Re: [PATCH v3] pasta: Don't try to watch namespaces in procfs with inotify, use timer instead

[Paul Holzinger <pholzing@redhat.com>]

Hi Stefano, thanks for the patch.

On 19/02/2024 09:05, Stefano Brivio wrote:
> We watch network namespace entries to detect when we should quit
> (unless --no-netns-quit is passed), and these might stored in a tmpfs
> typically mounted at /run/user/UID or /var/run/user/UID, or found in
> procfs at /proc/PID/ns/.
>
> Currently, we try to use inotify for any possible location of those
> entries, but inotify, of course, doesn't work on pseudo-filesystems
> (see inotify(7)).
>
> The man page reflects this: the description of --no-netns-quit
> implies that we won't quit anyway if the namespace is not "bound to
> the filesystem".
>
> Well, we won't quit, but, since commit 9e0dbc894813 ("More
> deterministic detection of whether argument is a PID, PATH or NAME"),
> we try. And, indeed, this is harmless, as the caveat from that
> commit message states.
>
> Now, it turns out that Buildah, a tool to create container images,
> sharing its codebase with Podman, passes a procfs entry to pasta, and
> expects pasta to exit once the network namespace is not needed
> anymore, that is, once the original Buildah process terminates.
Note we do not want to track buildah, we pass down the pid of the 
container process so the tree looks like this:

buildah
\_ container process
\_ pasta

We pass down the pid of the container process to pasta and it needs to 
exit with it. One buildah can spawn multiple pasta processes at once. 
Each RUN instruction in a Dockerfile will setup a new container and thus 
a new netns.

