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Date: Tue, 25 Mar 2014 06:35:02 +0000
From: Laurent Bercot <ska-dietlibc@...rnet.org>
To: musl@...ts.openwall.com
Subject: Re: Transition path for removing lazy init of thread pointer
On 25/03/2014 01:55, Rich Felker wrote:
> The mandatory syscall is set_thread_area or equivalent, e.g.
> arch_prctl on x86_64. It's there because most archs need a syscall to
> set the thread pointer used for accessing TLS. Even in single-threaded
> programs, there are reasons one may want to have it.
>
> The big reason is that, on most archs, stack protector's canary value
> is stored at a fixed offset from the thread pointer rather than in a
> global, so stack protector can't work without the thread pointer being
> initialized. Up to now we've tried to detect whether stack protector
> is used based on symbol references to __stack_chk_fail, but this check
> gives a false negative (and thus crashing programs) if gcc optimizes
> out the check to __stack_chk_fail but not the load of the canary, e.g.
> in the program: int main() { exit(0); }
That's a good reason indeed.
I take it you're still hell-bent against compile-time options ? Because
a musl compile-time option "I don't want this musl to support stack
protector, yes I know it will crash programs compiled with it, but I'm
a big boy and know what I'm doing" would be great for OCD people like
me who like their strace clean. :)
> The other main reason is that lazy initialization is a lot more
> expensive at runtime.
That's not a good reason for single-threaded programs.
> So despite always initializing the thread pointer kinda looking like
> "bloat" from a minimal-program standpoint, it's really a major step
> forward in debloating and simplifying lots of code.
I totally understand and approve for multi-threaded programs and
programs using stack protection. I just wish there were a special
optimization for "int main() { return 0; }".
--
Laurent
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