signalfd() not handling sigqueue() sigval data correctly

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Michael Kerrisk

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Apr 8, 2008, 5:20:11 PM4/8/08
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Hi Davide,

I was doing some playing about with signalfd(), and seem to have encountered a
bug: when a signalfd read() fetches data for a signal that was sent by
sigqueue(), the data accompanying the signal is not returned. Instead
ssi_int/ssi_ptr is zero.

I've not looked into the cause of the problem, but the programs below can be
used to demonstrate it.

Here's an example run:

# Run in the background, waiting for signal 44.
$ ./signalfd_sigval 44 &
./signalfd_sigval: PID = 14783
[1] 14783
# Send signal 44 to PID 14783 with accompanying data 123
$ ./sigqueue 14783 44 123
/home/mtk/alp/signals/sigqueue: PID = 14784
Got signal 44
ssi_code= -1
ssi_pid = 14784
ssi_uid = 1000
ssi_int = 0 <= should be 123 here
ssi_ptr = 0 <= and here the hex version of 123

Could you take a look at this?

Cheers,

Michael


/* signalfd_sigval.c */

#define _GNU_SOURCE
#include <sys/syscall.h>
#include <unistd.h>
#include <stdint.h>
#include <signal.h>
#if defined(__i386__)
#define __NR_signalfd 321
#endif

struct signalfd_siginfo {
uint32_t ssi_signo;
int32_t ssi_errno;
int32_t ssi_code;
uint32_t ssi_pid;
uint32_t ssi_uid;
int32_t ssi_fd;
uint32_t ssi_tid;
uint32_t ssi_band;
uint32_t ssi_overrun;
uint32_t ssi_trapno;
int32_t ssi_status;
int32_t ssi_int;
uint64_t ssi_ptr;
uint64_t ssi_utime;
uint64_t ssi_stime;
uint64_t ssi_addr;
uint8_t __pad[48];
};

static int
signalfd(int ufd, sigset_t const *mask)
{
#define SIZEOF_SIG (_NSIG / 8)
#define SIZEOF_SIGSET (SIZEOF_SIG > sizeof(sigset_t) ? \
sizeof(sigset_t): SIZEOF_SIG)
return syscall(__NR_signalfd, ufd, mask, SIZEOF_SIGSET);
}

// #include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
sigset_t mask;
int sfd;
struct signalfd_siginfo fdsi;
ssize_t s;

if (argc != 2) {
fprintf(stderr, "Usage: %s sig-num\n", argv[0]);
exit(EXIT_FAILURE);
}

printf("%s: PID = %ld\n", argv[0], (long) getpid());

sigemptyset(&mask);
sigaddset(&mask, atoi(argv[1]));

if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
errExit("sigprocmask");

sfd = signalfd(-1, &mask);
if (sfd == -1)
errExit("signalfd");

for (;;) {
s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
if (s != sizeof(struct signalfd_siginfo))
errExit("read");

printf("Got signal %d\n", fdsi.ssi_signo);
printf(" ssi_code= %d\n", fdsi.ssi_code);
printf(" ssi_pid = %d\n", fdsi.ssi_pid);
printf(" ssi_uid = %d\n", fdsi.ssi_uid);
printf(" ssi_int = %d\n", fdsi.ssi_int);
printf(" ssi_ptr = %llx\n", fdsi.ssi_ptr);
}
}

/* sigqueue.c */

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)

int
main(int argc, char *argv[])
{
union sigval sv;

if (argc != 4) {
fprintf(stderr, "Usage: %s pid sig-num data\n", argv[0]);
exit(EXIT_FAILURE);
}

printf("%s: PID = %ld\n", argv[0], (long) getpid());

sv.sival_int = atoi(argv[3]);
if (sigqueue(atoi(argv[1]), atoi(argv[2]), sv) == -1)
errExit("sigqueue");

exit(EXIT_SUCCESS);
}

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Davide Libenzi

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Apr 8, 2008, 7:40:06 PM4/8/08
to
On Tue, 8 Apr 2008, Michael Kerrisk wrote:

> Hi Davide,
>
> I was doing some playing about with signalfd(), and seem to have encountered a
> bug: when a signalfd read() fetches data for a signal that was sent by
> sigqueue(), the data accompanying the signal is not returned. Instead
> ssi_int/ssi_ptr is zero.

