Re: [PATCH] btrfs: bugfix: handle FS_IOC32_{GETFLAGS,SETFLAGS,GETVERSION} in btrfs_ioctl

[David Sterba]

On Thu, May 14, 2015 at 04:27:54PM +0000, Luke Dashjr wrote:
> On Thursday, May 14, 2015 2:06:17 PM David Sterba wrote:
> > On Wed, May 13, 2015 at 05:15:26PM +0000, Luke Dashjr wrote:
> > > 32-bit ioctl uses these rather than the regular FS_IOC_* versions. They
> > > can be handled in btrfs using the same code. Without this, 32-bit
> > > {ch,ls}attr fail.
> > 
> > Yes, but this has to be implemented in another way. See eg.
> > https://git.kernel.org/linus/e9750824114ff
> 
> I don't see what is different with that implementation. All f2fs_compat_ioctl 
> does is change cmd to the plain-IOC equivalent and call f2fs_ioctl with the 
> same arg (compat_ptr merely causes a cast to void* and back, which AFAIK is a 
> noop on 64-bit?). Am I missing something?

No, that's the idea. Add new calback for compat_ioctl, put it under
#ifdef CONFIG_COMPAT and do the same number switch.

> I could try to just imitate it, but 
> I'd rather know what is significant/going on to ensure I don't waste your time 
> with code I don't even properly understand myself.
> 
> Perhaps by coincidence, the patch does at least in practice work (although at 
> least `btrfs send` appears to be broken still, and I'm at a loss for how to 
> approach fixing that).

The 'receive' 32bit/64bit was broken due to size difference in the ioctl
structure that led to different ioctl. This is transparently fixed, see
BTRFS_IOC_SET_RECEIVED_SUBVOL_32 at the top of ioctl.c.

In what way is SEND broken? There are only u64/s64 members in
btrfs_ioctl_send_args, I don't see how this could break on 32/64
userspace/kernel.
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