Re: [PATCH] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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.

> 
> 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.
>