Are you able to fetch those info with, say, sigtimedwait(2)?

- Davide

Michael Kerrisk

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Apr 9, 2008, 2:40:13 AM4/9/08
to
On Wed, Apr 9, 2008 at 1:37 AM, Davide Libenzi <dav...@xmailserver.org> wrote:
> On Tue, 8 Apr 2008, Michael Kerrisk wrote:
>
> > Hi Davide,
> >
> > I was doing some playing about with signalfd(), and seem to have encountered a
> > bug: when a signalfd read() fetches data for a signal that was sent by
> > sigqueue(), the data accompanying the signal is not returned. Instead
> > ssi_int/ssi_ptr is zero.
>
> Are you able to fetch those info with, say, sigtimedwait(2)?

Yes.


--
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug? Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html

Davide Libenzi

unread,
Apr 9, 2008, 3:10:09 AM4/9/08
to
On Wed, 9 Apr 2008, Michael Kerrisk wrote:

> On Wed, Apr 9, 2008 at 1:37 AM, Davide Libenzi <dav...@xmailserver.org> wrote:
> > On Tue, 8 Apr 2008, Michael Kerrisk wrote:
> >
> > > Hi Davide,
> > >
> > > I was doing some playing about with signalfd(), and seem to have encountered a
> > > bug: when a signalfd read() fetches data for a signal that was sent by
> > > sigqueue(), the data accompanying the signal is not returned. Instead
> > > ssi_int/ssi_ptr is zero.
> >
> > Are you able to fetch those info with, say, sigtimedwait(2)?
>
> Yes.

Okie, found it. Tomorrow I'll post a patch. Thank you for reporting.

- Davide

Davide Libenzi

unread,
Apr 9, 2008, 4:00:14 PM4/9/08
to
On Tue, 8 Apr 2008, Michael Kerrisk wrote:

> Hi Davide,
>
> I was doing some playing about with signalfd(), and seem to have encountered a
> bug: when a signalfd read() fetches data for a signal that was sent by
> sigqueue(), the data accompanying the signal is not returned. Instead
> ssi_int/ssi_ptr is zero.

Michael, can you give the patch below a spin? I tested it on my x86_64 box
and seems to be working fine:

Got signal 44
ssi_code= -1

ssi_pid = 6314
ssi_uid = 1000
ssi_int = 123
ssi_ptr = 7b


- Davide


---
fs/signalfd.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6.mod/fs/signalfd.c
===================================================================
--- linux-2.6.mod.orig/fs/signalfd.c 2008-04-09 11:53:37.000000000 -0700
+++ linux-2.6.mod/fs/signalfd.c 2008-04-09 12:05:46.000000000 -0700
@@ -111,9 +111,14 @@
err |= __put_user(kinfo->si_uid, &uinfo->ssi_uid);
err |= __put_user((long) kinfo->si_ptr, &uinfo->ssi_ptr);
break;
- default: /* this is just in case for now ... */
+ default:
+ /*
+ * This case catches also the signals queued by sigqueue().
+ */
err |= __put_user(kinfo->si_pid, &uinfo->ssi_pid);
err |= __put_user(kinfo->si_uid, &uinfo->ssi_uid);
+ err |= __put_user((long) kinfo->si_ptr, &uinfo->ssi_ptr);
+ err |= __put_user(kinfo->si_int, &uinfo->ssi_int);
break;

Michael Kerrisk

unread,
Apr 10, 2008, 1:10:06 AM4/10/08
to
Hi Davide,

> > I was doing some playing about with signalfd(), and seem to have encountered a
> > bug: when a signalfd read() fetches data for a signal that was sent by
> > sigqueue(), the data accompanying the signal is not returned. Instead
> > ssi_int/ssi_ptr is zero.
>
> Michael, can you give the patch below a spin? I tested it on my x86_64 box
> and seems to be working fine:
>
> Got signal 44
> ssi_code= -1
> ssi_pid = 6314
> ssi_uid = 1000
> ssi_int = 123
> ssi_ptr = 7b

This works for me on x86-32. Thanks Davide!

