Hi Stephen,
Please see some comments below.
It's looking good ;)
Thanks,
Alex
On 12/18/20 5:58 PM, Stephen Kitt wrote:
> This documents close_range(2) based on information in
> 278a5fbaed89dacd04e9d052f4594ffd0e0585de,
> 60997c3d45d9a67daf01c56d805ae4fec37e0bd8, and
> 582f1fb6b721facf04848d2ca57f34468da1813e.
>
> Signed-off-by: Stephen Kitt <steve@xxxxxxx>
> ---
> V3: fix synopsis overflow
> copy notes from membarrier.2 re the lack of wrapper
> semantic newlines
> drop non-standard "USE CASES" section heading
> add code example
>
> V2: unsigned int to match the kernel declarations
> groff and grammar tweaks
> CLOSE_RANGE_UNSHARE unshares *and* closes
> Explain that EMFILE and ENOMEM can occur with C_R_U
> "Conforming to" phrasing
> Detailed explanation of CLOSE_RANGE_UNSHARE
> Reading /proc isn't common
>
> man2/close_range.2 | 266 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 266 insertions(+)
> create mode 100644 man2/close_range.2
>
> diff --git a/man2/close_range.2 b/man2/close_range.2
> new file mode 100644
> index 000000000..f8f2053ac
> --- /dev/null
> +++ b/man2/close_range.2
> @@ -0,0 +1,266 @@
> +.\" Copyright (c) 2020 Stephen Kitt <steve@xxxxxxx>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of this
> +.\" manual under the conditions for verbatim copying, provided that the
> +.\" entire resulting derived work is distributed under the terms of a
> +.\" permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date. The author(s) assume no
> +.\" responsibility for errors or omissions, or for damages resulting from
> +.\" the use of the information contained herein. The author(s) may not
> +.\" have taken the same level of care in the production of this manual,
> +.\" which is licensed free of charge, as they might when working
> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.\"
> +.TH CLOSE_RANGE 2 2020-12-08 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +close_range \- close all file descriptors in a given range
> +.SH SYNOPSIS
> +.nf
> +.B #include <linux/close_range.h>
> +.PP
> +.BI "int close_range(unsigned int " first ", unsigned int " last ,
> +.BI " unsigned int " flags );
> +.fi
> +.PP
> +.IR Note :
> +There is no glibc wrapper for this system call; see NOTES.
> +.SH DESCRIPTION
> +The
> +.BR close_range ()
> +system call closes all open file descriptors from
> +.I first
> +to
> +.I last
> +(included).
> +.PP
> +Errors closing a given file descriptor are currently ignored.
> +.PP
> +.I flags
> +can be 0 or set to one or both of the following:
> +.TP
> +.B CLOSE_RANGE_UNSHARE
> +unshares the range of file descriptors from any other processes,
> +before closing them,
> +avoiding races with other threads sharing the file descriptor table.
> +.TP
> +.BR CLOSE_RANGE_CLOEXEC " (since Linux 5.10)"
|sort
I prefer alphabetic order rather than adding new items at the bottom.
When lists grow, it becomes difficult to find what you're looking for.
CLOEXEC should go before UNSHARE.
> +sets the close-on-exec bit instead of immediately closing the file
> +descriptors.
[
sets the close-on-exec bit instead of
immediately closing the file descriptors.
]
> +.SH RETURN VALUE
> +On success,
> +.BR close_range ()
> +returns 0.
> +On error, \-1 is returned and
> +.I errno
> +is set to indicate the cause of the error.
> +.SH ERRORS
> +.TP
> +.B EINVAL
> +.I flags
> +is not valid, or
> +.I first
> +is greater than
> +.IR last .
> +.PP
> +The following can occur with
> +.B CLOSE_RANGE_UNSHARE
> +(when constructing the new descriptor table):
> +.TP
> +.B EMFILE
> +The per-process limit on the number of open file descriptors has been reached
> +(see the description of
> +.B RLIMIT_NOFILE
> +in
> +.BR getrlimit (2)).
> +.TP
> +.B ENOMEM
> +Insufficient kernel memory was available.
> +.SH VERSIONS
> +.BR close_range ()
> +first appeared in Linux 5.9.
> +.SH CONFORMING TO
> +.BR close_range ()
> +is a nonstandard function that is also present on FreeBSD.
> +.SH NOTES
> +Glibc does not provide a wrapper for this system call; call it using
> +.BR syscall (2).
> +.\" 278a5fbaed89dacd04e9d052f4594ffd0e0585de
> +.SS Closing all open file descriptors
The comment with the commit would be better inside the section it refers
to, so:
[
.SS Closing all open file descriptors
.\" 278a5fbaed89dacd04e9d052f4594ffd0e0585de
]
> +To avoid blindly closing file descriptors in the range of possible
> +file descriptors,
[
To avoid blindly closing file descriptors
in the range of possible file descriptors,
]
> +this is sometimes implemented (on Linux) by listing open file
> +descriptors in
[
this is sometimes implemented (on Linux)
by listing open file descriptors in
]
> +.I /proc/self/fd/
> +and calling
> +.BR close (2)
> +on each one.
> +.BR close_range ()
> +can take care of this without requiring
> +.I /proc
> +and with a single system call,
s/with/within/
> +which provides significant performance benefits.
