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Date: Wed, 5 Nov 2014 10:43:03 -0500
From: Rich Felker <>
Subject: Re: fixing -fPIE + -fstack-protector-all

On Wed, Nov 05, 2014 at 04:25:03PM +0100, John Spencer wrote:
> using -fPIE + -fstack-protector-all is currently broken for a number
> of architectures (most notably i386) in the default gcc setup
> (including the musl-cross patches), as it depends on a
> libssp_nonshared.a which provides __stack_chk_fail_local().

As discussed on IRC, I would _like_ to be able to simply add the
following to crt/i386/crti.s:

__stack_chk_fail_local: hlt

and equivalent for other archs. This has the added benefit of
effecting a crash without going through the PLT (whereas
libssp_nonshared.a's __stack_chk_fail_local calls __stack_chk_fail via
the PLT) so it's not vulnerable to attacks that have overwritten the
GOT with malicious pointers.

However, this proposed solution breaks one odd corner case: static
linking when all the source files were compiled with -fPIC or -fPIE.
In that case, there would be no references to __stack_chk_fail, only
to __stack_chk_fail_local, and thereby __init_ssp would not get
linked, and a zero canary would be used.

One possible way to handle this would be giving up the conditional
linking of ssp init and just always initializing it. The .o file is 78
bytes on i386 and 70 bytes on x86_64, but there would also be some
savings to offset the cost simply from having the code inline in
__init_libc rather than as an external function.

I'm open to other ideas too.


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