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getcwd.3: Note behavior for unreachable current working directory 

For the code that does this, see fs/dcache.c, search for
"prepend_unreachable".

Test case to demonstrate:

$ cat getcwd.c

int main(void) {
  unshare(CLONE_NEWUSER | CLONE_NEWNS);
  chdir("/usr");
  chroot("bin");

  printf("current directory: \"%s\"\n", get_current_dir_name());

  char *real = realpath(".", NULL);
  printf("realpath of .: \"%s\"\n", real ? real : "{none}");
  real = realpath("../home/jann/.ssh", NULL);
  printf("realpath of path: \"%s\"\n", real ? real : "{none}");

  return 0;
}
$ cat getcwd_test.c

int main(void) {
  unshare(CLONE_NEWUSER | CLONE_NEWNS);
  chdir("/usr");
  chroot("bin");
  printf("current directory: \"%s\"\n", get_current_dir_name());
  return 0;
}
$ gcc -o getcwd_test getcwd_test.c -Wall
$ ./getcwd_test
current directory: "(unreachable)/usr"

realpath.3 doesn't currently seem to handle this
case in a sane way, so I'm not going to document
its behavior yet. I'll report that as a bug instead.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Jann Horn 2015-04-05 17:44:48 +02:00 committed by Michael Kerrisk
parent a405066e3d
commit a2ac97c78b
1 changed files with 13 additions and 0 deletions
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13
man3/getcwd.3
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@ -80,6 +80,19 @@ The pathname is returned as the function result and via the argument
.IR buf ,
if present.
If the current directory is not below the root directory of the current
process (e.g. because the process set a new filesystem root
using
.BR chroot (2)
without changing its current directory into the new root), the returned
path will be prefixed with the string "(unreachable)". Such behavior can
also be caused by an unprivileged user by changing the current directory
into another mount namespace.
When dealing with paths from untrusted sources, callers of these
functions should consider checking whether the returned path starts
with '/' or '(' to avoid misinterpreting an unreachable path
as a relative path.
The
.BR getcwd ()
function copies an absolute pathname of the current working directory