Re: FIBMAP/FIEMAP discrepancy for CAP_SYS_RAWIO

[Florian Weimer <fw@deneb.enyo.de>]

* Andreas Dilger:

> On Sep 06, 2009  13:39 +0000, Florian Weimer wrote:
>> The FIBMAP ioctl requires CAP_SYS_RAWIO, but FIEMAP doesn't.  Why's
>> that?  Is it that there is no backwards-compatible way to introduce
>> locking on the bmap path?
>
> I'm not sure why there is a root-only requirement for FIBMAP, but the
> FIEMAP data is definitely useful even for non-root users for many
> reasons, such as optimized file copies/rsync/tar/etc skipping holes
> in sparse files easily.

I'm slightly worried because the generic FIEMAP-on-FIBMAP
implementation takes the inode mutex, but the FIBMAP ioctl doesn't.

> If you are implementing a tool to use this, I would code it to try
> FIEMAP first, then FIBMAP (if it is running as root, or it gets
> fixed in some future kernel), then just do without (as it most likely
> does already today).

If FIBMAP is unsafe, it's likely exposed by concurrent changes to the
file, so using it would still be unsafe for backup purposes.  And I
really only need the number of the first block.  (I want to optimize
reading of Maildir-style folders, mainly for backup purposes.)