fexecve(3) - Linux manual page (original) (raw)

fexecve(3) Library Functions Manual fexecve(3)

NAME top

   fexecve - execute program specified via file descriptor

LIBRARY top

   Standard C library (_libc_, _-lc_)

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)](../man2/execve.2.html), 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](../man3/errno.3.html)_ is set
   to indicate the error.

ERRORS top

   Errors are as for [execve(2)](../man2/execve.2.html), 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)](../man2/execveat.2.html) system call,
          and the _/proc_ filesystem could not be accessed.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌─────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├─────────────────────────────────────┼───────────────┼─────────┤
   │ **fexecve**()                           │ Thread safety │ MT-Safe │
   └─────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   POSIX.1-2008.

HISTORY top

   glibc 2.3.2.

   On Linux with glibc versions 2.26 and earlier, **fexecve**() is
   implemented using the [proc(5)](../man5/proc.5.html) 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)](../man2/execveat.2.html) system call,
   then **fexecve**() is implemented using that system call, with the
   benefit that _/proc_ does not need to be mounted.

NOTES top

   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)](../man2/execve.2.html) 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)](../man2/execve.2.html), [execveat(2)](../man2/execveat.2.html)

COLOPHON top

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Linux man-pages 6.9.1 2024-05-02 fexecve(3)

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