this was discussed on the mailing list and no consensus on the
preferred solution was reached, so in anticipation of a release, i'm
just committing a minimally-invasive solution that avoids the problem
by ensuring that multi-threaded-capable programs will always have
initialized the thread pointer before any signal handler can run.
in the long term we may switch to initializing the thread pointer at
program start time whenever the program has the potential to access
any per-thread data.
in gcc 3, the visibility attribute must be placed on both the
declaration and on the definition. if it's omitted from the
definition, the compiler fails to emit the ".hidden" directive in the
assembly, and the linker will either generate textrels (if supported,
such as on i386) or refuse to link (on targets where certain types of
textrels are forbidden or impossible without further assumptions about
memory layout, such as on x86_64).
this patch also unifies the decision about when to use visibility into
libc.h and makes the visibility in the utf-8 state machine tables
based on libc.h rather than a duplicate test.
even if pthread_create/exit code is not linked, run flag needs to be
checked and cleanup function potentially run on pop. thus, move the
code to the module that's always linked when pthread_cleanup_push/pop
is used.
the old abi was intended to duplicate glibc's abi at the expense of
being ugly and slow, but it turns out glib was not even using that abi
except on non-gcc-compatible compilers (which it doesn't even support)
and was instead using an exceptions-in-c/unwind-based approach whose
abi we could not duplicate anyway without nasty dwarf2/unwind
integration.
the new abi is copied from a very old glibc abi, which seems to still
be supported/present in current glibc. it avoids all unwinding,
whether by sjlj or exceptions, and merely maintains a linked list of
cleanup functions to be called from the context of pthread_exit. i've
made some care to ensure that longjmp out of a cleanup function should
work, even though it is not required to.
this change breaks abi compatibility with programs which were using
pthread cancellation, which is unfortunate, but that's why i'm making
the change now rather than later. considering that most pthread
features have not been usable until recently anyway, i don't see it as
a major issue at this point.
i'm not sure that it's "correct" for dlopen to block cancellation
when calling constructors for libraries it loads, but it sure seems
like the right thing. in any case, dlopen itself needs cancellation
blocked.
this is mainly in hopes of supporting c++ (not yet possible for other
reasons) but will also help applications/libraries which use (and more
often, abuse) the gcc __attribute__((__constructor__)) feature in "C"
code.
x86_64 and arm versions of the new startup asm are untested and may
have minor problems.
these don't work (or do anything at all) but at least make it possible
to static link programs that insist on "having" dynamic loading
support...as long as they don't actually need to use it.
adding real support for dlopen/dlsym with static linking is going to
be significantly more difficult...
it should be noted that only the actual underlying buffer flush and
fill operations are cancellable, not reads from or writes to the
buffer. this behavior is compatible with POSIX, which makes all
cancellation points in stdio optional, and it achieves the goal of
allowing cancellation of a thread that's "stuck" on IO (due to a
non-responsive socket/pipe peer, slow/stuck hardware, etc.) without
imposing any measurable performance cost.
even a single-threaded program can be cancellable, e.g. if it's called
pthread_cancel(pthread_self()). the correct predicate to check is not
whether multiple threads have been invoked, but whether pthread_self
has been invoked.
this fixes an issue using gold instead of gnu ld for linking. it also
should eliminate the need of the startup code to even load/pass the
got address to the dynamic linker.
based on patch submitted by sh4rm4 with minor cosmetic changes.
further cleanup will follow.
note that regardless of the name used, basename is always conformant.
it never takes on the bogus gnu behavior, unlike glibc where basename
is nonconformant when declared manually without including libgen.h.
CHUNK_SIZE macro was defined incorrectly and shaving off at least one
significant bit in the size of mmapped chunks, resulting in the test
for oldlen==newlen always failing and incurring a syscall. fortunately
i don't think this issue caused any other observable behavior; the
definition worked correctly for all non-mmapped chunks where its
correctness matters more, since their lengths are always multiples of
the alignment.
musl's dynamic linker does not support unloading dsos, so there's
nothing for this function to do. adding the symbol in case anything
depends on its presence..
