], [test "x$sha256_func_found" = xno])
if test "x$enable_external_sha256$sha256_func_found" = xyesno; then
AC_MSG_ERROR([--enable-external-sha256 was specified but no supported external SHA-256 implementation was found])
fi
# Check for SSE2 intrinsics.
AC_CHECK_DECL([_mm_movemask_epi8],

16
src/liblzma/check/check.h
View file

@ -15,13 +15,12 @@
#include "common.h"
// If either the data type or the function for external SHA-256 is missing,
// use the internal SHA-256 code.
#if !(defined(HAVE_CC_SHA256_CTX) || defined(HAVE_SHA256_CTX) \
|| defined(HAVE_SHA2_CTX)) \
|| !(defined(HAVE_CC_SHA256_INIT) \
|| defined(HAVE_SHA256_INIT) \
|| defined(HAVE_SHA256INIT))
// If the function for external SHA-256 is missing, use the internal SHA-256
// code. Due to how configure works, these defines can only get defined when
// both a usable header and a type have already been found.
#if !(defined(HAVE_CC_SHA256_INIT) \
|| defined(HAVE_SHA256_INIT) \
|| defined(HAVE_SHA256INIT))
# define HAVE_INTERNAL_SHA256 1
#endif
@ -35,9 +34,6 @@
#elif defined(HAVE_SHA2_H)
# include <sys/types.h>
# include <sha2.h>
#elif defined(HAVE_MINIX_SHA2_H)
# include <sys/types.h>
# include <minix/sha2.h>
#endif
#if defined(HAVE_INTERNAL_SHA256)