Robert O'Callahan robert at ocallahan.org
Mon Nov 10 18:24:38 PST 2014

On Tue, Nov 11, 2014 at 1:03 PM, Kyle Huey <me at kylehuey.com> wrote:

> The comment in Registers.h says
> > /**
> >  * This pseudo-register holds the system-call number when we get ptrace
> >  * enter-system-call and exit-system-call events. Setting it changes
> >  * the system-call executed when resuming after an enter-system-call
> >  * event.
> >  */
> Does that mean that we sometimes change which syscall we're executing
> when we get an enter-syscall-event?  Why do we do this?

I see three callers to set_original_syscallno:
* destroy_buffers does it for real. The current syscall is SYS_exit but we
need to do some work in the context of the exiting task before it exits, so
we smash the syscall number to something safe (SYS_gettid), complete the
syscall, do the work we need to do, and then do another SYS_exit for real.
* record_signal does it, but see the comment there; we're only restoring an
original syscallno that is sometimes lost for unclear reasons.
* maybe_restart_syscall does it when we're resuming an interrupted syscall.
I don't understand that case all that well.

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