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Date: Mon, 11 Nov 2013 18:05:51 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] audit needed for signal handlers
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Il 11/11/2013 17:56, Anthony Liguori ha scritto:
> On Mon, Nov 11, 2013 at 8:50 AM, Eric Blake <eblake@redhat.com> wrote:
>> Quick - identify the bug in this code (from ui/curses.c):
>>
>> static void curses_winch_handler(int signum)
>> {
>>     struct winsize {
>>         unsigned short ws_row;
>>         unsigned short ws_col;
>>         unsigned short ws_xpixel;   /* unused */
>>         unsigned short ws_ypixel;   /* unused */
>>     } ws;
>>
>>     /* terminal size changed */
>>     if (ioctl(1, TIOCGWINSZ, &ws) == -1)
>>         return;
>>
>>     resize_term(ws.ws_row, ws.ws_col);
>>     curses_calc_pad();
>>     invalidate = 1;
>>
>>     /* some systems require this */
>>     signal(SIGWINCH, curses_winch_handler);
>> }
>>
>> Here's a hint: ioctl() can clobber errno.  But if a signal handler is
>> called in the middle of other code that is using errno, then the handler
>> MUST restore the value of errno before returning, if it is to guarantee
>> that the interrupted context won't be corrupted.
> 
> Isn't this precisely why EINTR exists?

No.

    do {
        rc = read(...);
    } while (rc == -1 && errno == EINTR);
    /* signal handler runs here */
    if (errno == EAGAIN) {
        ...
    }

That said, aren't all signals in QEMU (except SIG_IPI) caught with
signalfd and the handlers run synchronously in the iothread?

Paolo