Hi

I've been playing around with signalfd() on linux and it seems to work but there also seems to be zero information other than the signal number in the signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler
set using sigaction with the siginfo_t passed to the handler, the below code returns 0 for everything in the signalfd_siginfo structure.

The relevant parts of the code are below. Have I done something wrong?

struct signalfd_siginfo siginfo;
:
:
sigemptyset(&sigset);
sigaddset(&sigset,SIGINT); /* Control-C */
sigaddset(&sigset,SIGTSTP); /* Control-Z */

if (sigprocmask(SIG_BLOCK,&sigset,NULL) == -1)
{
perror("ERROR: sigprocmask()");
return 1;
}

if ((fd = signalfd(-1,&sigset,0)) == -1)
{
perror("ERROR: signalfd()");
return 1;
}

:
:

while(1)
{
FD_ZERO(&mask);
FD_SET(fd,&mask);

switch(select(FD_SETSIZE,&mask,0,0,NULL))
:
:


if (FD_ISSET(fd,&mask))
{
len = read(fd,&siginfo,sizeof(siginfo));
if (len == -1)
{
perror("ERROR: read(fd)");
return 1;
}
if (len != sizeof(siginfo))
{
printf("WARNING: Expected %lu bytes, got %d\n",
sizeof(siginfo),len);
}
printf("RX signal %d:\n",siginfo.ssi_signo);
printf(" Error: %d\n",siginfo.ssi_errno);
printf(" PID : %d\n",siginfo.ssi_pid);
printf(" UID : %d\n",siginfo.ssi_uid);
printf(" UTime: %lu\n",siginfo.ssi_utime);
printf(" STime: %lu\n",siginfo.ssi_stime);
}
}

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Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but >there also seems to be zero information other than the signal number in the >signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler >set using sigaction with the siginfo_t passed to the handler, the below code >returns 0 for everything in the signalfd_siginfo structure.

I wonder if you still need to call sigaction with SA_SIGINFO before
using signalfd?

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Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but there also seems to be zero information other than the signal number in the signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler set using sigaction with the siginfo_t passed to the handler, the below code returns 0 for everything in the signalfd_siginfo structure.

The relevant parts of the code are below. Have I done something wrong?

struct signalfd_siginfo siginfo;
:
:
sigemptyset(&sigset);
sigaddset(&sigset,SIGINT); /* Control-C */
sigaddset(&sigset,SIGTSTP); /* Control-Z */

[...]

sigaction(2) documents the details regarding which fields in a siginfo structure will be set under which conditions and if were using C-c and
C-z to generate signals, the pid and uid values shouldn't be set. They
should be set when using kill(2) or sigqueue(2) to send signals.

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On Mon, 24 Feb 2025 14:18:44 GMT
scott@slp53.sl.home (Scott Lurndal) wibbled:

Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but >>there also seems to be zero information other than the signal number in the >>signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler >>set using sigaction with the siginfo_t passed to the handler, the below code >>returns 0 for everything in the signalfd_siginfo structure.

I wonder if you still need to call sigaction with SA_SIGINFO before
using signalfd?

Actually my mistake - the pid and uid are only zerop when it receives a signal from the terminal but when another process sends it it works ok. Thanks for
the idea though.

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On Mon, 24 Feb 2025 15:51:29 -0000 (UTC), Muttley wrote:

... the pid and uid are only zerop when it receives a
signal from the terminal but when another process sends it it works ok.

Just confirmed that with the following test program:

import asyncio
import linuxproc as lxp
from linuxproc import \
SIG, \
SigProcMask, \
SignalFile

async def main() :
to_block = SigProcMask({SIG.TERM, SIG.INT})
notif = SignalFile.create(to_block)
to_block.apply_replace()
while True :
evt = await notif.get_next_async(timeout = 1)
print(evt)
#end while
#end main

asyncio.run(main())

using my Python wrappers at <https://gitlab.com/ldo/python_linuxfs>.

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