execve.2, fexecve.3: SEE ALSO: add execveat(2)

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

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.TH FEXECVE 3 2014-04-20 "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 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.

If
.I fd
is a file descriptor that refers to an interpreter script
and has been marked as close-on-exec (see the discussion of the
.BR FD_CLOEXEC
in
.BR fcntl (2)),
.BR fexecve ()
will fail to execute the script, since,
by the time the script interpreter tries to access the script file,
.I fd
has already been closed.

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.
.SH SEE ALSO
.BR execve (2),
.BR execveat (2)