There are, sadly, many combinations of linkage in use throughout the tree.
The main differentiator, though, is between program/libraries related to
Gecko or not. Kind of. Some need mozglue, some don't. Some need dependent
linkage, some standalone.
Anyways, these new templates remove the need to manually define the
right dependencies against xpcomglue, nspr, mozalloc and mozglue
in most cases.
Places that build programs and were resetting MOZ_GLUE_PROGRAM_LDFLAGS
or that build libraries and were resetting MOZ_GLUE_LDFLAGS can now
just not use those Gecko-specific templates.
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.
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.
