lkml.org 
[lkml]   [2008]   [Feb]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Subject[PATCH] r/o bind mounts: stub functions
    From
    Date
    Linus,

    We've been causing Ted a lot of pain having to keep different copies of
    his patches for mainline and for -mm since -mm has these functions and
    mainline doesn't. I'm pretty darn sure at that these are the right API
    and we just need another run in -mm to make sure that the reset of the
    series in in good shape.

    Could we bump this one in a bit ahead of the rest to ease Ted's pain? I
    think the odds of this breaking anything by itself are pretty low.

    ---

    This patch adds two functions: mnt_want_write() and mnt_drop_write().
    These are used like a lock pair around and fs operations that might
    cause a write to the filesystem.

    Note that these will replace many of the uses of IS_RDONLY(inode)
    currently in the kernel.

    Before these can become useful, we must first cover each place in the VFS
    where writes are performed with a want/drop pair. When that is complete, we
    can actually introduce code that will safely check the counts before allowing
    r/w<->r/o transitions to occur.

    Acked-by: Serge Hallyn <serue@us.ibm.com>
    Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
    Signed-off-by: Christoph Hellwig <hch@infradead.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
    ---

    linux-2.6.git-dave/fs/namespace.c | 54 +++++++++++++++++++++++++++++++
    linux-2.6.git-dave/include/linux/mount.h | 3 +
    2 files changed, 57 insertions(+)

    diff -puN fs/namespace.c~r-o-bind-mounts-stub-functions fs/namespace.c
    --- linux-2.6.git/fs/namespace.c~r-o-bind-mounts-stub-functions 2008-02-15 13:25:45.000000000 -0800
    +++ linux-2.6.git-dave/fs/namespace.c 2008-02-15 13:25:45.000000000 -0800
    @@ -80,6 +80,60 @@ struct vfsmount *alloc_vfsmnt(const char
    return mnt;
    }

    +/*
    + * Most r/o checks on a fs are for operations that take
    + * discrete amounts of time, like a write() or unlink().
    + * We must keep track of when those operations start
    + * (for permission checks) and when they end, so that
    + * we can determine when writes are able to occur to
    + * a filesystem.
    + */
    +/**
    + * mnt_want_write - get write access to a mount
    + * @mnt: the mount on which to take a write
    + *
    + * This tells the low-level filesystem that a write is
    + * about to be performed to it, and makes sure that
    + * writes are allowed before returning success. When
    + * the write operation is finished, mnt_drop_write()
    + * must be called. This is effectively a refcount.
    + */
    +int mnt_want_write(struct vfsmount *mnt)
    +{
    + if (__mnt_is_readonly(mnt))
    + return -EROFS;
    + return 0;
    +}
    +EXPORT_SYMBOL_GPL(mnt_want_write);
    +
    +/**
    + * mnt_drop_write - give up write access to a mount
    + * @mnt: the mount on which to give up write access
    + *
    + * Tells the low-level filesystem that we are done
    + * performing writes to it. Must be matched with
    + * mnt_want_write() call above.
    + */
    +void mnt_drop_write(struct vfsmount *mnt)
    +{
    +}
    +EXPORT_SYMBOL_GPL(mnt_drop_write);
    +
    +/*
    + * __mnt_is_readonly: check whether a mount is read-only
    + * @mnt: the mount to check for its write status
    + *
    + * This shouldn't be used directly ouside of the VFS.
    + * It does not guarantee that the filesystem will stay
    + * r/w, just that it is right *now*. This can not and
    + * should not be used in place of IS_RDONLY(inode).
    + */
    +int __mnt_is_readonly(struct vfsmount *mnt)
    +{
    + return (mnt->mnt_sb->s_flags & MS_RDONLY);
    +}
    +EXPORT_SYMBOL_GPL(__mnt_is_readonly);
    +
    int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
    {
    mnt->mnt_sb = sb;
    diff -puN include/linux/mount.h~r-o-bind-mounts-stub-functions include/linux/mount.h
    --- linux-2.6.git/include/linux/mount.h~r-o-bind-mounts-stub-functions 2008-02-15 13:25:45.000000000 -0800
    +++ linux-2.6.git-dave/include/linux/mount.h 2008-02-15 13:25:45.000000000 -0800
    @@ -70,9 +70,12 @@ static inline struct vfsmount *mntget(st
    return mnt;
    }

    +extern int mnt_want_write(struct vfsmount *mnt);
    +extern void mnt_drop_write(struct vfsmount *mnt);
    extern void mntput_no_expire(struct vfsmount *mnt);
    extern void mnt_pin(struct vfsmount *mnt);
    extern void mnt_unpin(struct vfsmount *mnt);
    +extern int __mnt_is_readonly(struct vfsmount *mnt);

    static inline void mntput(struct vfsmount *mnt)
    {
    _


    \
     
     \ /
      Last update: 2008-02-16 02:35    [W:0.034 / U:62.060 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site