Re: [PATCH] passt, log: Call __openlog() earlier, log to stderr until we detach

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 13 Mar 2024 11:23:56 +0100
Paul Holzinger <pholzing@redhat.com> wrote:

> Thanks for the quick patch.
> 
> On 12/03/2024 23:34, Stefano Brivio wrote:
> > Paul reports that, with commit 15001b39ef1d ("conf: set the log level
> > much earlier"), early messages aren't reported to standard error
> > anymore.
> >
> > The reason is that, once the log mask is changed from LOG_EARLY, we
> > don't force logging to stderr, and this mechanism was abused to have
> > early errors on stderr. Now that we drop LOG_EARLY earlier on, this
> > doesn't work anymore.
> >
> > Call __openlog() as soon as we know the mode we're running as, using
> > LOG_PERROR. Then, once we detach, if we're not running from an
> > interactive terminal and logging to standard error is not forced,
> > drop LOG_PERROR from the options.
> >
> > While at it, make sure we don't print messages to standard error
> > reporting that we couldn't log to the system logger, if we didn't
> > open a connection yet. That's expected.
> >
> > Reported-by: Paul Holzinger <pholzing@redhat.com>
> > Fixes: 15001b39ef1d ("conf: set the log level much earlier")
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
>
> Tested-by: Paul Holzinger <pholzing@redhat.com>

And thanks for the quick test.

> > ---
> >   log.c   |  2 +-
> >   passt.c | 14 +++++++-------
> >   2 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/log.c b/log.c
> > index eafaca2..bdd31b4 100644
> > --- a/log.c
> > +++ b/log.c
> > @@ -174,7 +174,7 @@ void passt_vsyslog(int pri, const char *format, va_list ap)
> >   	if (log_opt & LOG_PERROR)
> >   		fprintf(stderr, "%s", buf + prefix_len);
> >   
> > -	if (send(log_sock, buf, n, 0) != n)
> > +	if (log_sock >= 0 && send(log_sock, buf, n, 0) != n)
> >   		fprintf(stderr, "Failed to send %i bytes to syslog\n", n);
> >   }
> >   
> > diff --git a/passt.c b/passt.c
> > index f430648..e4cb11e 100644
> > --- a/passt.c
> > +++ b/passt.c
> > @@ -225,6 +225,8 @@ int main(int argc, char **argv)
> >   	strncpy(argv0, argv[0], PATH_MAX - 1);
> >   	name = basename(argv0);
> >   	if (strstr(name, "pasta")) {
> > +		__openlog(log_name = "pasta", LOG_PERROR, LOG_DAEMON);
> > +
> >   		sa.sa_handler = pasta_child_handler;
> >   		if (sigaction(SIGCHLD, &sa, NULL)) {
> >   			die("Couldn't install signal handlers: %s",
> > @@ -237,18 +239,16 @@ int main(int argc, char **argv)
> >   		}
> >   
> >   		c.mode = MODE_PASTA;
> > -		log_name = "pasta";
> >   	} else if (strstr(name, "passt")) {
> > +		__openlog(log_name = "passt", LOG_PERROR, LOG_DAEMON);
> > +
> >   		c.mode = MODE_PASST;
> > -		log_name = "passt";
> >   	} else {
> >   		exit(EXIT_FAILURE);
> >   	}
> >   
> >   	madvise(pkt_buf, TAP_BUF_BYTES, MADV_HUGEPAGE);
> >   
> > -	__openlog(log_name, 0, LOG_DAEMON);
> > -
> >   	c.epollfd = epoll_create1(EPOLL_CLOEXEC);
> >   	if (c.epollfd == -1) {
> >   		perror("epoll_create1");
> > @@ -269,9 +269,6 @@ int main(int argc, char **argv)
> >   	conf(&c, argc, argv);
> >   	trace_init(c.trace);
> >   
> > -	if (c.force_stderr || isatty(fileno(stdout)))
> > -		__openlog(log_name, LOG_PERROR, LOG_DAEMON);
> > -
> >   	pasta_netns_quit_init(&c);
> >   
> >   	tap_sock_init(&c);
> > @@ -314,6 +311,9 @@ int main(int argc, char **argv)
> >   	if (isolate_prefork(&c))
> >   		die("Failed to sandbox process, exiting");
> >   
> > +	if (!c.force_stderr && !isatty(fileno(stdout)))
>
> I still have the feeling this should check the stderr fd instead of 
> stdout as it is logging to stderr. But I guess it shouldn't make a big 
> difference in real situations so not a big deal.

Now that you mention this, I'm quite undecided: true, we're checking if
we want to print to standard error, but what we want to check here is
if we're running from an interactive terminal, for which testing if
stdout is a terminal is quite idiomatic.

On the other hand:

$ cat isatty.c
#include <stdio.h>
#include <unistd.h>

int main()
{
    return isatty(fileno(stdout));
}
$ gcc -o isatty isatty.c 
$ ./isatty >/dev/null; echo $?
0
$ ./isatty 2>/dev/null; echo $?
1

so we would (fail to) print to stderr if it's redirected to /dev/null,
but not print to stderr if stdout is redirected.

At this point I would have a slight preference toward changing it like
you suggested.

-- 
Stefano