Re: [PATCH v3 04/38] fs: add mount_setattr()

Christian Brauner
2020-12-02T09:43:11Z

On Tue, Dec 01, 2020 at 11:49:07AM +0100, Christoph Hellwig wrote:

Sorry for not responding to this yesterday. I missed most of your mails
because they have been filtered into a dedicated folder (as they should
be) and I would've looked into that folder but somehow gmail let ~3
mails of you into my general inbox and so I didn't bother...

> Lots of crazy long lines in the patch. Remember that you should only
> go past 80 lines if it clearly improves readability, and I don't
> think it does anywhere in here.

Weird, I did reformat the patch to the 80 char limit and I have dual
display in vim, meaning I have a visible line at 80 chars and 100 chars
whenever I edit a file. I'll go through it again, thanks!

>
> > index a7cd0f64faa4..a5a6c470dc07 100644
> > --- a/fs/internal.h
> > +++ b/fs/internal.h
> > @@ -82,6 +82,14 @@ int may_linkat(struct path *link);
> > /*
> > * namespace.c
> > */
> > +struct mount_kattr {
> > + unsigned int attr_set;
> > + unsigned int attr_clr;
> > + unsigned int propagation;
> > + unsigned int lookup_flags;
> > + bool recurse;
> > +};
>
> Even with the whole series applied this structure is only used in
> namespace.c, so it might be worth moving there.

Good point. Will do.

>
> > +static inline int mnt_hold_writers(struct mount *mnt)
> > {
> > - int ret = 0;
> > -
> > mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
> > /*
> > * After storing MNT_WRITE_HOLD, we'll read the counters. This store
> > @@ -497,15 +495,29 @@ static int mnt_make_readonly(struct mount *mnt)
> > * we're counting up here.
> > */
> > if (mnt_get_writers(mnt) > 0)
> > - ret = -EBUSY;
> > - else
> > - mnt->mnt.mnt_flags |= MNT_READONLY;
> > + return -EBUSY;
> > +
> > + return 0;
> > +}
> > +
> > +static inline void mnt_unhold_writers(struct mount *mnt)
> > +{
> > /*
> > * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
> > * that become unheld will see MNT_READONLY.
> > */
> > smp_wmb();
> > mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
> > +}
> > +
> > +static int mnt_make_readonly(struct mount *mnt)
> > +{
> > + int ret;
> > +
> > + ret = mnt_hold_writers(mnt);
> > + if (!ret)
> > + mnt->mnt.mnt_flags |= MNT_READONLY;
> > + mnt_unhold_writers(mnt);
> > return ret;
> > }
> >
> > @@ -3438,6 +3450,33 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
> > return ret;
> > }
>
> This refactoring seems worth a little prep patch.

Will split into separate patch.

>
> >
> > +static int build_attr_flags(unsigned int attr_flags, unsigned int *flags)
> > +{
> > + unsigned int aflags = 0;
> > +
> > + if (attr_flags & ~(MOUNT_ATTR_RDONLY |
> > + MOUNT_ATTR_NOSUID |
> > + MOUNT_ATTR_NODEV |
> > + MOUNT_ATTR_NOEXEC |
> > + MOUNT_ATTR__ATIME |
> > + MOUNT_ATTR_NODIRATIME))
> > + return -EINVAL;
> > +
> > + if (attr_flags & MOUNT_ATTR_RDONLY)
> > + aflags |= MNT_READONLY;
> > + if (attr_flags & MOUNT_ATTR_NOSUID)
> > + aflags |= MNT_NOSUID;
> > + if (attr_flags & MOUNT_ATTR_NODEV)
> > + aflags |= MNT_NODEV;
> > + if (attr_flags & MOUNT_ATTR_NOEXEC)
> > + aflags |= MNT_NOEXEC;
> > + if (attr_flags & MOUNT_ATTR_NODIRATIME)
> > + aflags |= MNT_NODIRATIME;
> > +
> > + *flags = aflags;
> > + return 0;
> > +}
>
> Same for adding this helper.

Will do.

>
> > + *kattr = (struct mount_kattr){
>
> Missing whitespace before the {.

Good spot, thank you!

>
> > + switch (attr->propagation) {
> > + case MAKE_PROPAGATION_UNCHANGED:
> > + kattr->propagation = 0;
> > + break;
> > + case MAKE_PROPAGATION_UNBINDABLE:
> > + kattr->propagation = MS_UNBINDABLE;
> > + break;
> > + case MAKE_PROPAGATION_PRIVATE:
> > + kattr->propagation = MS_PRIVATE;
> > + break;
> > + case MAKE_PROPAGATION_DEPENDENT:
> > + kattr->propagation = MS_SLAVE;
> > + break;
> > + case MAKE_PROPAGATION_SHARED:
> > + kattr->propagation = MS_SHARED;
> > + break;
> > + default:
>
> Any reason to not just reuse the MS_* flags in the new API? Yes, your
> new names are more descriptive, but having different names for the same
> thing is also rather confusing.

I'm not really married to this so I don't see a reason why not.

>
> > + if (upper_32_bits(attr->attr_set))
> > + return -EINVAL;
> > + if (build_attr_flags(lower_32_bits(attr->attr_set), &kattr->attr_set))
> > + return -EINVAL;
> > +
> > + if (upper_32_bits(attr->attr_clr))
> > + return -EINVAL;
> > + if (build_attr_flags(lower_32_bits(attr->attr_clr), &kattr->attr_clr))
> > + return -EINVAL;
>
> What is so magic about the upper and lower 32 bits?

Nothing apart from the fact that they arent't currently valid. I can
think about reworking these lines. Or do you already have a preferred
way of doing this in mind?

>
> > + return -EINVAL;
> > + else if ((attr->attr_clr & MOUNT_ATTR__ATIME) &&
> > + ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME))
> > + return -EINVAL;
>
> No need for the else here.

Thanks!

>
> That being said I'd reword the thing to be a little more obvious:
>
> if (attr->attr_clr & MOUNT_ATTR__ATIME) {
> if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
> return -EINVAL;
>
> ... code doing the update of the atime flags here
> } else {
> if (attr->attr_set & MOUNT_ATTR__ATIME)
> return -EINVAL;
> }

Will do.

>
>
> > +/* Change propagation through mount_setattr(). */
> > +enum propagation_type {
> > + MAKE_PROPAGATION_UNCHANGED = 0, /* Don't change mount propagation (default). */
> > + MAKE_PROPAGATION_UNBINDABLE = 1, /* Make unbindable. */
> > + MAKE_PROPAGATION_PRIVATE = 2, /* Do not receive or send mount events. */
> > + MAKE_PROPAGATION_DEPENDENT = 3, /* Only receive mount events. */
> > + MAKE_PROPAGATION_SHARED = 4, /* Send and receive mount events. */
> > +};
>
> FYI, in uapis using defines instead of enums is usually the better
> choice, as that allows userspace to probe for later added defines.
>
> But if we use MS_* here that would be void anyway.

Indeed.

>
> > +/* List of all mount_attr versions. */
> > +#define MOUNT_ATTR_SIZE_VER0 24 /* sizeof first published struct */
> > +#define MOUNT_ATTR_SIZE_LATEST MOUNT_ATTR_SIZE_VER0
>
> The _LATEST things is pretty dangerous as there basically is no safe
> and correct way for userspace to use it.

Ok, I'll remove the _LATEST.

Thanks for the review (and sorry again for missing your mails)!

Christian