From 9a6ff5bc53dedbaa601a1a76cbaf8a76afd60c9f Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Mon, 15 Apr 2024 21:41:06 +0200 Subject: Adding upstream version 6.7. Signed-off-by: Daniel Baumann --- man5/proc_pid_fd.5 | 161 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 161 insertions(+) create mode 100644 man5/proc_pid_fd.5 (limited to 'man5/proc_pid_fd.5') diff --git a/man5/proc_pid_fd.5 b/man5/proc_pid_fd.5 new file mode 100644 index 0000000..0489169 --- /dev/null +++ b/man5/proc_pid_fd.5 @@ -0,0 +1,161 @@ +.\" Copyright (C) 1994, 1995, Daniel Quinlan +.\" Copyright (C) 2002-2008, 2017, Michael Kerrisk +.\" Copyright (C) 2023, Alejandro Colomar +.\" +.\" SPDX-License-Identifier: GPL-3.0-or-later +.\" +.TH proc_pid_fd 5 2023-08-15 "Linux man-pages 6.7" +.SH NAME +/proc/pid/fd/ \- file descriptors +.SH DESCRIPTION +.TP +.IR /proc/ pid /fd/ +This is a subdirectory containing one entry for each file which the +process has open, named by its file descriptor, and which is a +symbolic link to the actual file. +Thus, 0 is standard input, 1 standard output, 2 standard error, and so on. +.IP +For file descriptors for pipes and sockets, +the entries will be symbolic links whose content is the +file type with the inode. +A +.BR readlink (2) +call on this file returns a string in the format: +.IP +.in +4n +.EX +type:[inode] +.EE +.in +.IP +For example, +.I socket:[2248868] +will be a socket and its inode is 2248868. +For sockets, that inode can be used to find more information +in one of the files under +.IR /proc/net/ . +.IP +For file descriptors that have no corresponding inode +(e.g., file descriptors produced by +.BR bpf (2), +.BR epoll_create (2), +.BR eventfd (2), +.BR inotify_init (2), +.BR perf_event_open (2), +.BR signalfd (2), +.BR timerfd_create (2), +and +.BR userfaultfd (2)), +the entry will be a symbolic link with contents of the form +.IP +.in +4n +.EX +.RI anon_inode: file-type +.EE +.in +.IP +In many cases (but not all), the +.I file-type +is surrounded by square brackets. +.IP +For example, an epoll file descriptor will have a symbolic link +whose content is the string +.IR "anon_inode:[eventpoll]" . +.IP +.\"The following was still true as at kernel 2.6.13 +In a multithreaded process, the contents of this directory +are not available if the main thread has already terminated +(typically by calling +.BR pthread_exit (3)). +.IP +Programs that take a filename as a command-line argument, +but don't take input from standard input if no argument is supplied, +and programs that write to a file named as a command-line argument, +but don't send their output to standard output +if no argument is supplied, can nevertheless be made to use +standard input or standard output by using +.IR /proc/ pid /fd +files as command-line arguments. +For example, assuming that +.I \-i +is the flag designating an input file and +.I \-o +is the flag designating an output file: +.IP +.in +4n +.EX +.RB "$" " foobar \-i /proc/self/fd/0 \-o /proc/self/fd/1 ..." +.EE +.in +.IP +and you have a working filter. +.\" The following is not true in my tests (MTK): +.\" Note that this will not work for +.\" programs that seek on their files, as the files in the fd directory +.\" are not seekable. +.IP +.I /proc/self/fd/N +is approximately the same as +.I /dev/fd/N +in some UNIX and UNIX-like systems. +Most Linux MAKEDEV scripts symbolically link +.I /dev/fd +to +.IR /proc/self/fd , +in fact. +.IP +Most systems provide symbolic links +.IR /dev/stdin , +.IR /dev/stdout , +and +.IR /dev/stderr , +which respectively link to the files +.IR 0 , +.IR 1 , +and +.I 2 +in +.IR /proc/self/fd . +Thus the example command above could be written as: +.IP +.in +4n +.EX +.RB "$" " foobar \-i /dev/stdin \-o /dev/stdout ..." +.EE +.in +.IP +Permission to dereference or read +.RB ( readlink (2)) +the symbolic links in this directory is governed by a ptrace access mode +.B PTRACE_MODE_READ_FSCREDS +check; see +.BR ptrace (2). +.IP +Note that for file descriptors referring to inodes +(pipes and sockets, see above), +those inodes still have permission bits and ownership information +distinct from those of the +.IR /proc/ pid /fd +entry, +and that the owner may differ from the user and group IDs of the process. +An unprivileged process may lack permissions to open them, as in this example: +.IP +.in +4n +.EX +.RB "$" " echo test | sudo \-u nobody cat" +test +.RB "$" " echo test | sudo \-u nobody cat /proc/self/fd/0" +cat: /proc/self/fd/0: Permission denied +.EE +.in +.IP +File descriptor 0 refers to the pipe created by the shell +and owned by that shell's user, which is not +.IR nobody , +so +.B cat +does not have permission +to create a new file descriptor to read from that inode, +even though it can still read from its existing file descriptor 0. +.SH SEE ALSO +.BR proc (5) -- cgit v1.2.3