Re: [PATCH 0/6] Fix bug 215 and some related issues with fd handling

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 14 Jul 2026 19:29:20 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> Stefano, you called it correctly.  While working on bug 215, as usual,
> I found a bunch of adjacent things to clean up.

So, I finally finished reviewing the series.

Other than 1/6 and 2/6 on which I already commented (long story short,
I think we should avoid 1/6, and about 2/6, it would be nice to parse -F
just once in general but I think we shouldn't "force" it like that...
maybe just parse / get it outside conf()?), I don't see any substantial
issue with the other patches, but I have some general comments about the
approach.

As a detail, though, I would recommend Cc'ing everybody who might be
interested or affected by this (reporter of bug #215, Rich as he fixed
the original https://github.com/libguestfs/libguestfs/issues/360, and
Alyssa as author of aa1cc8922867 ("conf: allow --fd 0").

> I did start by attempting the appoach you suggested for bug 215 -
> remembering which of the low fds were standard streams and avoiding
> closing them in __daemon().  It is indeed shorter, but only by 1-2
> lines.  Looking at possible interactions with other things, I became
> more and more convinced that leaving anything other than the standard
> streams in fds 0-2 was an accident waiting to happen.

It did actually happen, see c66f0341d94d ("log: Don't report syslog
failures to stderr after initialisation"). I didn't consider that,
and it's indeed a strong argument in favour of this approach.

I still have some remarks and doubts about it though:

1. we might have users passing a given file descriptor with the
   expectation that it won't change its number as seen from procfs
   (and dup2() changes that, right?), even just for debugging, and 4/6
   breaks that. It's not a very legitimate expectation maybe but it
   might be one, we simply don't know

2. the reason behind my proposal (check if file descriptors are open
   when we start and avoid closing them) was simplicity and avoiding
   the risk of a number of side effects (more below).

   Yes, it's just a bit shorter (depending on how we count), but this
   diff (build tested only) should be equivalent to patches 4/6 and 5/6,
   which look considerably more complicated to me (even though the
   simplification in close_open_files() is significant... but we could
   get the same outcome also with just 4/6):

---
diff --git a/passt.c b/passt.c
index 65a07d7..e2ea613 100644
--- a/passt.c
+++ b/passt.c
@@ -330,8 +330,9 @@ static void passt_worker(void *opaque, int nfds, struct epoll_event *events)
 int main(int argc, char **argv)
 {
 	struct epoll_event events[NUM_EPOLL_EVENTS];
+	bool close_low_fd[STDERR_FILENO + 1];
+	int nfds, devnull_fd = -1, i;
 	struct ctx *c = &passt_ctx;
-	int nfds, devnull_fd = -1;
 	struct rlimit limit;
 	struct timespec now;
 	struct sigaction sa;
@@ -339,6 +340,9 @@ int main(int argc, char **argv)
 	if (clock_gettime(CLOCK_MONOTONIC, &log_start))
 		die_perror("Failed to get CLOCK_MONOTONIC time");
 
+	for (i = STDIN_FILENO; i <= STDERR_FILENO; i++)
+		close_low_fd[i] = !fcntl(i, F_GETFD);
+
 	arch_avx2_exec(argv);
 
 	isolate_initial(argc, argv);
@@ -419,7 +423,10 @@ int main(int argc, char **argv)
 		die("Failed to sandbox process, exiting");
 
 	if (!c->foreground) {
-		__daemon(c->pidfile_fd, devnull_fd);
+		if (c->fd_tap != -1 && c->fd_tap < STDERR_FILENO)
+			close_low_fd[c->fd_tap] = false;
+
+		__daemon(close_low_fd, c->pidfile_fd, devnull_fd);
 		close(c->pidfile_fd);
 		c->pidfile_fd = -1;
 		log_stderr = false;
diff --git a/util.c b/util.c
index 4bc5d6f..57d42e1 100644
--- a/util.c
+++ b/util.c
@@ -501,13 +501,14 @@ int output_file_open(const char *path, int flags)
 
 /**
  * __daemon() - daemon()-like function writing PID file before parent exits
+ * @close_fd:	Standard stream descriptors numbers to close
  * @pidfile_fd:	Open PID file descriptor
  * @devnull_fd:	Open file descriptor for /dev/null
  *
  * Return: 0 in the child process on success. The parent process exits.
  * 	   Does not return in either process on failure (calls _exit).
  */
-int __daemon(int pidfile_fd, int devnull_fd)
+int __daemon(bool close_fd[STDERR_FILENO + 1], int pidfile_fd, int devnull_fd)
 {
 	pid_t pid = fork();
 
@@ -522,9 +523,9 @@ int __daemon(int pidfile_fd, int devnull_fd)
 	}
 
 	if (setsid()				< 0 ||
-	    dup2(devnull_fd, STDIN_FILENO)	< 0 ||
-	    dup2(devnull_fd, STDOUT_FILENO)	< 0 ||
-	    dup2(devnull_fd, STDERR_FILENO)	< 0 ||
+	    (close_fd[STDIN_FILENO]  && dup2(devnull_fd, STDIN_FILENO)  < 0) ||
+	    (close_fd[STDOUT_FILENO] && dup2(devnull_fd, STDOUT_FILENO)	< 0) ||
+	    (close_fd[STDERR_FILENO] && dup2(devnull_fd, STDERR_FILENO)	< 0) ||
 	    close(devnull_fd))
 		passt_exit(EXIT_FAILURE);
 
diff --git a/util.h b/util.h
index 90e8a20..246ac67 100644
--- a/util.h
+++ b/util.h
@@ -160,7 +160,7 @@ bool ns_is_init(void);
 int open_in_ns(const struct ctx *c, const char *path, int flags);
 int output_file_open(const char *path, int flags);
 void pidfile_write(int fd, pid_t pid);
-int __daemon(int pidfile_fd, int devnull_fd);
+int __daemon(bool close_fd[STDERR_FILENO + 1], int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int ilog2(unsigned long x);
 int write_file(const char *path, const char *buf);
---

3. I have a generic worry that LSMs might get in the way. This would be solved
   by testing your series against current AppArmor and SELinux policies but I
   didn't get the chance yet (it would be nice if you could...)

4. if the concern is a misused fprintf() or printf(), shouldn't we prevent
   direct usage anyway with, say:

#define printf(x) @ "Don't call printf() directly, use err() / warn() / debug() etc."

   and similar for fprintf(), that could only be called directly from
   FPRINTF() and wherever we really need it? At that point I'm not sure
   we would have any remaining concern about risks of using standard
   streams by mistake

5. assuming we go with both 4/6 and 5/6: can we finally assume that sockets
   will never be numbered 0 and save a lot of initialisations to -1 and
   related checks, at that point? If we can achieve that as side effect,
   that would be another argument in favour of it in my eyes

> Much of the
> rest of the series is, for example, dealing with the possibility of
> --fd [012].

...well, yes, but the possibility of --fd [12] was introduced by the
series itself. :)

Anyway, summing up my feedback, *maybe* other advantages outweigh 1. (I
haven't checked what happens in procfs though), and once we check that
3. is not a problem, I'm fine with this approach (even though still a
bit reluctant because we're adding substantial changes for a problem
that doesn't even exist anymore as it's already fixed in libguestfs).

-- 
Stefano