Cheers,

Michael

> ---
> fs/signalfd.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.mod/fs/signalfd.c
> ===================================================================
> --- linux-2.6.mod.orig/fs/signalfd.c 2008-04-09 11:53:37.000000000 -0700
> +++ linux-2.6.mod/fs/signalfd.c 2008-04-09 12:05:46.000000000 -0700
> @@ -111,9 +111,14 @@
> err |= __put_user(kinfo->si_uid, &uinfo->ssi_uid);
> err |= __put_user((long) kinfo->si_ptr, &uinfo->ssi_ptr);
> break;
> - default: /* this is just in case for now ... */
> + default:
> + /*
> + * This case catches also the signals queued by sigqueue().
> + */
> err |= __put_user(kinfo->si_pid, &uinfo->ssi_pid);
> err |= __put_user(kinfo->si_uid, &uinfo->ssi_uid);
> + err |= __put_user((long) kinfo->si_ptr, &uinfo->ssi_ptr);
> + err |= __put_user(kinfo->si_int, &uinfo->ssi_int);
> break;
> }
>
>

--

Michael Kerrisk
Maintainer of the Linux man-pages project: http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug? Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html

Michael Kerrisk

unread,
Apr 10, 2008, 6:40:23 AM4/10/08
to
On Thu, Apr 10, 2008 at 7:03 AM, Michael Kerrisk
<mtk.ma...@googlemail.com> wrote:
> Hi Davide,
>
> > > I was doing some playing about with signalfd(), and seem to have encountered a
> > > bug: when a signalfd read() fetches data for a signal that was sent by
> > > sigqueue(), the data accompanying the signal is not returned. Instead
> > > ssi_int/ssi_ptr is zero.
> >
> > Michael, can you give the patch below a spin? I tested it on my x86_64 box
> > and seems to be working fine:
> >
> > Got signal 44
> > ssi_code= -1
> > ssi_pid = 6314
> > ssi_uid = 1000
> > ssi_int = 123
> > ssi_ptr = 7b
>
> This works for me on x86-32. Thanks Davide!

Davide,

Are you going to get this pushed into 2.6.26? I assume we can't get
it into 2.6.25...

Davide Libenzi

unread,
Apr 10, 2008, 1:30:13 PM4/10/08
to
On Thu, 10 Apr 2008, Michael Kerrisk wrote:

> On Thu, Apr 10, 2008 at 7:03 AM, Michael Kerrisk
> <mtk.ma...@googlemail.com> wrote:
> > Hi Davide,
> >
> > > > I was doing some playing about with signalfd(), and seem to have encountered a
> > > > bug: when a signalfd read() fetches data for a signal that was sent by
> > > > sigqueue(), the data accompanying the signal is not returned. Instead
> > > > ssi_int/ssi_ptr is zero.
> > >
> > > Michael, can you give the patch below a spin? I tested it on my x86_64 box
> > > and seems to be working fine:
> > >
> > > Got signal 44
> > > ssi_code= -1
> > > ssi_pid = 6314
> > > ssi_uid = 1000
> > > ssi_int = 123
> > > ssi_ptr = 7b
> >
> > This works for me on x86-32. Thanks Davide!
>
> Davide,
>
> Are you going to get this pushed into 2.6.26? I assume we can't get
> it into 2.6.25...

I'll be sending it Andrew-bound today. I dunno where it'll land mainline.


- Davide

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