fexecve(3) Library Functions Manual fexecve(3)
NAME
fexecve - execute program specified via file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int fexecve(int fd, char *const argv[], char *const envp[]);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fexecve():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
DESCRIPTION
fexecve() performs the same task as execve(2), with the difference that
the file to be executed is specified via a file descriptor, fd, 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.
RETURN VALUE
A successful call to fexecve() never returns. On error, the function
does return, with a result value of -1, and errno is set to indicate
the error.
ERRORS
Errors are as for execve(2), with the following additions:
EINVAL fd is not a valid file descriptor, or argv is NULL, or envp is
NULL.
ENOENT The close-on-exec flag is set on fd, and fd refers to a script.
See BUGS.
ENOSYS The kernel does not provide the execveat(2) system call, and the
/proc filesystem could not be accessed.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ fexecve() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
glibc 2.3.2.
On Linux with glibc versions 2.26 and earlier, fexecve() is implemented
using the proc(5) filesystem, so /proc needs to be mounted and avail-
able at the time of the call. Since glibc 2.27, if the underlying ker-
nel supports the execveat(2) system call, then fexecve() is implemented
using that system call, with the benefit that /proc does not need to be
mounted.
NOTES
The idea behind 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 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 to 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
on 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.)
BUGS
If 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 char-
acters #!) and the close-on-exec flag has been set for fd, then fex-
ecve() fails with the error ENOENT. This error occurs because, by the
time the script interpreter is executed, fd has already been closed be-
cause of the close-on-exec flag. Thus, the close-on-exec flag can't be
set on fd if it refers to a script, leading to the problems described
in NOTES.
SEE ALSO
execve(2), execveat(2)
Linux man-pages 6.7 2023-10-31 fexecve(3)
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