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

[Daniel P . Berrangé]

On Thu, Apr 06, 2023 at 07:20:41PM +0800, 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.

Even though O_CLOEXEC has existed for a long time, there is plenty
of code that doesn't use it reliably. While QEMU can control its
own code, we use a huge number of 3rd party libraries and we don't
trust them to reliably be using O_CLOEXEC on everything they open.

> 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) {
> -        int open_max = sysconf(_SC_OPEN_MAX), i;
> -
> -        for (i = 3; i < open_max; i++) {
> -            if (i != fd) {
> -                close(i);
> -            }
> -        }
>          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);
> -            }
> -        }

BSD has closefrom(3) we could use here, while modern Linux has
close_range(3, open_max)

We should probe for those two funtions and use them preferentially,
only falling back to the current manual loop where they don't exist.


> -
>          fd_buf = g_strdup_printf("%s%d", "--fd=", sv[1]);
>  
>          if (strrchr(helper, ' ') || strrchr(helper, '\t')) {
> -- 
> 2.34.1
> 
> 

With regards,
Daniel
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