>
> Get this to work by using the timer fallback mechanism if the
> namespace name is passed as a path belonging to a pseudo-filesystem.
> This is expected to be procfs, but I covered sysfs and devpts
> pseudo-filesystems as well, because nothing actually prevents
> creating this kind of directory structure and links there.
>
> Note that fstatfs(), according to some versions of man pages, was
> apparently "deprecated" by the LSB. My reasoning for using it is
> essentially this:
>    https://lore.kernel.org/linux-man/f54kudgblgk643u32tb6at4cd3kkzha6hslahv24szs4raroaz@ogivjbfdaqtb/t/#u
>
> ...that is, there was no such thing as an LSB deprecation, and
> anyway there's no other way to get the filesystem type.
>
> Also note that, while it might sound more obvious to detect the
> filesystem type using fstatfs() on the file descriptor itself
> (c->pasta_netns_fd), the reported filesystem type for it is nsfs, no
> matter what path was given to pasta. If we use the parent directory,
> we'll typically have either tmpfs or procfs reported.
>
> If the target naemsapce is given as a PID, or as a PID-based procfs
typo namespace
> entry, we don't risk races if this PID is recycled: our handle on
> /proc/PID/ns will always refer to the original namespace associated
> with that PID, and we don't re-open this entry from procfs to check
> it.
>
> Instead of directly monitoring the target namespace, we could have
> tried to monitor a process with a given PID, using pidfd_open() to
> get a handle on it, to decide when to terminate.
>
> But it's not guaranteed that the parent process is actually the one
> associated to the network namespace we operate on, and if we get a
> PID file descriptor for a PID (parent or not) or path that was given
> on our command line, this inherently causes a race condition as that
> PID might have been recycled by the time we call pidfd_open().
Well I mean the race is always there regardless of pidfd_open, already 
when you open /proc/$PID/ns/net the race exists as it may no longer be 
the correct pid if the process exited (and was reaped) before it can 
open the path. I think if we really want a race free interface then it 
would make the most sense to have the parent pass down a pidfd to pasta, 
this allows pasta to poll the fd to see the exit and also to call setns 
on that fd.
>
> Even assuming the process we want to watch is the parent process, and
> a race-free usage of pidfd_open() would have been possible, I'm not
> very enthusiastic about enabling yet another system call in the
> seccomp profile just for this, while openat() is needed anyway.
If the parent opens the pidfd then this would not be an issue.
>
> Update the man page to reflect that, even if the target network
> namespace is passed as a procfs path or a PID, we'll now quit when
> the procfs entry is gone.
>
> Reported-by: Paul Holzinger <pholzing@redhat.com>
> Link: https://github.com/containers/podman/pull/21563#issuecomment-1948200214
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
> v3: Given that we now open c->netns_dir before checking the
>      filesystem type, we could as well pass this file descriptor to
>      fstatfs() to do the check, instead of statfs() on the path.
>
>      Fix a couple of paragraphs in the commit message.
>
> v2: Coverity Scan isn't happy if we "check" (kind of) c->netns_dir
>      with statfs() before opening it in a non-atomic way. Just to make
>      things clear, false positive or not: open it, check it, close it
>      if it wasn't needed: we don't rely on the check.
>
>   passt.1 |  8 ++++++--
>   pasta.c | 24 +++++++++++++++++++-----
>   2 files changed, 25 insertions(+), 7 deletions(-)
>
> diff --git a/passt.1 b/passt.1
> index dc2d719..de6e3bf 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -550,8 +550,12 @@ without \-\-userns.
>   
>   .TP
>   .BR \-\-no-netns-quit
> -If the target network namespace is bound to the filesystem (that is, if PATH or
> -NAME are given as target), do not exit once the network namespace is deleted.
> +If the target network namespace is bound to the filesystem, do not exit once
> +that path is deleted.
> +
> +If the target network namespace is represented by a procfs entry, do not exit
> +once that entry is removed from procfs (representing the fact that a process
> +with the given PID terminated).
>   
>   .TP
>   .BR \-\-config-net
> diff --git a/pasta.c b/pasta.c
> index 01d1511..61feaa9 100644
> --- a/pasta.c
> +++ b/pasta.c
> @@ -33,6 +33,7 @@
>   #include <sys/timerfd.h>
>   #include <sys/types.h>
>   #include <sys/stat.h>
> +#include <sys/statfs.h>
>   #include <fcntl.h>
>   #include <sys/wait.h>
>   #include <signal.h>
> @@ -41,6 +42,7 @@
>   #include <netinet/in.h>
>   #include <net/ethernet.h>
>   #include <sys/syscall.h>
> +#include <linux/magic.h>
>   
>   #include "util.h"
>   #include "passt.h"
> @@ -390,12 +392,21 @@ void pasta_netns_quit_init(const struct ctx *c)
>   	union epoll_ref ref = { .type = EPOLL_TYPE_NSQUIT_INOTIFY };
>   	struct epoll_event ev = { .events = EPOLLIN };
>   	int flags = O_NONBLOCK | O_CLOEXEC;
> -	int fd;
> +	struct statfs s = { 0 };
> +	bool try_inotify = true;
> +	int fd = -1, dir_fd;
>   
>   	if (c->mode != MODE_PASTA || c->no_netns_quit || !*c->netns_base)
>   		return;
>   
> -	if ((fd = inotify_init1(flags)) < 0)
> +	if ((dir_fd = open(c->netns_dir, O_CLOEXEC | O_RDONLY)) < 0)
> +		die("netns dir open: %s, exiting", strerror(errno));
> +
> +	if (fstatfs(dir_fd, &s)          || s.f_type == DEVPTS_SUPER_MAGIC ||
> +	    s.f_type == PROC_SUPER_MAGIC || s.f_type == SYSFS_MAGIC)
> +		try_inotify = false;
> +
> +	if (try_inotify && (fd = inotify_init1(flags)) < 0)
>   		warn("inotify_init1(): %s, use a timer", strerror(errno));
>   
>   	if (fd >= 0 && inotify_add_watch(fd, c->netns_dir, IN_DELETE) < 0) {
> @@ -409,11 +420,11 @@ void pasta_netns_quit_init(const struct ctx *c)
>   		if ((fd = pasta_netns_quit_timer()) < 0)
>   			die("Failed to set up fallback netns timer, exiting");
>   
> -		ref.nsdir_fd = open(c->netns_dir, O_CLOEXEC | O_RDONLY);
> -		if (ref.nsdir_fd < 0)
> -			die("netns dir open: %s, exiting", strerror(errno));
> +		ref.nsdir_fd = dir_fd;
>   
>   		ref.type = EPOLL_TYPE_NSQUIT_TIMER;
> +	} else {
> +		close(dir_fd);
>   	}
>   
>   	if (fd > FD_REF_MAX)
> @@ -463,6 +474,9 @@ void pasta_netns_quit_timer_handler(struct ctx *c, union epoll_ref ref)
>   
>   	fd = openat(ref.nsdir_fd, c->netns_base, O_PATH | O_CLOEXEC);
>   	if (fd < 0) {
> +		if (errno == EACCES)	/* Expected for existing procfs entry */
> +			return;
> +
>   		info("Namespace %s is gone, exiting", c->netns_base);
>   		exit(EXIT_SUCCESS);
>   	}