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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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.

> 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')) {