From: Jonathan Nieder <jrnieder@gmail.com>
Date: Thu, 17 May 2012 18:49:00 -0500
Subject: 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.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
 configure.ac                |    5 +++++
 src/liblzma/common/common.c |   40 ++++++++++++++++++++++++++++++++++++++--
 src/liblzma/common/common.h |    4 ++++
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index f18f8959..1476c8e2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -479,6 +479,11 @@ if test "x$enable_threads" = xyes; then
 fi
 AM_CONDITIONAL([COND_THREADS], [test "x$ax_pthread_ok" = xyes])
 
+# As a Debian-specific hack, liblzma uses dlopen() to check if extra
+# paranoia is needed because unversioned symbols from liblzma.so.2 are
+# present in the same process.  See src/liblzma/common/common.c.
+AC_SEARCH_LIBS([dlopen], [dl])
+
 echo
 echo "Initializing Libtool:"
 LT_PREREQ([2.2])
diff --git a/src/liblzma/common/common.c b/src/liblzma/common/common.c
index 85ae96a9..f1693a01 100644
--- a/src/liblzma/common/common.c
+++ b/src/liblzma/common/common.c
@@ -12,6 +12,8 @@
 
 #include "common.h"
 
+#include <dlfcn.h>
+
 
 /////////////
 // Version //
@@ -141,6 +143,17 @@ lzma_next_end(lzma_next_coder *next, lzma_allocator *allocator)
 // External to internal API wrapper //
 //////////////////////////////////////
 
+static bool
+liblzma2_loaded(void)
+{
+	void *handle = dlopen("liblzma.so.2", RTLD_LAZY | RTLD_NOLOAD);
+	if (handle) {
+		dlclose(handle);
+		return true;
+	}
+	return false;
+}
+
 extern lzma_ret
 lzma_strm_init(lzma_stream *strm)
 {
@@ -154,6 +167,7 @@ lzma_strm_init(lzma_stream *strm)
 			return LZMA_MEM_ERROR;
 
 		strm->internal->next = LZMA_NEXT_CODER_INIT;
+		strm->internal->liblzma2_compat = liblzma2_loaded();
 	}
 
 	memzero(strm->internal->supported_actions,
@@ -168,6 +182,24 @@ lzma_strm_init(lzma_stream *strm)
 }
 
 
+// Before v5.0.0~6 (liblzma: A few ABI tweaks to reserve space in
+// structures, 2010-10-23), the reserved fields in lzma_stream were:
+//
+//	void *reserved_ptr1;
+//	void *reserved_ptr2;
+//	uint64_t reserved_int1;
+//	uint64_t reserved_int2;
+//	lzma_reserved_enum reserved_enum1;
+//	lzma_reserved_enum reserved_enum2;
+//
+// Nowadays there are two more pointers between reserved_ptr2 and
+// reserved_int1 and two size_t fields between reserved_int2 and
+// reserved_enum1.
+//
+// When strm->internal->liblzma2_compat is set, limit the checks of
+// reserved fields to fields that were present in the old ABI to avoid
+// segfaults and spurious "Unsupported options" from callers sharing
+// the process image that expect the old ABI.
 extern LZMA_API(lzma_ret)
 lzma_code(lzma_stream *strm, lzma_action action)
 {
@@ -185,8 +217,12 @@ lzma_code(lzma_stream *strm, lzma_action action)
 	if (strm->reserved_ptr1 != NULL
 			|| strm->reserved_ptr2 != NULL
 			|| strm->reserved_ptr3 != NULL
-			|| strm->reserved_ptr4 != NULL
-			|| strm->reserved_int1 != 0
+			|| strm->reserved_ptr4 != NULL)
+		return LZMA_OPTIONS_ERROR;
+
+	if (strm->internal->liblzma2_compat)
+		; /* Enough checks. */
+	else if (strm->reserved_int1 != 0
 			|| strm->reserved_int2 != 0
 			|| strm->reserved_int3 != 0
 			|| strm->reserved_int4 != 0
diff --git a/src/liblzma/common/common.h b/src/liblzma/common/common.h
index 5c92af27..de482b38 100644
--- a/src/liblzma/common/common.h
+++ b/src/liblzma/common/common.h
@@ -216,6 +216,10 @@ struct lzma_internal_s {
 	/// If true, lzma_code will return LZMA_BUF_ERROR if no progress was
 	/// made (no input consumed and no output produced by next.code).
 	bool allow_buf_error;
+
+	/// Indicates whether we are sharing a process image with
+	/// liblzma.so.2 and need to tread carefully.
+	bool liblzma2_compat;
 };
 
 
-- 
1.7.10.2