mirror of https://github.com/mkerrisk/man-pages
233 lines
5.8 KiB
Groff
233 lines
5.8 KiB
Groff
.\" Copyright (c) 2014 Google, Inc., written by David Drysdale
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.\" and Copyright (c) 2015, Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH EXECVEAT 2 2015-01-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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execveat \- execute program relative to a directory file descriptor
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.SH SYNOPSIS
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.B #include <unistd.h>
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.sp
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.BI "int execveat(int " dirfd ", const char *" pathname ","
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.br
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.BI " char *const " argv "[], char *const " envp "[],"
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.br
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.BI " int " flags );
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.SH DESCRIPTION
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.\" commit 51f39a1f0cea1cacf8c787f652f26dfee9611874
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The
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.BR execveat ()
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system call executes the program referred to by the combination of
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.I dirfd
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and
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.IR pathname .
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It operates in exactly the same way as
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.BR execve (2),
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except for the differences described in this manual page.
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If the pathname given in
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.I pathname
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is relative, then it is interpreted relative to the directory
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referred to by the file descriptor
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.I dirfd
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(rather than relative to the current working directory of
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the calling process, as is done by
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.BR execve (2)
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for a relative pathname).
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If
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.I pathname
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is relative and
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.I dirfd
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is the special value
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.BR AT_FDCWD ,
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then
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.I pathname
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is interpreted relative to the current working
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directory of the calling process (like
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.BR execve (2)).
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If
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.I pathname
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is absolute, then
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.I dirfd
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is ignored.
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If
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.I pathname
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is an empty string and the
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.BR AT_EMPTY_PATH
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flag is specified, then the file descriptor
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.I dirfd
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specifies the file to be executed (i.e.,
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.IR dirfd
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refers to an executable file, rather than a directory).
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The
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.I flags
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argument is a bit mask that can include zero or more of the following flags:
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.TP
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.BR AT_EMPTY_PATH
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If
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.I pathname
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is an empty string, operate on the file referred to by
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.IR dirfd
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(which may have been obtained using the
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.BR open (2)
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.B O_PATH
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flag).
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.TP
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.B AT_SYMLINK_NOFOLLOW
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If the file identified by
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.I dirfd
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and a non-NULL
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.I pathname
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is a symbolic link, then the call fails with the error
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.BR ELOOP .
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.SH RETURN VALUE
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On success,
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.BR execveat ()
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does not return.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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The same errors that occur for
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.BR execve (2)
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can also occur for
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.BR execveat ().
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The following additional errors can occur for
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.BR execveat ():
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.TP
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.B EBADF
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.I dirfd
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is not a valid file descriptor.
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.TP
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.B EINVAL
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Invalid flag specified in
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.IR flags .
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.TP
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.B ELOOP
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.I flags
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includes
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.BR AT_SYMLINK_NOFOLLOW
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and the file identified by
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.I dirfd
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and a non-NULL
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.I pathname
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is a symbolic link.
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.TP
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.B ENOENT
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The program identified by
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.I dirfd
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and
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.I pathname
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requires the use of an interpreter program
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(such as a script starting with "#!"), but the file descriptor
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.I dirfd
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was opened with the
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.B O_CLOEXEC
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flag, with the result that
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the program file is inaccessible to the launched interpreter.
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See BUGS.
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.TP
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.B ENOTDIR
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.I pathname
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is relative and
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.I dirfd
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is a file descriptor referring to a file other than a directory.
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.SH VERSIONS
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.BR execveat ()
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was added to Linux in kernel 3.19.
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GNU C library support is pending.
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.\" FIXME . check for glibc support in a future release
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.SH CONFORMING TO
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The
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.BR execveat ()
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system call is Linux-specific.
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.SH NOTES
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In addition to the reasons explained in
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.BR openat (2),
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the
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.BR execveat ()
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system call is also needed to allow
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.BR fexecve (3)
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to be implemented on systems that do not have the
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.I /proc
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filesystem mounted.
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When asked to execute a script file, the
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.IR argv[0]
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that is passed to the script interpreter is a string of the form
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.IR /dev/fd/N
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or
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.IR /dev/fd/N/P ,
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where
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.I N
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is the number of the file descriptor passed via the
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.IR dirfd
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argument.
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A string of the first form occurs when
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.BR AT_EMPTY_PATH
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is employed.
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A string of the second form occurs when the script is specified via both
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.IR dirfd
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and
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.IR pathname ;
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in this case,
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.IR P
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is the value given in
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.IR pathname .
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For the same reasons described in
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.BR fexecve (3),
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the natural idiom when using
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.BR execveat ()
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is to set the close-on-exec flag on
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.IR dirfd .
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(But see BUGS.)
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.SH BUGS
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The
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.B ENOENT
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error described above means that it is not possible to set the
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close-on-exec flag on the file descriptor given to a call of the form:
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execveat(fd, "", argv, envp, AT_EMPTY_PATH);
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However, the inability to set the close-on-exec flag means that a file
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descriptor referring to the script leaks through to the script itself.
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As well as wasting a file descriptor,
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this leakage can lead to file-descriptor exhaustion in scenarios
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where scripts recursively employ
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.BR execveat ().
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.\" For an example, see Michael Kerrisk's 2015-01-10 reply in this LKML
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.\" thread (http://thread.gmane.org/gmane.linux.kernel/1836105/focus=20229):
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.\"
|
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.\" Subject: [PATCHv10 man-pages 5/5] execveat.2: initial man page.\" for execveat(2
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.\" Date: Mon, 24 Nov 2014 11:53:59 +0000
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.SH SEE ALSO
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.BR execve (2),
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.BR openat (2),
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.BR fexecve (3)
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