Re: [PATCH] net: tap: Drop the close of fds for child process

[Bin Meng]

On Thu, Apr 6, 2023 at 8:34 PM Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> On 6/4/23 13:20, Bin Meng wrote:
> > Current codes using a brute-force traversal of all file descriptors
> > do not scale on a system where the maximum number of file descriptors
> > are set to a very large value (e.g.: in a Docker container of Manjaro
> > distribution it is set to 1073741816). QEMU just looks freezed during
> > start-up.
> >
> > The close-on-exec flag was introduced since a faily old Linux kernel
> > (2.6.23). With recent newer kernels that QEMU supports, we don't need
> > to manually close the fds for child process as the proper O_CLOEXEC
> > flag should have been set properly on files that we don't want child
> > process to see.
>
> But this file is common to all POSIX implementations, not only Linux.

Yes, this file is used for Linux, BSD and Solaris.
I checked that O_CLOEXEC is available on Linux (2.6.23), FreeBSD
(8.3), OpenBSD 5.0, Solaris 11. This flag is part of POSIX.1-2008.

Question is do we still need to support OSes that are older and do not
have this support?

>
> > Reported-by: Zhangjin Wu <falcon@tinylab.org>
> > Signed-off-by: Bin Meng <bmeng@tinylab.org>
> > ---
> >
> >   net/tap.c | 14 --------------
> >   1 file changed, 14 deletions(-)
> >
> > diff --git a/net/tap.c b/net/tap.c
> > index 1bf085d422..49e1915484 100644
> > --- a/net/tap.c
> > +++ b/net/tap.c
> > @@ -446,13 +446,6 @@ static void launch_script(const char *setup_script, 
> > const char *ifname,
> >           return;
> >       }
> >       if (pid == 0) {
>
> Maybe guard with #ifndef O_CLOEXEC
>
> > -        int open_max = sysconf(_SC_OPEN_MAX), i;
> > -
> > -        for (i = 3; i < open_max; i++) {
> > -            if (i != fd) {
> > -                close(i);
> > -            }
> > -        }
>
> or add qemu_close_cloexec() in util/osdep.c similar to qemu_open_cloexec()?
>
> >           parg = args;
> >           *parg++ = (char *)setup_script;
> >           *parg++ = (char *)ifname;
> > @@ -536,17 +529,10 @@ static int net_bridge_run_helper(const char *helper, 
> > const char *bridge,
> >           return -1;
> >       }
> >       if (pid == 0) {
> > -        int open_max = sysconf(_SC_OPEN_MAX), i;
> >           char *fd_buf = NULL;
> >           char *br_buf = NULL;
> >           char *helper_cmd = NULL;
> >
> > -        for (i = 3; i < open_max; i++) {
> > -            if (i != sv[1]) {
> > -                close(i);
> > -            }
> > -        }
> > -
> >           fd_buf = g_strdup_printf("%s%d", "--fd=", sv[1]);
> >
> >           if (strrchr(helper, ' ') || strrchr(helper, '\t')) {

Regards,
Bin