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Re: Saving errno around signal handlers


On 03/01/2014 01:31 AM, Rich Felker wrote:
On Fri, Feb 28, 2014 at 05:58:54PM +0100, Florian Weimer wrote:
On 02/28/2014 05:49 PM, Richard Henderson wrote:
On 02/28/2014 03:49 AM, Florian Weimer wrote:

In all cases (kernel and glibc implementation), we can use a tail call to make
sure that the stack layout looks like it is at present (at least on
architectures with an available callee-saved register), and we would not have
to adjust GDB and other tools.

How's that?  A tail call would not be able to "restore" anything afterward.

We already have the restorer that calls sigreturn, and could add

         movq    errno@gottpoff(%rip), %rdi
         movl    %r12d, %fs:(%rdi)

right before that.

This does not support nested signal handlers. The saved errno must be
on the stack frame of the trampoline calling the real signal handler,
not a fixed TLS location.

In this example, the saved errno value is in the %r12d register, which is callee-saved, and I suppose that should address your concern.

--
Florian Weimer / Red Hat Product Security Team


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