[PATCH v2] close_range.2: new page documenting close_range(2)

[Stephen Kitt]

This documents close_range(2) based on information in
278a5fbaed89dacd04e9d052f4594ffd0e0585de and
60997c3d45d9a67daf01c56d805ae4fec37e0bd8.

Signed-off-by: Stephen Kitt <steve@xxxxxxx>
---
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 | 138 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 138 insertions(+)
 create mode 100644 man2/close_range.2

diff --git a/man2/close_range.2 b/man2/close_range.2
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+++ b/man2/close_range.2
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+.\" 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+.\" manual page may be incorrect or out-of-date.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 ", unsigned int " flags );
+.fi
+.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 set to
+.B CLOSE_RANGE_UNSHARE
+to unshare the range of file descriptors from any other processes,
+before closing them, avoiding races with other threads sharing the
+file descriptor table.
+.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
+Currently, there is no glibc wrapper for this system call; call it using
+.BR syscall (2).
+.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 (the common case when
+.I last
+is
+.BR ~0U ),
+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.
+.SH USE CASES
+.\" 278a5fbaed89dacd04e9d052f4594ffd0e0585de
+.\" 60997c3d45d9a67daf01c56d805ae4fec37e0bd8
+.SS Closing file descriptors before exec
+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
+.SS Closing all open 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
+.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, which provides significant performance
+benefits.
+.SH SEE ALSO
+.BR close (2)

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2.20.1