fexecve (3)

#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:

_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L

Before glibc 2.10:

_GNU_SOURCE

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, 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:

EINVAL

fd is not a valid file descriptor, or argv is NULL, or envp is NULL.

ENOSYS

The /proc filesystem could not be accessed.

fexecve() is implemented since glibc 2.3.2.

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, fexecve() is implemented using the proc(5) filesystem, so /proc needs to be mounted and available at the time of the call.

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

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.

execve(2)

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