> +.\" 60997c3d45d9a67daf01c56d805ae4fec37e0bd8
> +.SS Closing file descriptors before exec
[
.SS Closing file descriptors before exec
.\" 60997c3d45d9a67daf01c56d805ae4fec37e0bd8
]
> +File descriptors can be closed safely using
> +.PP
> +.in +4n
> +.EX
> +/* we don't want anything past stderr here */
> +close_range(3, ~0U, CLOSE_RANGE_UNSHARE);
> +execve(....);> +.EE
> +.in
> +.PP
> +.B CLOSE_RANGE_UNSHARE
> +is conceptually equivalent to
> +.PP
> +.in +4n
> +.EX
> +unshare(CLONE_FILES);
> +close_range(first, last, 0);
> +.EE
> +.in
> +.PP
> +but can be more efficient:
> +if the unshared range extends past the current maximum number of file
> +descriptors allocated in the caller's file descriptor table
[
if the unshared range extends past
the current maximum number of file descriptors allocated
in the caller's file descriptor table
]
> +(the common case when
> +.I last
> +is
> +.BR ~0U ),
Literal values are not (usually) formatted.
[
.I last
is ~0U),
]
> +the kernel will unshare a new file descriptor table for the caller up
> +to
[
the kernel will unshare a new file descriptor table for the caller up to
]
> +.IR first .
> +This avoids subsequent close calls entirely;
> +the whole operation is complete once the table is unshared.
> +.\" 582f1fb6b721facf04848d2ca57f34468da1813e
> +.SS Closing files on \fBexec\fP
[
.SS Closing files on \fBexec\fP
.\" 582f1fb6b721facf04848d2ca57f34468da1813e
]
> +This is particularly useful in cases where multiple
> +.RB pre- exec
> +setup steps risk conflicting with each other.
> +For example, setting up a
> +.BR seccomp (2)
> +profile can conflict with a
> +.B close_range
.BR close_range ()
> +call:
> +if the file descriptors are closed before the seccomp profile is set
.BR seccomp (2)
> +up,
Please, split at a different point.
> +the profile setup can't use them control their closure;
I don't understand what you wanted to say. them?
> +if the file descriptors are closed afterwards,
> +the seccomp profile can't block the
> +.B close_range
.BR close_range ()
> +call or any fallbacks.
> +Using
> +.B CLOSE_RANGE_CLOEXEC
> +avoids this:
> +the descriptors can be marked before the seccomp profile is set up,
.BR seccomp (2)
> +and the profile can control access to
> +.B close_range
.BR close_range ()
> +without affecting the calling process.
> +.SH EXAMPLES
> +The following program is designed to be execed by the second program
> +below.
> +It lists its open file descriptors:
> +.PP
> +.in +4n
> +.EX
> +/* listopen.c */
> +
> +#include <stdio.h>
> +#include <sys/stat.h>
> +
> +int
> +main(int argc, char *argv[])
> +{
> + int i;
We use C99 declarations for loop indices.
> + struct stat buf;
> +
> + for (i = 0; i < 100; i++) {
for (int i = 0; i < 100; i++) {
> + if (!fstat(i, &buf))
> + printf("FD %d is open.\n", i);
s/\\/\\e/
see: d1a719857b7eb68f5e5c1c965089038dee683240
I sometimes forget to fix those after copying the program to the page.
My solution is to copy the rendered text from the man page to a file
and then compile, and those errors become obvious ;)
> + }
> +
> + exit(EXIT_SUCCESS);
> +)
> +.EE
> +.in
> +.PP
> +This program executes the command given on its command-line after
> +opening the files listed after the command,
> +and then using
s/using/uses/
> +.B close_range
.BR close_range ()
> +to close them:
> +.PP
> +.in +4n
> +.EX
> +/* close_range.c */
> +
> +#include <fcntl.h>
> +#include <linux/close_range.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/stat.h>
> +#include <sys/syscall.h>
> +#include <sys/types.h>
> +#include <unistd.h>
> +
> +int
> +main(int argc, char *argv[])
> +{
> + char *newargv[] = { NULL };
> + char *newenviron[] = { NULL };
> + int i;
dd
> +
> + if (argc < 3) {
> + fprintf(stderr, "Usage: %s <command-to-run> <files-to-open>\n", argv[0]);
s/\\/\\e/
> + exit(EXIT_FAILURE);
> + }
> +
> + for (i = 2; i < argc; i++) {
for (int i = 2; i < argc; i++) {
> + if (open(argv[i], O_RDONLY) == -1) {
> + perror(argv[i]);
> + exit(EXIT_FAILURE);
> + }
> + }
> +
> + if (syscall(__NR_close_range, 3, ~0U, CLOSE_RANGE_UNSHARE) == -1) {
> + perror("close_range");
> + exit(EXIT_FAILURE);
> + }
> +
> + execve(argv[1], newargv, newenviron);
> + perror("execve");
> + exit(EXIT_FAILURE);
> +}
> +.EE
> +.in
> +.PP
> +We can use the second program to exec the first as follows:
> +.PP
> +.in +4n
> +.EX
> +.RB "$" " make listopen close_range"
> +.RB "$" " ./close_range ./listopen /dev/null /dev/zero"
> +FD 0 is open.
> +FD 1 is open.
> +FD 2 is open.
> +.EE
> +.in
> +.PP
> +Removing the call to
> +.B close_range
.BR close_range ()
> +will show different output, with the file descriptors for the named
> +files still open.
[
will show different output,
with the file descriptors for the named files still open.
]
> +.SH SEE ALSO
> +.BR close (2)
>
> base-commit: b5dae3959625f5ff378e9edf9139057d1c06bb55
>
--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
