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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.

       ENOSYS 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).

       ┌──────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────┼───────────────┼─────────┤
       │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

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