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Re: [Qemu-devel] audit needed for signal handlers
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] audit needed for signal handlers |
Date: |
Tue, 12 Nov 2013 13:24:40 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20131028 Thunderbird/17.0.10 |
On 11/11/13 19:03, Max Filippov wrote:
> On Mon, Nov 11, 2013 at 8:50 PM, Eric Blake <address@hidden> 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)
>
> An unsafe function is called in a signal. See man 7 signal,
> section 'Async-signal-safe functions'. This should be avoided.
>
>> 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.
>
> I believe it cannot, at least in linux, as technically the signal
> handler is always called in a new thread, specifically created
> to only handle that signal, and errno should be thread-local.
That's incorrect (*). The handler runs on a new *stack frame*, but
inside the same thread. You can specify an alternate stack for signal
handlers to run on in advance (see SA_ONSTACK / sigaltstack()), in which
case the new stack frame will be "allocated" there. Otherwise the system
will just use the normal stack. The handler indeed runs like an
"unexpected", "out-of-the-blue" normal function call.
It is actually pretty vital that the handler is run by the specific
thread that the signal has been delivered to.
(*) Example code (not a correct/portable program due to the race on
"errno", but it does disprove the idea that errno is "protected" on Linux):
#define _XOPEN_SOURCE 600
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <stdio.h>
static void ringring(int signum)
{
errno = -12;
}
int main(void)
{
sigaction(SIGALRM,
&(struct sigaction){ .sa_handler = &ringring },
NULL);
alarm(1);
errno = 0;
/* pause() would clobber errno */
while (errno == 0)
;
printf("%d\n", errno);
return 0;
}
Thanks
Laszlo
- Re: [Qemu-devel] audit needed for signal handlers, (continued)
- Re: [Qemu-devel] audit needed for signal handlers, Paolo Bonzini, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers, Peter Maydell, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers, Eric Blake, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers, Paolo Bonzini, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers, Gerd Hoffmann, 2013/11/12
- Re: [Qemu-devel] audit needed for signal handlers, Laszlo Ersek, 2013/11/12
- Re: [Qemu-devel] audit needed for signal handlers, Eric Blake, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers, Paolo Bonzini, 2013/11/11
Re: [Qemu-devel] audit needed for signal handlers, Peter Maydell, 2013/11/11
Re: [Qemu-devel] audit needed for signal handlers, Max Filippov, 2013/11/11
- Re: [Qemu-devel] audit needed for signal handlers,
Laszlo Ersek <=