Section (3) fexecve
fexecve — execute program specified via file descriptor
|char *const argv,|
|char *const envp
fexecve() performs the same
task as execve(2), with the
difference that the file to be executed is specified via a
rather than via a pathname. The file descriptor
fd must be opened read-only
O_RDONLY) or with the
O_PATH flag and the caller must
have permission to execute the file that it refers to.
A successful call to
fexecve() never returns. On error, the
function does return, with a result value of −1, and
errno is set appropriately.
Errors are as for execve(2), with the following additions:
fdis not a valid file descriptor, or
argvis NULL, or
The close-on-exec flag is set on
fdrefers to a script. See BUGS.
The kernel does not provide the execveat(2) system call, and the
/procfilesystem could not be accessed.
For an explanation of the terms used in this section, see attributes(7).
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.
On Linux with glibc versions 2.26 and earlier,
fexecve() is implemented using
the proc(5) filesystem, so
/proc needs to be mounted and
available at the time of the call. Since glibc 2.27, if the
underlying kernel supports the execveat(2) system call,
fexecve() is implemented
using that system call, with the benefit that
/proc does not need to be mounted.
The idea behind
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 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).
fexecve() does not mitigate the
problem that the
contents of a
file could be changed between the checksumming and the call
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
fexecve() is to set the close-on-exec flag
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
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.)
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
#!) and the
close-on-exec flag has been set for
fexecve() fails with the error ENOENT. This error occurs because, by the
time the script interpreter is executed,
fd has already been closed
because of the close-on-exec flag. Thus, the close-on-exec
flag can_zsingle_quotesz_t be set on
fd if it refers to a script,
leading to the problems described in NOTES.
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Copyright (c) 2006, 2014, Michael Kerrisk
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