[PATCH v2 0/3] Support opening a symlink with O_PATH | O_NOFOLLOW

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Jan 13 15:28:00 GMT 2020

Hi Ken,

On Dec 29 17:56, Ken Brown wrote:
> Currently, opening a symlink with O_NOFOLLOW fails with ELOOP.
> Following Linux, the first patch in this series allows the call to
> succeed if O_PATH is also specified.
> According to the Linux man page for 'open', the file descriptor
> returned by the call should be usable as the dirfd argument in calls
> to fstatat and readlinkat with an empty pathname, to have
> the calls operate on the symbolic link.  The second and third patches
> achieve this.  For fstatat, we do this by adding support
> for the AT_EMPTY_PATH flag.
> Note: The man page mentions fchownat and linkat also.  linkat already
> supports the AT_EMPTY_PATH flag, so nothing needs to be done.  But I
> don't understand how this could work for fchownat, because fchown
> fails with EBADF if its fd argument was opened with O_PATH.  So I
> haven't touched fchownat.

It was never supposed to work that way.  We can make fchownat work
with AT_EMPTY_PATH, but using it on a file opened with O_PATH
contradicts the Linux open(2) man page, afaics:

 O_PATH (since Linux 2.6.39)
  Obtain a file descriptor that can be used for two  purposes:  to
  indicate a location in the filesystem tree and to perform opera‐
  tions that act purely at the file descriptor  level.   The  file
  itself  is not opened, and other file operations (e.g., read(2),
  write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
  fail with the error EBADF.
  ^^^^^^^^^           ^^^^^

That'd from the current F31 man pages.

> Am I missing something?

Good question.  Let me ask in return, did *I* now miss something?


Corinna Vinschen
Cygwin Maintainer
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