[Bug 1000801] Synthetic target crash with -fstack-protector

[bugzilla-daemon at ecoscentric dot com]

http://bugs.ecos.sourceware.org/show_bug.cgi?id=1000801





--- Comment #1 from John Dallaway <john@dallaway.org.uk>  2009-06-30 09:48:08 ---
-------- Original Message --------
Subject: Re: GCC stack protector with linux synthetic target
Date: Mon, 29 Jun 2009 10:49:47 +0100
From: Bart Veer <bartv@ecoscentric.com>

Basically -fstack-protector depends on some extra work done by the
glibc startup code. Of course the synthetic target does not use glibc
so that extra bit of initialization does not happen. The extra init is
not straightforward. It involves manipulating the x86 segment
registers via a Linux system call. I could find no documentation for
exactly what has to be done, and I could not immediately figure it out
from the glibc sources. Straightforward copying from glibc is not
acceptable either because of licensing issues.

Stack protection support was added to the compiler a few years back,
offhand I don't know exactly when. However the default setting varies
between distributions. Fedora and its ilk default to
-fno-stack-protector, so everything just works. Debian and its ilk
default to -fstack-protector, so things go wrong.

Possible solutions are:

1) add -fno-stack-protector to the default flags for synth. This
should work fine with all current distros. However it will break the
world for anybody using an older distro where the gcc being used did
not yet know about this compiler flag, or for anybody deliberately
using an older gcc e.g. to use the same compiler version for synth and
for real embedded targets. This was not really acceptable when we last
looked at this. It may be more acceptable now, but is still not ideal.

2) try to do some run-time detection to figure out whether or not
-fno-stack-protector should be added. There are various complications,
e.g. if people change the COMMAND_PREFIX setting.

3) fix the problem properly by doing the segment register
initialization. This should solve the problem irrespective of the
distro or the version of gcc being used. It would also mean that the
synthetic targets gains whatever benefits -fstack-protector offers.

Since I run Fedora on most of my systems I am not affected by any of
this, so sorting it out is a low priority for me. If you want to look
at option (3), that would be great. I would much prefer that to option
(1), since if you go for that then there will never be any incentive
to do the job properly.


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