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Date: Thu, 10 Sep 2020 15:23:40 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Rich Felker' <dalias@...c.org>, "linux-api@...r.kernel.org" <linux-api@...r.kernel.org> CC: Alexander Viro <viro@...iv.linux.org.uk>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: RE: [PATCH] vfs: add fchmodat2 syscall From: Rich Felker > Sent: 10 September 2020 15:24 ... > index 9af548fb841b..570a21f4d81e 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -610,15 +610,30 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode) > return err; > } > > -static int do_fchmodat(int dfd, const char __user *filename, umode_t mode) > +static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int flags) > { > struct path path; > int error; > unsigned int lookup_flags = LOOKUP_FOLLOW; > + > + if (flags & AT_SYMLINK_NOFOLLOW) > + lookup_flags &= ~LOOKUP_FOLLOW; > + if (flags & ~AT_SYMLINK_NOFOLLOW) > + return -EINVAL; I think I'd swap over those two tests. So unsupported flags are clearly errored. > retry: > error = user_path_at(dfd, filename, lookup_flags, &path); > if (!error) { > - error = chmod_common(&path, mode); > + /* 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. > + * Userspace emulation of AT_SYMLINK_NOFOLLOW in > + * glibc and musl blocked it too, for same reason. */ > + if (S_ISLNK(path.dentry->d_inode->i_mode) > + && (flags & AT_SYMLINK_NOFOLLOW)) > + error = -EOPNOTSUPP; Again swap the order of the tests. I think it reads better as: if ((flags & AT_SYMLINK_NOFOLLOW) && S_ISLNK(path.dentry->d_inode->i_mode)) error = -EOPNOTSUPP; As well as saving a few clock cycles. > + else > + error = chmod_common(&path, mode); > path_put(&path); > if (retry_estale(error, lookup_flags)) { > lookup_flags |= LOOKUP_REVAL; ... David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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