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Message-ID: <53312396.5060307@skarnet.org> 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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