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From: Alex Barcelo <abarcelo@ac.upc.edu>
Date: Sat, 4 Feb 2012 15:26:24 +0100
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Subject: [Qemu-devel] sigaltstack bug in qemu (testing on qemu-ppc)
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To: qemu-devel <qemu-devel@nongnu.org>

I am barely able to understand this inline function:

static inline int sas_ss_flags(unsigned long sp)
{
    return (target_sigaltstack_used.ss_size == 0 ? SS_DISABLE
            : on_sig_stack(sp) ? SS_ONSTACK : 0);
}
(signal.c @97)

... and it seems wrong to me when used in the following function. I
have a test program that uses sigaltstack to do some stack
manipulation. It doesn't work.

The function:
    ...
    if ((ka->sa_flags & TARGET_SA_ONSTACK) &&
        (/* here maybe a "!" */ sas_ss_flags(oldsp))) {
    ....
(signal.c, get_sigframe @4121)

Forcing a true value makes everything work (not that I'm claiming it
as the solution, obviously).

I think that it lacks an "!". Either flag SS_DISABLE or SS_ONSTACK are
flags that should *disable* the stack change, so 1 (SS_ONSTACK) and 2
(SS_DISABLE) should not enter the if. And a 0 value means that it
should be ok to do a stack change. It makes sense, but I'm not sure if
I'm oversimplifying things too much. And I wasn't sure if just sending
a patch was ok, given that I don't fully understand the code.