Re: [PATCH] new syscall: flink

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <1049663559.1602.46.camel@dhcp22.swansea.linux.org.uk>
By author:    Alan Cox <alan@lxorguk.ukuu.org.uk>
In newsgroup: linux.dev.kernel
> 
> It is actually rather more complicated. Suppose I give you a pipe
> pair handle. You can flink what was a private object and has no
> meaning as a name.
> 
> Suppose I give you a socket what does the call man ?
> 
> Suppose I give you a handle to an anonymous mapping ?
> 

-EXDEV

> Suppose I give you a handle to data, how do you know what disk
> it belongs to ?

f_ino->i_sb should give you that information.

> Suppose I give you an O_RDONLY handle to a file which you then
> flink and gain write access too ?

This, I believe, is the real issue.  However, we already have that
problem:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <limits.h>

int main(int argc, char *argv[])
{
  int rfd, wfd;
  char filebuf[PATH_MAX];

  rfd = open("testfile", O_RDONLY|O_CREAT, 0666);
  /* Now rfd is a read-only file descriptor */

  sprintf(filebuf, "/proc/self/fd/%d", rfd);
  wfd = open(filebuf, O_RDWR);
  /* Now wfd is a read-write file descriptor */

  write(wfd, "Tjo fidelittan hatt!\n", 21);

  return 0;
}


	-hpa
-- 
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