fexecve - execute program specified via file descriptor
int fexecve(int fd, char *const argv, char *const envp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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.
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:
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
fexecve() is implemented since glibc 2.3.2.
For an explanation of the terms used in this section, see attributes(7).
- fd is not a valid file descriptor, or argv is NULL, or
envp is NULL.
- The /proc filesystem could not be accessed.
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, then fexecve() is
implemented using that system call, with the benefit that /proc does
not need to be mounted.
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
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 characters
#!) and the close-on-exec flag has been set for fd, then
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't be set on fd if it refers to a script, leading to the problems
described in NOTES.
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