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    SubjectRe: [PATCH] x86_64: fix delayed signals


    On Thu, 10 Jul 2008, Roland McGrath wrote:
    >
    > There are many different scenarios that could hit this bug, most of
    > them races. The simplest one to demonstrate does not require any
    > race: when one signal has done handler setup at the check before
    > returning from a syscall, and there is another signal pending that
    > should be handled. The second signal's handler should interrupt the
    > first signal handler before it actually starts (so the interrupted PC
    > is still at the handler's entry point). Instead, it runs away until
    > the next kernel entry (next syscall, tick, etc).

    I have this dim memory of at least _some_ of this being on purpose.

    If you look at old kernels (_really_ old ones - I think it's way before
    even the historical git archive, but I didn't take a look), we used to set
    up several stack frames at once, so that we'd nest the stack frames
    completely.

    In other words, the code in do_signal() used to literally be a loop,
    something like

    while ((signr = get_signal_to_deliver(&info, &ka, regs, NULL)) > 0) {
    .. setup signal frame ..

    (No, I don't think that's at all accurate of the actual code we used to
    have - I just took the current do_signal() code as an example)

    And that explicit loop was removed in order for us to have just a single
    outstanding signal at a time. I forget the exact details why.

    But if you really want that behaviour, then re-introducing the loop would
    likely be the better approach (or should be combined), since I think you
    effectively just re-introduced it (at a much bigger granularity).

    Hmm.

    Linus


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