Re: [PATCH] fs: generic_copy_file_checks: Do not adjust count based on file size

[Nicolas Boichat]

On Thu, Jan 28, 2021 at 1:57 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
>
> On Thu, Jan 28, 2021 at 08:46:04AM +0800, Nicolas Boichat wrote:
[snip]
> > Okay, so, based on this and Al's reply, I see 2 things we can do:
> >  1. Go should probably not use copy_file_range in a common library
> > function, as I don't see any easy way to detect this scenario
> > currently (detect 0 size? sure, but that won't work with the example
> > you provide above). And the man page should document this behaviour
> > more explicitly to prevent further incorrect usage.
> >  2. Can procfs/sysfs/debugfs and friends explicitly prevent usage of
> > copy_file_range? (based on Al's reply, there seems to be no way to
> > implement it correctly as seeking in such files will not work in case
> > of short writes)
>
> One /could/ make those three provide a phony CFR implementation that
> would return -EOPNOTSUPP, though like others have said, it's weird to
> have regular files that aren't quite regular.  Not sure where that
> leaves them, though...

Not that simple, as the issue happens on cross-filesystem operations
where file_operations->copy_file_range is not called (and also, that'd
require modifying operations for every single generated file...

Anyway, made an attempt here:
https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@xxxxxxxxxxxx/T/#t

> --D
>
> >
> > Thanks,
> >
> > >
> > > Cheers,
> > >
> > > Dave.
> > > --
> > > Dave Chinner
> > > david@xxxxxxxxxxxxx