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

[Stefano Brivio <sbrivio@redhat.com>]

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.

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 statfs(), 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 robust to detect the
filesystem type using fstatfs() on the file descriptor
(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.

The timer, however, still uses the file descriptor of the parent
directory later, as it has no access to the filesystem, and that
avoids possible races if the PID is recycled: if the original process
terminates, the handle we have on /proc/PID/ns still refers to it,
not to any other process with the same PID.

We could have used pidfd_open() to get a handle on the parent
process. 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().

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.

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>
---
 passt.1 |  8 ++++++--
 pasta.c | 15 +++++++++++++--
 2 files changed, 19 insertions(+), 4 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..4110917 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 +392,18 @@ 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;
 
 	if (c->mode != MODE_PASTA || c->no_netns_quit || !*c->netns_base)
 		return;
 
-	if ((fd = inotify_init1(flags)) < 0)
+	if (statfs(c->netns_dir, &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) {
@@ -463,6 +471,9 @@ 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);
 	}
-- 
@@ -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 +392,18 @@ 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;
 
 	if (c->mode != MODE_PASTA || c->no_netns_quit || !*c->netns_base)
 		return;
 
-	if ((fd = inotify_init1(flags)) < 0)
+	if (statfs(c->netns_dir, &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) {
@@ -463,6 +471,9 @@ 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);
 	}
-- 
2.39.2