fexecve(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       fexecve - execute program specified via file descriptor

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       fexecve() is implemented since glibc 2.3.2.

ATTRIBUTES         top

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

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

CONFORMING TO         top

       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         top

       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         top

       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         top

       execve(2), execveat(2)

COLOPHON         top

       This page is part of release 5.13 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                          2021-03-22                     FEXECVE(3)

Pages that refer to this page: execve(2)execveat(2)open(2)exec(3)posix_spawn(3)signal-safety(7)