Re: [PATCH v2 1/2] vfs: block chmod of symlinks

[Greg KH]

On Tue, Sep 15, 2020 at 08:22:54PM -0400, Rich Felker wrote:
> It was discovered while implementing userspace emulation of fchmodat
> AT_SYMLINK_NOFOLLOW (using O_PATH and procfs magic symlinks; otherwise
> it's not possible to target symlinks with chmod operations) that some
> filesystems erroneously allow access mode of symlinks to be changed,
> but return failure with EOPNOTSUPP (see glibc issue #14578 and commit
> a492b1e5ef). This inconsistency is non-conforming and wrong, and the
> consensus seems to be that it was unintentional to allow link modes to
> be changed in the first place.
> 
> Signed-off-by: Rich Felker <dalias@xxxxxxxx>
> ---
>  fs/open.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 9af548fb841b..cdb7964aaa6e 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -570,6 +570,12 @@ int chmod_common(const struct path *path, umode_t mode)
>  	struct iattr newattrs;
>  	int error;
>  
> +	/* Block chmod from getting to fs layer. Ideally the fs would either
> +	 * allow it or fail with EOPNOTSUPP, but some are buggy and return
> +	 * an error but change the mode, which is non-conforming and wrong. */
> +	if (S_ISLNK(inode->i_mode))
> +		return -EOPNOTSUPP;

I still fail to understand why these "buggy" filesystems can not be
fixed.  Why are you papering over a filesystem-specific-bug with this
core kernel change that we will forever have to keep?

thanks,

greg k-h