[lkml]   [2021]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH v6] close_range.2: new page documenting close_range(2)
This documents close_range(2) based on information in
60997c3d45d9a67daf01c56d805ae4fec37e0bd8, and

Signed-off-by: Stephen Kitt <>
V6: bit mask, close-on-exec flag language improvements
another close(2) reference
only include one example program
ensure the example code doesn't wrap

V5: clarification of the open/close_range/execve sequence

V4: sort flags alphabetically
move commit references inside the corresponding section
more semantic newlines
unformat numeric constants
more formatting for function references
escape C backslashes
C99 loop indices

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 | 236 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 236 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..5abb73990
--- /dev/null
+++ b/man2/close_range.2
@@ -0,0 +1,236 @@
+.\" Copyright (c) 2020 Stephen Kitt <>
+.\" 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+.\" manual page may be incorrect or out-of-date.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.
+.TH CLOSE_RANGE 2 2020-12-08 "Linux" "Linux Programmer's Manual"
+close_range \- close all file descriptors in a given range
+.B #include <linux/close_range.h>
+.BI "int close_range(unsigned int " first ", unsigned int " last ,
+.BI " unsigned int " flags );
+.IR Note :
+There is no glibc wrapper for this system call; see NOTES.
+.BR close_range ()
+system call closes all open file descriptors from
+.I first
+.I last
+Errors closing a given file descriptor are currently ignored.
+.I flags
+is a bit mask containing 0 or more of the following:
+.BR CLOSE_RANGE_CLOEXEC " (since Linux 5.11)"
+sets the file descriptor's close-on-exec flag instead of
+immediately closing the file descriptors.
+unshares the range of file descriptors from any other processes,
+before closing them,
+avoiding races with other threads sharing the file descriptor table.
+On success,
+.BR close_range ()
+returns 0.
+On error, \-1 is returned and
+.I errno
+is set to indicate the cause of the error.
+.I flags
+is not valid, or
+.I first
+is greater than
+.IR last .
+The following can occur with
+(when constructing the new descriptor table):
+The per-process limit on the number of open file descriptors has been reached
+(see the description of
+.BR getrlimit (2)).
+Insufficient kernel memory was available.
+.BR close_range ()
+first appeared in Linux 5.9.
+.BR close_range ()
+is a nonstandard function that is also present on FreeBSD.
+Glibc does not provide a wrapper for this system call; call it using
+.BR syscall (2).
+.SS Closing all open file descriptors
+.\" 278a5fbaed89dacd04e9d052f4594ffd0e0585de
+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 within a single system call,
+which provides significant performance benefits.
+.SS Closing file descriptors before exec
+.\" 60997c3d45d9a67daf01c56d805ae4fec37e0bd8
+File descriptors can be closed safely using
+.PP +4n
+/* we don't want anything past stderr here */
+close_range(3, ~0U, CLOSE_RANGE_UNSHARE);
+is conceptually equivalent to
+.PP +4n
+close_range(first, last, 0);
+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 ~0U),
+the kernel will unshare a new file descriptor table for the caller up to
+.IR first .
+This avoids subsequent
+.BR close (2)
+calls entirely;
+the whole operation is complete once the table is unshared.
+.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
+.BR close_range ()
+if the file descriptors are closed before the
+.BR seccomp (2)
+profile is set up,
+the profile setup can't use them itself,
+or control their closure;
+if the file descriptors are closed afterwards,
+the seccomp profile can't block the
+.BR close_range ()
+call or any fallbacks.
+avoids this:
+the descriptors can be marked before the
+.BR seccomp (2)
+profile is set up,
+and the profile can control access to
+.BR close_range ()
+without affecting the calling process.
+The following program executes the command given on its command-line,
+after opening the files listed after the command and then using
+.BR close_range ()
+to close them:
+.PP +4n
+/* 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>
+main(int argc, char *argv[])
+ char *newargv[] = { NULL };
+ char *newenviron[] = { NULL };
+ if (argc < 3) {
+ fprintf(stderr, "Usage: %s <command> <file>...\en", argv[0]);
+ }
+ for (int i = 2; i < argc; i++) {
+ if (open(argv[i], O_RDONLY) == -1) {
+ perror(argv[i]);
+ }
+ }
+ if (syscall(__NR_close_range, 3, ~0U, 0) == -1) {
+ perror("close_range");
+ }
+ execve(argv[1], newargv, newenviron);
+ perror("execve");
+Running any program with the above, with files to open:
+.PP +4n
+.RB "$" " ./close_range " <program> " /dev/null /dev/zero"
+and inspecting the open files in the resulting process will show that
+the files have indeed been closed.
+.BR close (2)
base-commit: fb0d03d11cec04da7720a80a1373605d81fbb432

 \ /
  Last update: 2021-01-23 17:30    [W:0.076 / U:0.076 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site