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Re: GCC stack protector with linux synthetic target

>>>>> "John" == John Dallaway <> writes:

    John> Hi Rutger
    John> Rutger Hofman wrote:

    >> An aside: I run Ubuntu. At first, I couldn't run synth at all.
    >> Applications would crash, and gdb would crash on the
    >> application too! After some list searching, I found out that
    >> this probably is Ubuntu-specific. We need to include
    >> -fno-stack-protector in the GLOBAL_CFLAGS configure flag.
    >> Request: cannot this be automated for synth building? My guess
    >> is that it will not harm on systems other than Ubuntu, and it
    >> will save Ubuntu users effort.

    John> Do you understand _why_ eCos is crashing with the GCC stack
    John> protector enabled? Is it due to an assumption made in eCos
    John> regarding the stack frame which could be corrected? We
    John> should ensure we understand the nature of the failure before
    John> disabling this feature.

This was investigated some time ago - look at the archives.

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.


Bart Veer                                   eCos Configuration Architect
eCosCentric Limited    The eCos experts
Barnwell House, Barnwell Drive, Cambridge, UK.      Tel: +44 1223 245571
Registered in England and Wales: Reg No 4422071.

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