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prior to linux 2.6.22, futex wait could fail with EINTR even for
non-interrupting (SA_RESTART) signals. this was no problem provided
the caller simply restarted the wait, but sem_[timed]wait is required
by POSIX to return when interrupted by a signal. commit
a113434cd68ce30642c4995b1caadcd084be6f09 introduced this behavior, and
commit c0ed5a201b2bdb6d1896064bec0020c9973db0a1 reverted it based on a
mistaken belief that it was not required. this belief stems from a bug
in the specification: the description requires the function to return
when interrupted, but the errors section marks EINTR as a "may fail"
condition rather than a "shall fail" one.
since there does seem to be significant value in the change made in
commit c0ed5a201b2bdb6d1896064bec0020c9973db0a1, making it so that
programs that call sem_wait without checking for EINTR don't silently
make forward progress without obtaining the semaphore or treat it as a
fatal error and abort, add a behind-the-scenes mechanism in the
__timedwait backend to suppress EINTR in programs that have never
installed interrupting signal handlers, and have sigaction track and
report this state. this way the semaphore code is not cluttered by
workarounds and can be updated (to be done in next commit) to reflect
the high-level logic for conforming behavior.
these changes are based loosely on a patch by Markus Wichmann, with
the main changes being atomic update to flag object and moving the
workaround from sem_timedwait to the __timedwait futex backend.
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this further reduces the number of source files which need to include
libc.h and thereby be potentially exposed to libc global state and
internals.
this will also facilitate further improvements like adding an inline
fast-path, if we want to do so later.
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this cleans up what had become widespread direct inline use of "GNU C"
style attributes directly in the source, and lowers the barrier to
increased use of hidden visibility, which will be useful to recovering
some of the efficiency lost when the protected visibility hack was
dropped in commit dc2f368e565c37728b0d620380b849c3a1ddd78f, especially
on archs where the PLT ABI is costly.
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this code in sigaction was the only place where sizeof was being
applied to the kernel sigaction's mask member to get the size argument
to pass to the kernel. everywhere else, _NSIG/8 is used for this
purpose.
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Linux makes this surprisingly difficult, but it can be done. the trick
here is using the fact that we control the implementation of sigaction
to prevent changing the disposition of SIGABRT to anything but SIG_DFL
after abort has tried and failed to terminate the process simply by
calling raise(SIGABRT).
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this error case was overlooked in the old range checking logic. new
check is moved out of __libc_sigaction to the public wrapper in order
to unify the error path and reduce code size.
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the restorer function pointer provided in the kernel sigaction
structure is interpreted by the kernel as a raw code address, not a
function descriptor.
this commit moves the declarations of the __restore and __restore_rt
symbols to ksigaction.h so that arch versions of the file can override
them, and introduces a version for sh which declares them as objects
rather than functions.
an alternate solution would have been defining SA_RESTORER to 0 so
that the functions are not used, but this both requires executable
stack (since the sh kernel does not have a vdso page with permanent
restorer functions) and crashes on qemu user-level emulation.
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this is the first step in an overhaul aimed at greatly simplifying and
optimizing everything dealing with thread-local state.
previously, the thread pointer was initialized lazily on first access,
or at program startup if stack protector was in use, or at certain
random places where inconsistent state could be reached if it were not
initialized early. while believed to be fully correct, the logic was
fragile and non-obvious.
in the first phase of the thread pointer overhaul, support is retained
(and in some cases improved) for systems/situation where loading the
thread pointer fails, e.g. old kernels.
some notes on specific changes:
- the confusing use of libc.main_thread as an indicator that the
thread pointer is initialized is eliminated in favor of an explicit
has_thread_pointer predicate.
- sigaction no longer needs to ensure that the thread pointer is
initialized before installing a signal handler (this was needed to
prevent a situation where the signal handler caused the thread
pointer to be initialized and the subsequent sigreturn cleared it
again) but it still needs to ensure that implementation-internal
thread-related signals are not blocked.
- pthread tsd initialization for the main thread is deferred in a new
manner to minimize bloat in the static-linked __init_tp code.
- pthread_setcancelstate no longer needs special handling for the
situation before the thread pointer is initialized. it simply fails
on systems that cannot support a thread pointer, which are
non-conforming anyway.
- pthread_cleanup_push/pop now check for missing thread pointer and
nop themselves out in this case, so stdio no longer needs to avoid
the cancellable path when the thread pointer is not available.
a number of cases remain where certain interfaces may crash if the
system does not support a thread pointer. at this point, these should
be limited to pthread interfaces, and the number of such cases should
be fewer than before.
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the trick here is that sigaction can track for us which signals have
ever had a signal handler set for them, and only those signals need to
be considered for reset. this tracking mask may have false positives,
since it is impossible to remove bits from it without race conditions.
false negatives are not possible since the mask is updated with atomic
operations prior to making the sigaction syscall.
implementation-internal signals are set to SIG_IGN rather than SIG_DFL
so that a signal raised in the parent (e.g. calling pthread_cancel on
the thread executing pthread_spawn) does not have any chance make it
to the child, where it would cause spurious termination by signal.
this change reduces the minimum/typical number of syscalls in the
child from around 70 to 4 (including execve). this should greatly
improve the performance of posix_spawn and other interfaces which use
it (popen and system).
to facilitate these changes, sigismember is also changed to return 0
rather than -1 for invalid signals, and to return the actual status of
implementation-internal signals. POSIX allows but does not require an
error on invalid signal numbers, and in fact returning an error tends
to confuse applications which wrongly assume the return value of
sigismember is boolean.
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in signal() it is needed since __sigaction uses restrict in parameters
and sharing the buffer is technically an aliasing error. do the same
for the syscall, as at least qemu-user does not handle it properly.
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this mirrors the stdio_impl.h cleanup. one header which is not
strictly needed, errno.h, is left in pthread_impl.h, because since
pthread functions return their error codes rather than using errno,
nearly every single pthread function needs the errno constants.
in a few places, rather than bringing in string.h to use memset, the
memset was replaced by direct assignment. this seems to generate much
better code anyway, and makes many functions which were previously
non-leaf functions into leaf functions (possibly eliminating a great
deal of bloat on some platforms where non-leaf functions require ugly
prologue and/or epilogue).
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this is necessary because posix_spawn calls sigaction after vfork, and
if the thread pointer is not already initialized, initializing it in
the child corrupts the parent process's state.
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to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
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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.
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the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.
note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.
the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.
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otherwise we cannot support an application's desire to use
asynchronous cancellation within the callback function. this change
also slightly debloats pthread_create.c.
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