Replacing a_crash() ?

[Rich Felker <dalias@libc.org>]

Now that we have an abort() that reliably terminates with uncatchable
SIGABRT, I've been thinking about replacing the a_crash() calls in
musl (which are usually an instruction generating SIGILL or SIGSEGV)
with calls to the uncatchable tail of abort(), which I would factor
off as a __forced_abort() function.

In case it's not clear, the reason for not just calling abort() is
that too many programs catch it, and catching it is even encouraged.
Catchability is a problem with the current approach too, since
a_crash() is used in places where process state is known to be
dangerously corrupt and likely under attacker control; eliminating it
is one of the potential goals of switching to __forced_abort().

Are there any objections to making such a change? So far I've gotten
mostly positive feedback -- SIGABRT is more telling of what's happened
than SIGSEGV/SIGILL. It would also get rid of the ugly misplacement of
a_crash() (no longer needed) in "atomic.h" and the inclusion of
"atomic.h" in some files where it makes no sense without knowing it's
where a_crash() is defined.

For i386, some nontrivial work would be needed to make abort's tail
perform syscalls with int $128 rather than the vdso, which is unsafe
since the pointer to it may have been subverted. On other archs,
inline syscalls are fully inline. I'd probably add a
NEED_FAILSAFE_SYSCALL macro to define before including "syscall.h" and
have arch/i386/syscall_arch.h adjust the asm string based on it; this
is more maintainable than writing an asm version of the function.

Rich