dlinfo.3: ATTRIBUTES: Note function that is thread-safe

[thirdparty/man-pages.git] / man3 / fexecve.3

.\" Copyright (c) 2006, 2014, Michael Kerrisk
.\"
.\" %%%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.\" manual page may be incorrect or out-of-date.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 FEXECVE 3 2015-07-23 "Linux" "Linux Programmer's Manual"
.SH NAME
fexecve \- execute program specified via file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int fexecve(int " fd ", char *const " argv "[], char *const " envp []);
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR fexecve ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_GNU_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
.BR fexecve ()
performs the same task as
.BR execve (2),
with the difference that the file to be executed
is specified via a file descriptor,
.IR fd ,
rather than via a pathname.
The file descriptor
.I fd
must be opened read-only,
and the caller must have permission to execute the file that it refers to.
.\" POSIX.1-2008 specifies the O_EXEC flag for open as an alternative,
.\" but Linux doesn't support this flag yet.
.SH RETURN VALUE
A successful call to
.BR fexecve ()
never returns.
On error, the function does return, with a result value of \-1, and
.I errno
is set appropriately.
.SH ERRORS
Errors are as for
.BR execve (2),
with the following additions:
.TP
.B EINVAL
.I fd
is not a valid file descriptor, or
.I argv
is NULL, or
.I envp
is NULL.
.TP
.B ENOSYS
The
.I /proc
filesystem could not be accessed.
.SH VERSIONS
.BR fexecve ()
is implemented since glibc 2.3.2.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR fexecve ()
T}	Thread safety	MT-Safe
.TE

.SH CONFORMING TO
POSIX.1-2008.
This function is not specified in POSIX.1-2001,
and is not widely available on other systems.
It is specified in POSIX.1-2008.
.SH NOTES
On Linux,
.BR fexecve ()
is implemented using the
.BR proc (5)
filesystem, so
.I /proc
needs to be mounted and available at the time of the call.
.\" FIXME .
.\" With the addition of the execveat(2), fexecve() can be implemented
.\" even where /proc is unavailable. Review future glibc releases to
.\" see if the implementation is changed to use execveat(2).

The idea behind
.BR fexecve ()
is to allow the caller to verify (checksum) the contents of
an executable before executing it.
Simply opening the file, checksumming the contents, and then doing an
.BR execve (2)
would not suffice, since, between the two steps, the filename,
or a directory prefix of the pathname, could have been exchanged
(by, for example, modifying the target of a symbolic link).
.BR fexecve ()
does not mitigate the problem that the
.I contents
of a file could be changed between the checksumming and the call to
.BR fexecve ();
for that, the solution is to ensure that the permissions on the file
prevent it from being modified by malicious users.

The natural idiom when using
.BR fexecve ()
is to set the close-on-exec flag on
.IR fd ,
so that the file descriptor does not leak through to the program
that is executed.
This approach is natural for two reasons.
First, it prevents file descriptors being consumed unnecessarily.
(The executed program normally has no need of a file descriptor
that refers to the program itself.)
Second, if
.BR fexecve ()
is used recursively,
employing the close-on-exec flag prevents the file descriptor exhaustion
that would result from the fact that each step in the recursion would
cause one more file descriptor to be passed to the new program.
(But see BUGS.)
.SH BUGS
If
.I fd
refers to a script (i.e., it is an executable text file that names
a script interpreter with a first line that begins with the characters
.IR #! )
and the close-on-exec flag has been set for
.IR fd ,
then
.BR fexecve ()
fails with the error
.BR ENOENT .
This error occurs because,
by the time the script interpreter is executed,
.I fd
has already been closed because of the close-on-exec flag.
Thus, the close-on-exec flag can't be set on
.I fd
if it refers to a script, leading to the problems described in NOTES.
.SH SEE ALSO
.BR execve (2),
.BR execveat (2)