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Re: [RFC] sigaltstack + longjmp to protect from C stack overflow

From: Paul Eggert
Subject: Re: [RFC] sigaltstack + longjmp to protect from C stack overflow
Date: Mon, 25 Aug 2014 08:06:09 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0

Thanks for looking into this.  A few comments:

'handle_sigsegv' tries to guess direction of stack growth, but shouldn't stack growth direction be determined at startup? That should be more reliable than guessing from what may be a stray pointer reference.

init_signals need not use xmalloc to allocate the alternate stack, since it's of fixed size and Emacs always needs it. We should be able to allocate the stack statically.

sys/resource.h is not available everywhere; include it only if HAVE_SYS_RESOURCE_H is defined. Similarly, use struct rlimit and getrlimit only if HAVE_GETRLIMIT is defined, and use sigaltstack only if HAVE_SIGALTSTACK is defined (you'll need to modify configure.ac for that). And the code should do the right thing (i.e., stick with thread_fatal_action instead of sigsegv_action) if SA_SIGINFO and/or SA_ONSTACK are not defined. Come to think of it, check for all the above issues and do the sigsegv_action stuff only if all the macros check out.

There's no need to abort if sigaltstack fails. Just let Emacs go on as it does now, and use thread_fatal_action.

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