On Fri, Feb 21, 2020 at 12:21:42PM +1100, Aleksa Sarai wrote: > On 2020-02-13, Ross Zwisler <zwisler@xxxxxxxxxx> wrote: > > On Thu, Feb 06, 2020 at 12:10:45PM -0700, Ross Zwisler wrote: <> > > > As far as I can tell, SB_SUBMOUNT doesn't actually have any dependence on > > > MS_SUBMOUNT. Nothing ever sets or checks MS_SUBMOUNT from within the kernel, > > > and whether or not it's set from userspace has no bearing on how SB_SUBMOUNT > > > is used. SB_SUBMOUNT is set independently inside of the kernel in > > > vfs_submount(). > > > > > > I agree that their association seems to be historical, introduced in this > > > commit from David Howells: > > > > > > e462ec50cb5fa VFS: Differentiate mount flags (MS_*) from internal superblock flags > > > > > > In that commit message David notes: > > > > > > (1) Some MS_* flags get translated to MNT_* flags (such as MS_NODEV -> > > > MNT_NODEV) without passing this on to the filesystem, but some > > > filesystems set such flags anyway. > > > > > > I think this is sort of what we are trying to do with MS_NOSYMFOLLOW: have a > > > userspace flag that translates to MNT_NOSYMFOLLOW, but which doesn't need an > > > associated SB_* flag. Is it okay to reclaim the bit currently owned by > > > MS_SUBMOUNT and use it for MS_NOSYMFOLLOW. > > > > > > A second option would be to choose one of the unused MS_* values from the > > > middle of the range, such as 256 or 512. Looking back as far as git will let > > > me, I don't think that these flags have been used for MS_* values at least > > > since v2.6.12: > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/fs.h?id=1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 > > > > > > I think maybe these used to be S_WRITE and S_APPEND, which weren't filesystem > > > mount flags? > > > > > > https://sites.uclouvain.be/SystInfo/usr/include/sys/mount.h.html > > > > > > A third option would be to create this flag using the new mount system: > > > > > > https://lwn.net/Articles/753473/ > > > https://lwn.net/Articles/759499/ > > > > > > My main concern with this option is that for Chrome OS we'd like to be able to > > > backport whatever solution we come up with to a variety of older kernels, and > > > if we go with the new mount system this would require us to backport the > > > entire new mount system to those kernels, which I think is infeasible. > > > > > > David, what are your thoughts on this? Of these three options for supporting > > > a new MS_NOSYMFOLLOW flag: > > > > > > 1) reclaim the bit currently used by MS_SUBMOUNT > > > 2) use a smaller unused value for the flag, 256 or 512 > > > 3) implement the new flag only in the new mount system > > > > > > do you think either #1 or #2 are workable? If so, which would you prefer? > > > > Gentle ping on this - do either of the options using the existing mount API > > seem possible? Would it be useful for me to send out example patches in one > > of those directions? Or is it out of the question, and I should spend my time > > on making patches using the new mount system? Thanks! > > I think (1) or (2) sound reasonable, but I'm not really the right person > to ask. Cool, I appreciate the feedback. :) I'll go ahead and implement #2 and send it out, along with example man page updates.
