Re: [PATCH 1/2] open: add close_range()

[Christian Brauner <christian@brauner.io>]

On Tue, May 21, 2019 at 04:00:06PM +0100, Al Viro wrote:
> On Tue, May 21, 2019 at 01:34:47PM +0200, Christian Brauner wrote:
> 
> > This adds the close_range() syscall. It allows to efficiently close a range
> > of file descriptors up to all file descriptors of a calling task.
> > 
> > The syscall came up in a recent discussion around the new mount API and
> > making new file descriptor types cloexec by default. During this
> > discussion, Al suggested the close_range() syscall (cf. [1]). Note, a
> > syscall in this manner has been requested by various people over time.
> > 
> > First, it helps to close all file descriptors of an exec()ing task. This
> > can be done safely via (quoting Al's example from [1] verbatim):
> > 
> >         /* that exec is sensitive */
> >         unshare(CLONE_FILES);
> >         /* we don't want anything past stderr here */
> >         close_range(3, ~0U);
> >         execve(....);
> > 
> > The code snippet above is one way of working around the problem that file
> > descriptors are not cloexec by default. This is aggravated by the fact that
> > we can't just switch them over without massively regressing userspace. For
> > a whole class of programs having an in-kernel method of closing all file
> > descriptors is very helpful (e.g. demons, service managers, programming
> > language standard libraries, container managers etc.).
> > (Please note, unshare(CLONE_FILES) should only be needed if the calling
> >  task is multi-threaded and shares the file descriptor table with another
> >  thread in which case two threads could race with one thread allocating
> >  file descriptors and the other one closing them via close_range(). For the
> >  general case close_range() before the execve() is sufficient.)
> > 
> > Second, it allows userspace to avoid implementing closing all file
> > descriptors by parsing through /proc/<pid>/fd/* and calling close() on each
> > file descriptor. From looking at various large(ish) userspace code bases
> > this or similar patterns are very common in:
> > - service managers (cf. [4])
> > - libcs (cf. [6])
> > - container runtimes (cf. [5])
> > - programming language runtimes/standard libraries
> >   - Python (cf. [2])
> >   - Rust (cf. [7], [8])
> > As Dmitry pointed out there's even a long-standing glibc bug about missing
> > kernel support for this task (cf. [3]).
> > In addition, the syscall will also work for tasks that do not have procfs
> > mounted and on kernels that do not have procfs support compiled in. In such
> > situations the only way to make sure that all file descriptors are closed
> > is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE,
> > OPEN_MAX trickery (cf. comment [8] on Rust).
> > 
> > The performance is striking. For good measure, comparing the following
> > simple close_all_fds() userspace implementation that is essentially just
> > glibc's version in [6]:
> > 
> > static int close_all_fds(void)
> > {
> >         DIR *dir;
> >         struct dirent *direntp;
> > 
> >         dir = opendir("/proc/self/fd");
> >         if (!dir)
> >                 return -1;
> > 
> >         while ((direntp = readdir(dir))) {
> >                 int fd;
> >                 if (strcmp(direntp->d_name, ".") == 0)
> >                         continue;
> >                 if (strcmp(direntp->d_name, "..") == 0)
> >                         continue;
> >                 fd = atoi(direntp->d_name);
> >                 if (fd == 0 || fd == 1 || fd == 2)
> >                         continue;
> >                 close(fd);
> >         }
> > 
> >         closedir(dir); /* cannot fail */
> >         return 0;
> > }
> > 
> > to close_range() yields:
> > 1. closing 4 open files:
> >    - close_all_fds(): ~280 us
> >    - close_range():    ~24 us
> > 
> > 2. closing 1000 open files:
> >    - close_all_fds(): ~5000 us
> >    - close_range():   ~800 us
> > 
> > close_range() is designed to allow for some flexibility. Specifically, it
> > does not simply always close all open file descriptors of a task. Instead,
> > callers can specify an upper bound.
> > This is e.g. useful for scenarios where specific file descriptors are
> > created with well-known numbers that are supposed to be excluded from
> > getting closed.
> > For extra paranoia close_range() comes with a flags argument. This can e.g.
> > be used to implement extension. Once can imagine userspace wanting to stop
> > at the first error instead of ignoring errors under certain circumstances.
> > There might be other valid ideas in the future. In any case, a flag
> > argument doesn't hurt and keeps us on the safe side.
> > 
> > >From an implementation side this is kept rather dumb. It saw some input
> > from David and Jann but all nonsense is obviously my own!
> > - Errors to close file descriptors are currently ignored. (Could be changed
> >   by setting a flag in the future if needed.)
> > - __close_range() is a rather simplistic wrapper around __close_fd().
> >   My reasoning behind this is based on the nature of how __close_fd() needs
> >   to release an fd. But maybe I misunderstood specifics:
> >   We take the files_lock and rcu-dereference the fdtable of the calling
> >   task, we find the entry in the fdtable, get the file and need to release
> >   files_lock before calling filp_close().
> >   In the meantime the fdtable might have been altered so we can't just
> >   retake the spinlock and keep the old rcu-reference of the fdtable
> >   around. Instead we need to grab a fresh reference to the fdtable.
> >   If my reasoning is correct then there's really no point in fancyfying
> >   __close_range(): We just need to rcu-dereference the fdtable of the
> >   calling task once to cap the max_fd value correctly and then go on
> >   calling __close_fd() in a loop.
> 
> > +/**
> > + * __close_range() - Close all file descriptors in a given range.
> > + *
> > + * @fd:     starting file descriptor to close
> > + * @max_fd: last file descriptor to close
> > + *
> > + * This closes a range of file descriptors. All file descriptors
> > + * from @fd up to and including @max_fd are closed.
> > + */
> > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> > +{
> > +	unsigned int cur_max;
> > +
> > +	if (fd > max_fd)
> > +		return -EINVAL;
> > +
> > +	rcu_read_lock();
> > +	cur_max = files_fdtable(files)->max_fds;
> > +	rcu_read_unlock();
> > +
> > +	/* cap to last valid index into fdtable */
> > +	if (max_fd >= cur_max)
> > +		max_fd = cur_max - 1;
> > +
> > +	while (fd <= max_fd)
> > +		__close_fd(files, fd++);
> > +
> > +	return 0;
> > +}
> 
> Umm...  That's going to be very painful if you dup2() something to MAX_INT and
> then run that; roughly 2G iterations of bouncing ->file_lock up and down,
> without anything that would yield CPU in process.
> 
> If anything, I would suggest something like
> 
> 	fd = *start_fd;
> 	grab the lock
>         fdt = files_fdtable(files);
> more:
> 	look for the next eviction candidate in ->open_fds, starting at fd
> 	if there's none up to max_fd
> 		drop the lock
> 		return NULL
> 	*start_fd = fd + 1;
> 	if the fscker is really opened and not just reserved
> 		rcu_assign_pointer(fdt->fd[fd], NULL);
> 		__put_unused_fd(files, fd);
> 		drop the lock
> 		return the file we'd got
> 	if (unlikely(need_resched()))
> 		drop lock
> 		cond_resched();
> 		grab lock
> 		fdt = files_fdtable(files);
> 	goto more;
> 
> with the main loop being basically
> 	while ((file = pick_next(files, &start_fd, max_fd)) != NULL)
> 		filp_close(file, files);

That's obviously much more clever than what I had.
I honestly have never thought about using open_fds before this. Seemed
extremely localized to file.c
Thanks for the pointers!

Christian