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Date:   Sat, 31 Aug 2019 14:35:37 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Matthew Wilcox <willy@infradead.org>,
        linux-fsdevel@vger.kernel.org,
        Octavian Purdila <octavian.purdila@intel.com>,
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        Kai =?iso-8859-1?Q?M=E4kisara?= <Kai.Makisara@kolumbus.fi>,
        linux-scsi@vger.kernel.org
Subject: Re: [RFC] Re: broken userland ABI in configfs binary attributes
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On Sat, Aug 31, 2019 at 10:32:41AM +0200, Christoph Hellwig wrote:
> On Mon, Aug 26, 2019 at 08:28:19PM +0100, Al Viro wrote:
> > For configfs bin_attr it won't work, simply because it wants the entire
> > thing to be present - callback parses the data.  For SCSI tape...  Maybe,
> > but you'll need to take care of the overlaps with ->write().  Right now
> > it can't happen (the last reference, about to be dropped right after
> > st_flush() returns); if we do that on each ->flush(), we will have to
> > cope with that fun and we'll need to keep an error (if any) for the
> > next call of st_flush() to pick and return.  I'm not saying it can't
> > be done, but that's really a question for SCSI folks.
> 
> So for the one real life example of the configfs attribute life
> actually is simpler.  acpi_table_aml_write verifies early on that
> the size matches what it expects.  So if we document that any future
> instance needs to be able to do that as well we should be able to
> get away with just writing it from ->flush.

I'm not sure I understand what you mean...  Do you want them to recognize
incomplete data and quietly bugger off when called on too early ->flush()?