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SubjectRe: [PATCH 0/6 v1 resend] statfs: handle mount propagation
On Fri, May 25, 2018 at 02:48:19PM +0200, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@ubuntu.com>
>
> Hey,
>
> This is v1 of this patchset. All changes from v0 to v1 are non-functional.
> Specifically, the commit messages and justification have been extended as
> requested by Linus and Al.

Hey everyone,

Just a ping whether the requested changes have made this series suitable
for inclusion. Would be excellent if someone could do another review. :)

Thanks!
Christian


>
> This little series does the following:
>
> - unify the definition of constants in statfs.h and fs.h:
> The definitions for MS_* flags are currently a mixture between hex values
> and bit-shifts. All higher values are already initialized with bit-shifts
> for MS_* constants starting with (1<<16). This patch switches the
> definitions for MS_* constants over to uniformly use bit-shifts and
> alignes the definitions of ST_* flags too.
> Initializing them identically let's userspace easily detect when flags
> indicate the same property but use a different value in doing so.
>
> - extend statfs to handle mount propagation:
> For all cases the only way to do this right now is by parsing
> /proc/<pid>/mountinfo. Yes, it is doable but still it is somewhat costly
> and annoying as e.g. those mount propagation fields are optional.
> 1. prevent propagation from happening:
> From a userspace perspective we often run into the case where we
> simply want to know whether a given mountpoint is MS_SHARED or is
> MS_SLAVE. If it is we remount it as MS_PRIVATE to prevent any
> propagation from happening. We don't care about the peer
> relationship or how the propagation is exactly setup. We only want
> to prevent any propagation from happening.
> These mountpoints might be known in advance so parsing
> /proc/<pid>/mountinfo should not be needed.
> 2. differentiate between MS_SLAVE and MS_SHARED mountpoints:
> Mountpoints that are MS_SLAVE are kept intact and mountpoints that
> are MS_SHARED are made MS_PRIVATE. These mountpoint might be known in
> advance so parsing /proc/<pid>/mountinfo should not be needed.
> 3. retrieve propagation properties when procfs is not mounted:
> When the set of interesting mountpoints is known and /proc is not
> mounted calling statfs() is the only good way to reliably determine
> the propagation property of a mountpoint.
> 4. inspecting file descriptors to mountpoints for propagation
> properties:
> When file descriptors to mountpoints are passed around between
> processes it is useful to have fstatvfs() handle mount propagation
> properties too.
> To this end the flags:
> - ST_UNBINDABLE
> - ST_SHARED
> - ST_PRIVATE
> - ST_SLAVE
> are added. They have the same value as their MS_* counterparts.
>
> - Testing:
> I verified that now userspace can do e.g.
>
> int ret;
> char *s = "/some/path";
> struct statvfs sb;
>
> ret = statvfs(s, &sb);
> if (ret < 0)
> return false;
>
> if (sb.f_flag & ST_SHARED) {
> ret = mount("", s, NULL, MS_SLAVE | MS_REC, NULL);
> if (ret < 0)
> return -1;
> }
>
> Thanks!
> Christian
>
> Christian Brauner (6):
> fs: use << for MS_* flags
> statfs: use << to align with fs header
> statfs: add ST_UNBINDABLE
> statfs: add ST_SHARED
> statfs: add ST_SLAVE
> statfs: add ST_PRIVATE
>
> fs/statfs.c | 16 +++++++++++++++-
> include/linux/statfs.h | 30 +++++++++++++++++-------------
> include/uapi/linux/fs.h | 33 +++++++++++++++++----------------
> 3 files changed, 49 insertions(+), 30 deletions(-)
>
> --
> 2.17.0
>

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