liblzma: skip ABI-incompatible check when liblzma.so.2 is loaded
When liblzma started using ELF symbol versioning at the same time
as a soname bump (2 → 5) and a small increase in the reserved space at
the end of the lzma_stream structure checked by lzma_code, introducing
an unversioned compatibility symbol to ease the transition seemed like
a great idea. After all:
- most applications only use one version of the library (liblzma.so.2
or liblzma.so.5) and would obviously work fine
- applications linking to the new version of the library
(liblzma.so.5) should use the default, versioned lzma_code symbol so
errors initializing the reserved space can be noticed
- symbol versioning should ensure application/library mixtures
independently making use of both versions of the library also
work. Calls using the unversioned symbol names would be resolved
using the old symbol from liblzma.so.2 or the compatibility symbol
from liblzma.so.5, avoiding segfaults and spurious
LZMA_OPTIONS_ERROR errors.
- application/library mixtures using both versions of the library and
passing lzma_stream objects between the two would break, but that
was never supposed to be supported, anyway.
Three toolchain bugs dash that plan.
Current (2.22) versions of the gold linker cannot be used to build
libraries providing versioned and unversioned symbols with the same
name. On the other hand, BFD ld doesn't mind. So GNU gold refuses to
link versions of liblzma including the compatibility symbol (PR12261):
/usr/bin/ld: error: symbol lzma_code has undefined version
Annoying, but livable. liblzma with the compatibility symbol just
has to be built with BFD ld.
More importantly, gold does not support linking to libraries providing
versioned and unversioned symbols with the same name. If I link some
code to a version of liblzma with the compatibility symbol:
ld -o demo demo.o -llzma
then the documented behavior, implemented by BFD ld, is to interpret
calls to lzma_code as referring to the default version
(lzma_code@XZ_5.0). Current versions of GNU gold treat such calls as
referring to whichever symbol comes first in liblzma.so.5's symbol
table. If the unversioned symbol comes first (and in Debian liblzma5
5.1.1alpha+20110809-3 it does), GNU gold will mislink new applications
to use the unversioned compatibility symbol (PR13521):
$ ld.gold -o test.gold test.o -llzma
$ eu-readelf -s test.gold | grep lzma_code
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UNDEF lzma_code
5: 0000000000000000 0 FUNC GLOBAL DEFAULT UNDEF lzma_code
There is no warning.
Worse, ld.so from glibc unpredictably will sometimes use the versioned
symbol to resolve references to the unversioned base version when both
are present (PR12977).
Clearly no one has been testing mixtures of versioned and
unversioned symbols at all, and we cannot trust the symbol resolution
process to do anything in particular for them.
This patch implements an alternative method to implement the same
compatibility goals described above.
- No more compatibility symbol. liblzma.so.5 will define lzma_code
only once, with version XZ_5.0.
- When initializing an lzma_stream object, use dlopen("liblzma.so.2",
RTLD_NOLOAD) to detect whether the caller might have been expecting
the old ABI, and store that information in the private
stream->internal->liblzma2_compat field.
- In lzma_code, when checking reserved fields, skip fields past the
old end of the lzma_stream structure ifying reserved fields if and
only if this->internal->liblzma2_compat is false.
That's it. Hopefully this time it will work reliably.
Thanks to Eduard Bloch for noticing PR13521 and to Ian Lance Taylor
for PR12977.
Patch-Name: liblzma-skip-ABI-incompatible-check-when-liblzma.so.patch
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
3 files changed
