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[RFC] Possible new execveat(2) Linux syscall


Over at the LKML[1] we've been discussing a possible new syscall, execveat(2),
and it would be good to hear a glibc perspective about it (and whether there
are any interface changes that would make it easier to use from userspace).

The syscall prototype is:
  int execveat(int fd, const char *pathname,
                      char *const argv[],  char *const envp[],
                      int flags); /* AT_EMPTY_PATH, AT_SYMLINK_NOFOLLOW */
and it works similarly to execve(2) except:
 - the executable to run is identified by the combination of fd+pathname, like
   other *at(2) syscalls
 - there's an extra flags field to control behaviour.
(I've attached a text version of the suggested man page below)

One particular benefit of this is that it allows an fexecve(3) implementation
that doesn't rely on /proc being accessible, which is useful for sandboxed
applications.  (However, that does only work for non-interpreted programs:
the name passed to a script interpreter is of the form "/dev/fd/<fd>/<path>"
or "/dev/fd/<fd>", so the executed interpreter will normally still need /proc
access to load the script file).

How does this sound from a glibc perspective?


[1], with earlier discussions at,


EXECVEAT(2)              Linux Programmer's Manual             EXECVEAT(2)

       execveat - execute program relative to a directory file descriptor

       #include <unistd.h>

       int execveat(int fd, const char *pathname,
                    char *const argv[],  char *const envp[],
                    int flags);

       The  execveat()  system call executes the program pointed to by the
       combination of fd and pathname.  The execveat() system  call  operâ
       ates  in  exactly the same way as execve(2), except for the differâ
       ences described in this manual page.

       If the pathname given in pathname is relative, then  it  is  interâ
       preted relative to the directory referred to by the file descriptor
       fd (rather than relative to the current working  directory  of  the
       calling process, as is done by execve(2) for a relative pathname).

       If  pathname is relative and fd is the special value AT_FDCWD, then
       pathname is interpreted relative to the current  working  directory
       of the calling process (like execve(2)).

       If pathname is absolute, then fd is ignored.

       If pathname is an empty string and the AT_EMPTY_PATH flag is speciâ
       fied, then the file descriptor fd specifies the  file  to  be  exeâ

       flags can either be 0, or include the following flags:

              If pathname is an empty string, operate on the file referred
              to by fd (which may have been  obtained  using  the  open(2)
              O_PATH flag).

              If  the  file  identified by fd and a non-NULL pathname is a
              symbolic link, then the call fails with the error EINVAL.

       On success, execveat() does not return. On error  -1  is  returned,
       and errno is set appropriately.

       The  same  errors  that  occur  for  execve(2)  can  also occur for
       execveat().   The  following  additional  errors  can   occur   for

       EBADF  fd is not a valid file descriptor.

       ENOENT The  program  identified by fd and pathname requires the use
              of an interpreter program (such as a  script  starting  with
              "#!")  but  the  file  descriptor  fd  was  opened  with the
              O_CLOEXEC flag and so the program file  is  inaccessible  to
              the launched interpreter.

       EINVAL Invalid flag specified in flags.

              pathname  is  relative and fd is a file descriptor referring
              to a file other than a directory.

       execveat() was added to Linux in kernel 3.???.

       In addition to the reasons explained in openat(2),  the  execveat()
       system call is also needed to allow fexecve(3) to be implemented on
       systems that do not have the /proc filesystem mounted.

       execve(2), fexecve(3)

Linux                           2014-04-02                     EXECVEAT(2)

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