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

Christian Brauner
2019-05-23T16:33:46Z

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\x10353#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 30e07fa6e652aac32c775b8c6e005844c3c4eb;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.