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    SubjectRe: [PATCH v3] Check for immutable/append flag in fallocate path
    On Mon, Mar 07, 2011 at 10:38:37PM -0700, Andreas Dilger wrote:
    > On 2011-03-07, at 10:11 PM, Dave Chinner wrote:
    > > On Sat, Mar 05, 2011 at 10:37:45AM +0100, Marco Stornelli wrote:
    > >> From: Marco Stornelli <marco.stornelli@gmail.com>
    > >>
    > >> In the fallocate path the kernel doesn't check for the immutable/append
    > >> flag. It's possible to have a race condition in this scenario: an
    > >> application open a file in read/write and it does something, meanwhile
    > >> root set the immutable flag on the file, the application at that point
    > >> can call fallocate with success. In addition, we don't allow to do any
    > >> unreserve operation on an append only file but only the reserve one.
    > >>
    > >> Signed-off-by: Marco Stornelli <marco.stornelli@gmail.com>
    > >> ---
    > >> Patch is against 2.6.38-rc7
    > >>
    > >> ChangeLog:
    > >> v3: Modified do_fallocate instead of every single fs
    > >> v2: Added the check for append-only file for XFS
    > >> v1: First draft
    > >>
    > >> --- open.c.orig 2011-03-01 22:55:12.000000000 +0100
    > >> +++ open.c 2011-03-04 15:28:43.000000000 +0100
    > >> @@ -233,6 +233,14 @@ int do_fallocate(struct file *file, int
    > >>
    > >> if (!(file->f_mode & FMODE_WRITE))
    > >> return -EBADF;
    > >> +
    > >> + /* It's not possible punch hole on append only file */
    > >> + if (mode & FALLOC_FL_PUNCH_HOLE && IS_APPEND(inode))
    > >> + return -EPERM;
    > >
    > > Seeing as I didn't get an answer in before you reposted, I still
    > > think punching an append-only file is a valid thing to want to do.
    > >
    > > I've seen this done in the past for application-level transaction
    > > journal files. The journal file is append only so new transactions
    > > can only be written at the end of the file i.e. you cannot overwrite
    > > (and therefore corrupt) existing transactions. However, once a
    > > transaction is complete and the changes flushed to disk, the
    > > transaction is punched out of the file to zero the range so it
    > > doesn't get replayed during recovery after a system crash.
    >
    > To my thinking "append only" means just that - only new data can
    > be written at the end of the file, and existing data cannot be
    > modified. Allowing hole punch on such a file (e.g. range 0 .. ~0)
    > would allow erasing all of the data, entirely bypassing the
    > append-only flag.

    Not worth arguing over. XFS_IOC_UNRESVSP won't get changed, so the
    applications already doing this can just keep using that interface...

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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