Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Nicolas Boichat]

On Mon, Feb 15, 2021 at 9:12 AM Ian Lance Taylor <iant@xxxxxxxxxx> wrote:
>
> On Sun, Feb 14, 2021 at 4:38 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Feb 12, 2021 at 03:54:48PM -0800, Darrick J. Wong wrote:
> > > On Sat, Feb 13, 2021 at 10:27:26AM +1100, Dave Chinner wrote:
> > >
> > > > If you can't tell from userspace that a file has data in it other
> > > > than by calling read() on it, then you can't use cfr on it.
> > >
> > > I don't know how to do that, Dave. :)
> >
> > If stat returns a non-zero size, then userspace knows it has at
> > least that much data in it, whether it be zeros or previously
> > written data. cfr will copy that data. The special zero length
> > regular pipe files fail this simple "how much data is there to copy
> > in this file" check...
>
> This suggests that if the Go standard library sees that
> copy_file_range reads zero bytes, we should assume that it failed, and
> should use the fallback path as though copy_file_range returned
> EOPNOTSUPP or EINVAL.  This will cause an extra system call for an
> empty file, but as long as copy_file_range does not return an error
> for cases that it does not support we are going to need an extra
> system call anyhow.
>
> Does that seem like a possible mitigation?  That is, are there cases
> where copy_file_range will fail to do a correct copy, and will return
> success, and will not return zero?

I'm a bit worried about the sysfs files that report a 4096 bytes file
size, for 2 reasons:
 - I'm not sure if the content _can_ actually be longer (I couldn't
find a counterexample)
 - If you're unlucky enough to have a partial write in the output
filesystem, you'll get a non-zero return value and probably corrupted
content.

>
> Ian