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    SubjectRe: [braindump][RFC] signals and syscall restarts (Re: [PATCH v2 19/44] metag: Signal handling)
    On 06/12/12 22:09, Al Viro wrote:
    > On Thu, Dec 06, 2012 at 11:17:34AM +0000, James Hogan wrote:
    >
    >> Agreed, it looks wrong. Looking at the sh version, is there a particular
    >> reason to only check for -EFAULT and not the other errors that
    >> do_sigaltstack can return (-EPERM, -EINVAL, and -ENOMEM)?
    >
    > See commit fae2ae2a900a5c7bb385fe4075f343e7e2d5daa2

    Thanks :)

    >
    >>> BTW, what's to stop the syscall restart triggering if you catch a signal
    >>> while in rt_sigreturn(2)?
    >>
    >> I'm not sure I understand how that could cause a problem. Could you
    >> elaborate the sequence of events?
    >>
    >> The signal restart is triggered by the return value register, so
    >> rt_sigreturn would have to return -ERESTART*. This could happen if the
    >> signal handler overwrote the return value in the sigcontext (which as
    >> far as I can tell could also happen on ARM), or if the syscall that was
    >> originally interrupted by the signal has -ERESTARTNOINTR ||
    >> (-ERESTARTSYS && SA_RESTART), but in that case rt_sigreturn has already
    >> switched back to the context of the original syscall so that's the right
    >> thing to do isn't it? I've probably missed something important :-)
    >
    > [we probably need something along the lines of braindump below in
    > somewhere in Documentation/*; comments and improvements are very
    > welcome - this is just a starting point. We *do* need some coherent
    > explanation of signal semantics, judging by how often people step on
    > the same landmines...]

    Thanks, this helps a lot.

    Cheers
    James



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