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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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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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these changes still do not yield a fully-conforming abort, but they
fix two known issues:
- per POSIX, termination via SIGKILL is not "abnormal", but both ISO C
and POSIX require abort to yield abnormal termination.
- raising SIGKILL fails to do anything to pid 1 in some containers.
now, the trapping instruction produced by a_crash() is expected to
produce abnormal termination, without the risk of invoking a signal
handler since SIGILL and SIGSEGV are blocked, and _Exit, which
contains an infinite loop analogous to the one being removed from
abort itself, is used as a last resort.
this implementation still fails to produce an exit status as if the
process terminated via SIGABRT in cases where SIGABRT is blocked or
ignored, but fixing that is not easy; the obvious pseudo-solutions all
have subtle race conditions where a concurrent fork or exec can expose
incorrect signal state.
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Per POSIX, "The abort() function shall cause abnormal process
termination to occur, unless the signal SIGABRT is being caught and
the signal handler does not return."
If SIGABRT is blocked or if a signal handler is installed and does
return, abort is still required to cause abnormal program termination.
We cannot use a_crash() to do this, since a SIGILL handler could also
be installed (and might even longjmp out of the abort, not expecting
to be invoked from within abort), nor can we rely on resetting the
signal handler and re-raising the signal (this has race conditions in
multi-threaded programs). On the other hand, SIGKILL is a perfectly
safe, unblockable way to obtain abnormal program termination, and it
requires no ugly loop-and-retry logic.
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