Re: [PATCH] new syscall: flink

[H. Peter Anvin]

Followup to:  <b6qo2a$ecl$[e-mail]>
By author:    "H. Peter Anvin" <[e-mail]>
In newsgroup: linux.dev.kernel
> 
> This, I believe, is the real issue.  However, we already have that
> problem:
> 

[Sample code]

Here is a better piece of sample code that actually shows a
permissions violation happening:

[...]
mkdir("testdir", 0700)                  = 0
open("testdir/testfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
write(3, "Ansiktsburk\n", 12)           = 12
close(3)                                = 0
open("testdir/testfile", O_RDONLY)      = 3
chmod("testdir", 0)                     = 0
open("/proc/self/fd/3", O_RDWR)         = 4
write(4, "Tjo fidelittan hatt!\n", 21)  = 21
exit_group(0)                           = ?

----snip----

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

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

  mkdir("testdir", 0700);

  /* Create the file legitimately */
  sfd = open("testdir/testfile", O_WRONLY|O_CREAT|O_TRUNC, 0666);
  write(sfd, "Ansiktsburk\n", 12);
  close(sfd);

  /* Open read-only file descriptor */
  rfd = open("testdir/testfile", O_RDONLY);

  /* Make directory inaccessible */
  chmod("testdir", 0);

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

  /* Clobber file */
  write(wfd, "Tjo fidelittan hatt!\n", 21);

  return 0;
}
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