Re: [PATCH v1 1/2] open: add close_range()

[Christian Brauner <christian@brauner.io>]

On Thu, May 23, 2019 at 07:22:17PM +0300, Konstantin Khlebnikov wrote:
> On 22.05.2019 18:52, 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)
> > {
> >          int dir_fd;
> >          DIR *dir;
> >          struct dirent *direntp;
> >
> >          dir = opendir("/proc/self/fd");
> >          if (!dir)
> >                  return -1;
> >          dir_fd = dirfd(dir);
> >          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 == dir_fd || fd == 0 || fd == 1 || fd == 2)
> >                          continue;
> >                  close(fd);
> >          }
> >          closedir(dir);
> >          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.
> 
> Here is another strange but real-live scenario: crash handler for dumping core.
> 
> If applications has network connections it would be better to close them all,
> otherwise clients will wait until end of dumping process or timeout.
> Also closing normal files might be a good idea for releasing locks.
> 
> But simple closing might race with other threads - closed fd will be reused
> while some code still thinks it refers to original file.
> 
> Our solution closes files without freeing fd: it opens /dev/null and
> replaces all opened descriptors using dup2.
> 
> So, special flag for close_range() could close files without clearing bitmap.
> Effect should be the same - fd wouldn't be reused.
> 
> Actually two flags for two phases: closing files and releasing fd.
> 
> >
> >  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.
> >
> > /* References */
> > [1]: https://lore.kernel.org/lkml/20190516165021.GD17978@ZenIV.linux.org.uk/
> > [2]: https://github.com/python/cpython/blob/9e4f2f3a6b8ee995c365e86d976937c141d867f8/Modules/_posixsubprocess.c#L220
> > [3]: https://sourceware.org/bugzilla/show_bug.cgi?id=10353#c7
> > [4]: https://github.com/systemd/systemd/blob/5238e9575906297608ff802a27e2ff9effa3b338/src/basic/fd-util.c#L217
> > [5]: https://github.com/lxc/lxc/blob/ddf4b77e11a4d08f09b7b9cd13e593f8c047edc5/src/lxc/start.c#L236
> > [6]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/grantpt.c;h=2030e07fa6e652aac32c775b8c6e005844c3c4eb;hb=HEAD#l17
> >       Note that this is an internal implementation that is not exported.
> >       Currently, libc seems to not provide an exported version of this
> >       because of missing kernel support to do this.
> > [7]: https://github.com/rust-lang/rust/issues/12148
> > [8]: https://github.com/rust-lang/rust/blob/5f47c0613ed4eb46fca3633c1297364c09e5e451/src/libstd/sys/unix/process2.rs#L303-L308
> >       Rust's solution is slightly different but is equally unperformant.
> >       Rust calls getdtablesize() which is a glibc library function that
> >       simply returns the current RLIMIT_NOFILE or OPEN_MAX values. Rust then
> >       goes on to call close() on each fd. That's obviously overkill for most
> >       tasks. Rarely, tasks - especially non-demons - hit RLIMIT_NOFILE or
> >       OPEN_MAX.
> >       Let's be nice and assume an unprivileged user with RLIMIT_NOFILE set
> >       to 1024. Even in this case, there's a very high chance that in the
> >       common case Rust is calling the close() syscall 1021 times pointlessly
> >       if the task just has 0, 1, and 2 open.
> >
> > Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
> > Signed-off-by: Christian Brauner <christian@brauner.io>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Jann Horn <jannh@google.com>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Dmitry V. Levin <ldv@altlinux.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Florian Weimer <fweimer@redhat.com>
> > Cc: linux-api@vger.kernel.org
> > ---
> > v1:
> > - Linus Torvalds <torvalds@linux-foundation.org>:
> >    - add cond_resched() to yield cpu when closing a lot of file descriptors
> > - Al Viro <viro@zeniv.linux.org.uk>:
> >    - add cond_resched() to yield cpu when closing a lot of file descriptors
> > ---
> >   arch/alpha/kernel/syscalls/syscall.tbl      |  1 +
> >   arch/arm/tools/syscall.tbl                  |  1 +
> >   arch/arm64/include/asm/unistd32.h           |  2 +
> >   arch/ia64/kernel/syscalls/syscall.tbl       |  1 +
> >   arch/m68k/kernel/syscalls/syscall.tbl       |  1 +
> >   arch/microblaze/kernel/syscalls/syscall.tbl |  1 +
> >   arch/mips/kernel/syscalls/syscall_n32.tbl   |  1 +
> >   arch/mips/kernel/syscalls/syscall_n64.tbl   |  1 +
> >   arch/mips/kernel/syscalls/syscall_o32.tbl   |  1 +
> >   arch/parisc/kernel/syscalls/syscall.tbl     |  1 +
> >   arch/powerpc/kernel/syscalls/syscall.tbl    |  1 +
> >   arch/s390/kernel/syscalls/syscall.tbl       |  1 +
> >   arch/sh/kernel/syscalls/syscall.tbl         |  1 +
> >   arch/sparc/kernel/syscalls/syscall.tbl      |  1 +
> >   arch/x86/entry/syscalls/syscall_32.tbl      |  1 +
> >   arch/x86/entry/syscalls/syscall_64.tbl      |  1 +
> >   arch/xtensa/kernel/syscalls/syscall.tbl     |  1 +
> >   fs/file.c                                   | 63 ++++++++++++++++++---
> >   fs/open.c                                   | 20 +++++++
> >   include/linux/fdtable.h                     |  2 +
> >   include/linux/syscalls.h                    |  2 +
> >   include/uapi/asm-generic/unistd.h           |  4 +-
> >   22 files changed, 100 insertions(+), 9 deletions(-)
> >
> 
> It would be better to split arch/ wiring into separate patch for better readability.

Ok. You mean only do x86 - seems to be the standard - and then move the
others into a separate patch? Doesn't seem worth to have a patch
per-arch, I'd think.