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 5.03 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)