fexecve

FEXECVE(3)                  Linux Programmer's Manual                 FEXECVE(3)



NAME
       fexecve - execute program specified via file descriptor

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 appropriately.

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.

VERSIONS
       fexecve() is implemented since glibc 2.3.2.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface Attribute     Value   │
       ├──────────┼───────────────┼─────────┤
       │fexecve() │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

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.

NOTES
       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 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 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.

SEE ALSO
       execve(2), execveat(2)

COLOPHON
       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2019-10-10                         FEXECVE(3)