On Mon, May 22, 2023 at 01:41:57AM +0200, Stefano Brivio wrote:
> If we want /proc contents to be consistent after pasta spawns a child
> process in a new PID namespace (only for operation without a
> pre-existing namespace), we need to mount /proc after the clone(2)
> call with CLONE_NEWPID, and we enable the child to do that by
> passing, in the same call, the CLONE_NEWNS flag, as described by
> pid_namespaces(7).
> 
> This is not really a remount: in fact, passing MS_REMOUNT to mount(2)
> would make the call fail. We're in another mount namespace now, so
> it's a fresh mount that has the effect of hiding the existing one.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  pasta.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/pasta.c b/pasta.c
> index 3a4d704..b30ce70 100644
> --- a/pasta.c
> +++ b/pasta.c
> @@ -29,6 +29,7 @@
>  #include <syslog.h>
>  #include <sys/epoll.h>
>  #include <sys/inotify.h>
> +#include <sys/mount.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
>  #include <fcntl.h>
> @@ -172,6 +173,10 @@ static int pasta_spawn_cmd(void *arg)
>  	const struct pasta_spawn_cmd_arg *a;
>  	sigset_t set;
>  
> +	/* We run in a detached PID and mount namespace: mount /proc over */
> +	if (mount("", "/proc", "proc", 0, NULL))
> +		warn("Couldn't mount /proc: %s", strerror(errno));
> +
>  	if (write_file("/proc/sys/net/ipv4/ping_group_range", "0 0"))
>  		warn("Cannot set ping_group_range, ICMP requests might fail");
>  
> @@ -243,7 +248,7 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
>  	pasta_child_pid = do_clone(pasta_spawn_cmd, ns_fn_stack,
>  				   sizeof(ns_fn_stack),
>  				   CLONE_NEWIPC | CLONE_NEWPID | CLONE_NEWNET |
> -				   CLONE_NEWUTS | SIGCHLD,
> +				   CLONE_NEWUTS | CLONE_NEWNS  | SIGCHLD,
>  				   (void *)&arg);
>  
>  	if (pasta_child_pid == -1) {

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson