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SubjectRe: [PATCH v2 1/2] vfs: block chmod of symlinks
On Wed, Sep 16, 2020 at 08:18:15AM +0200, Greg KH wrote:
> 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@libc.org>
> > ---
> > 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?

Because checking this once in the VFS is much saner than trying to
patch up a gazillion file systems.

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