Re: [Qemu-devel] [for-2.10 PATCH v2] 9pfs: local: fix fchmodat_nofollow() limitations

[Michael Tokarev]

09.08.2017 17:23, Greg Kurz wrote:
> This function has to ensure it doesn't follow a symlink that could be used
> to escape the virtfs directory. This could be easily achieved if fchmodat()
> on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but
> it doesn't.
> 
> The current implementation covers most use-cases, but it notably fails if:
> - the target path has access rights equal to 0000 (openat() returns EPERM),
>   => once you've done chmod(0000) on a file, you can never chmod() again
> - the target path is UNIX domain socket (openat() returns ENXIO)
>   => bind() of UNIX domain sockets fails if the file is on 9pfs
> 
> The solution is to use O_PATH: openat() now succeeds in both cases, and we
> can ensure the path isn't a symlink with fstat(). The associated entry in
> "/proc/self/fd" can hence be safely passed to the regular chmod() syscall.

How we can ensure the path isn't a symlink using fstat() ?

As far as I understand, fstat NEVER, EVER will return S_ISLINK, because
we can't actually "open" a symlink itsef, only the target of the symlink.

/mjt