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; Our usualy place for this would be setattr_prepare. Also the comment style is off, and I don't think we should talk about buggy file systems here, but a policy to not allow the chmod. I also suspect the right error value is EINVAL - EOPNOTSUPP isn't really used in normal posix file system interfaces.
