Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Boris Burkov <boris@bur.io>]

On Wed, Apr 08, 2026 at 07:22:44AM +0930, Qu Wenruo wrote:
> 
> 
> 在 2026/4/8 03:43, Mark Harmstone 写道:
> > I think all three of us are confusing each other a little here.
> > 
> > The ioctl answers the question: if I were to read X bytes of data from a
> > file at Y offset and calculated the csums manually, what would the value
> > be? To which the kernel responds either with the values, that the read
> > is guaranteed to return zero and thus we can use the precomputed csum
> > for the zero sector, or that the value isn't known and userspace has to
> > do it anyway.
> > 
> > The value isn't known if it's a nodatasum file or if it's compressed. We
> > store the csums of compressed extents, but crucially it's over the
> > compressed data. So there's no one-to-one mapping between file blocks
> > and compressed sectors (by definition, because it's compressed), and
> > bookending means that it might be data we don't have access to.
> > 
> > We absolutely can't give non-root users csums to arbitrary data, that's
> > definitely a security breach.
> 
> If getting csums for random logical is a security breach, I do not think the
> new GET_CSUM ioctl is any better.
> 

Isn't it better because you have to use a file we do permissions checks on?

So it's not an arbitrary logical, it's a logical used by a file you have
access to? That might still be insecure against some attack, though, what
do I know..

> > 
> > Userspace can already obtain the csums from the disk for a file by using
> > FIEMAP and the tree search ioctl. But I believe the consensus around the
> > tree search ioctl is a) that it's very difficult to use, as you need to
> > know the internals of btrfs,
> 
> I completely agree with this part, furthermore due to the layout of csum
> tree, one has to workaround by searching with a much smaller value than the
> bytenr as the min_key, which means possible unnecessary reads of previous
> leaves.
> 
> Thanks,
> Qu
> 
> > and b) it requires CAP_SYS_ADMIN, at a time when containerization and
> > finer-grained access controls means this is frowned upon.
> > 
> > This ioctl is a simpler way of doing the csum lookup, and without
> > requiring root.
> >