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    SubjectRe: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler
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    01.02.2016 21:52, Oleg Nesterov пишет:
    > Stas, I probably missed something, but I don't understand your concerns,
    >
    > On 02/01, Stas Sergeev wrote:
    >> 01.02.2016 21:04, Oleg Nesterov пишет:
    >>> Yes, and SS_FORCE means "I know what I do", looks very simple.
    >> But to me its not because I don't know what to do with
    >> uc_stack after SS_FORCE is applied.
    > Nothing? restore_sigaltstack() should work as expected?
    That's likely the reason for EPERM: restore_sigaltstack()
    does the job, so manual modifications are disallowed.
    Allowing them will bring in the surprises where the changes
    done by the user are ignored.

    >>> I won't argue, but to me it would be better to keep this EPERM if !force.
    >>> Just because we should avoid the incompatible changes if possible.
    >> Ok then. Lets implement SS_FORCE.
    >> What semantic should it have wrt uc_stack?
    >>
    >> sigaltstack(SS_DISABLE | SS_FORCE);
    >> swapcontext();
    >> sigaltstack(set up new_sas);
    >> rt_sigreturn();
    > Yes, or
    >
    > sigaltstack({ DISABLE | FORCE}, &old_ss);
    > swapcontext();
    > sigaltstack(&old_ss, NULL);
    > rt_sigreturn();
    >
    > and if you are going to return from sighandler you do not even need the 2nd
    > sigaltstack(), you can rely on sigreturn.
    Yes, that's what I do in my app already.
    But its only there when SA_SIGINFO is used.

    >> What's at the end? Do we want a surprise for the user
    >> that he's new_sas got ignored?
    > Can't understand.... do you mean "set up new_sas" will be ignored because
    > rt_sigreturn() does restore_sigaltstack() ? I see no problem here...
    Allowing the modifications that were previously EPERMed
    but will now be silently ignored, may be seen as a problem.
    But if it isn't - fine, lets code that.

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