This allows to resolve weak symbols from some Android device's libc that
dlsym() won't. This is effectively an alternative fix to bug 791419, without
requiring wrapping symbols.
The new class is kind of like SystemElf, but using our linker's own symbol
resolution. This also adds some initialization from ELF program headers that
weren't done previously for self_elf, as well as registration as for CustomElf
instances.
When a library fails to load with CustomElf before it's registered, the
unregistration that does happen in CustomElf's destructor uses link_map,
so it needs to be initialized.
With bug 1077366, the linker makes the library containing it a fake
LD_PRELOAD. As a consequence, instead of, in the linker itself,
explicitely special-casing the symbols that disappeared in Android 4.4
that the flash plugin uses, it is now possible to use normal symbol
resolution to stubs defined separately in libmozglue.
Since essentially everything is linked to libmozglue and libmozglue takes
precedence in symbol resolution in our dynamic linker, there is no need
to wrap most symbols. PR_GetEnv/PR_SetEnv still needs wrapping because
there's no other way to actually wrap the calls from NSPR itself and NSS,
as well as the symbols wrapped because our dynamic linker can't find them
in system libraries on some devices because they're weak.
In order to avoid adding more dlsym overhead than there already is, resolve
symbols directly in the library containing the linker. (GetSymbolPtr is
essentially free ; dlsym makes the system linker compule a ElfHash itself,
and that's quite expensive to do on all symbols)
This also paves the way for direct symbol resolution in all system libraries.
It's necessary to delay it because for the second part, we need to call dlopen,
and until recently bionic's linker dead-locked when using dlopen from a static
initializer.
Android L added a libsigchain library it LD_PRELOADs. That library exposes
a different sigaction than libc's. It's used for ART.
faulty.lib gets its sigaction from libsigchain, but after bug 874708, the
libraries it loads simply use libc's sigaction, assuming it would be the
hooked one. In turn, this means libraries loaded by faulty.lib may
override faulty.lib's handler, which is definitely not the intent.
This essentially restores some of the code that bug 874708 removed.
An alternative fix would be to add support for LD_PRELOAD, but that has more
implications and feels more risky. This could be done, if necessary, as a
followup.
OS_LIBS for libraries that are not part of the gecko tree, EXTRA_LIBS for
libraries, such as NSPR, that are in the tree, but are not handled by
moz.build just yet. Those EXTRA_LIBS may also come from a system library.
However, in cases where the expanded variables are always empty for the
in-tree case, OS_LIBS is used (as for, e.g. MOZ_ZLIB_LIBS). OS_LDFLAGS is
used exclusively for non-library linker flags.
Always pass EXTRA_LIBS before OS_LIBS on linker command lines.
Forbid EXTRA_DSO_LDOPTS, SHARED_LIBRARY_LIBS and LIBS in Makefiles.
