Re: [PATCH 01/11] vfs: copy_file_range source range over EOF should fail

J. Bruce Fields
2018-12-05T16:51:52Z

On Wed, Dec 05, 2018 at 09:31:02AM +1100, Dave Chinner wrote:
> That's what the code does right now and *exactly what I'm trying to
> fix* because it EINVAL is ambiguous and not an indicator that we've
> reached the end of the source file. EINVAL can indicate several
> different errors, so it really has to be treated as a "copy failed"
> error by applications.
>
> Have a look at read/pread() - they return 0 in this case to indicate
> a short read, and the value of zero is explicitly defined as meaning
> "read position is beyond EOF". Applications know straight away that
> there is no more data to be read and there was no error, so can
> terminate on a successful short read.
>
> We need to allow applications to terminate copy loops on a
> successful short copy.

I'm a little confused by your definition of "short copy" and "short
read". Are you using that to mean a copy/read that returns zero? I
usually see it used to mean any successful call that returned less than
the requested amount. I'd expect a zero return to terminate a copy
loop, but not any positive return.

--b.

> IOWs, applications need to either:
>
> - get an immediate error saying the range is invalid rather
> than doing a short copy (as per the man page); or
> - have an explicit marker to say "no more data to be copied"
>
> Applications need the "no more data to copy" case to be explicit and
> unambiguous so they can make sane decisions about whether a short
> copy was successful because the file was shorter than expected or
> whether a short copy was a result of a real error being encountered.
> The current behaviour is largely unusable for applications because
> they have to guess at the reason for EINVAL part way through a
> copy....
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com