On Wed, Aug 12, 2020 at 12:36 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > On Tue, Aug 11, 2020 at 04:28:03PM -0600, Ross Zwisler wrote: > > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h > > index 96a0240f23fed..dd8306ea336c1 100644 > > --- a/include/uapi/linux/mount.h > > +++ b/include/uapi/linux/mount.h > > @@ -16,6 +16,7 @@ > > #define MS_REMOUNT 32 /* Alter flags of a mounted FS */ > > #define MS_MANDLOCK 64 /* Allow mandatory locks on an FS */ > > #define MS_DIRSYNC 128 /* Directory modifications are synchronous */ > > +#define MS_NOSYMFOLLOW 256 /* Do not follow symlinks */ > > #define MS_NOATIME 1024 /* Do not update access times. */ > > #define MS_NODIRATIME 2048 /* Do not update directory access times */ > > #define MS_BIND 4096 > > Does this need to be added to MS_RMT_MASK too? I don't think so, because IIUC that is only for "superblock flags", i.e. flags which are part of the sb_flags definition in do_mount()/path_mount()? https://github.com/torvalds/linux/blob/master/fs/namespace.c#L3172 With the current code I'm able to remount and flip nosymfollow both directions without issue. Similarly, MS_NOEXEC can be turned on and off at will, and it's not part of MS_RMT_MASK nor sb_flags. Interestingly, though, if you change MS_RMT_MASK to be 0, I would expect us to be unable to twiddle all the flags which are currently part of it, but that isn't the case. I was still able to change between RO/RW, and turn on lazytime. So, I think this flag is working as expected, but that there is probably a bug in there somewhere WRT the handling of MS_RMT_MASK.
