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Date:   Thu, 5 May 2022 06:48:47 +0900
From:   asmadeus@codewreck.org
To:     Christian Schoenebeck <linux_oss@crudebyte.com>
Cc:     David Howells <dhowells@redhat.com>,
        David Kahurani <k.kahurani@gmail.com>, davem@davemloft.net,
        ericvh@gmail.com, kuba@kernel.org, linux-kernel@vger.kernel.org,
        lucho@ionkov.net, netdev@vger.kernel.org,
        v9fs-developer@lists.sourceforge.net, Greg Kurz <groug@kaod.org>
Subject: Re: 9p EBADF with cache enabled (Was: 9p fs-cache tests/benchmark
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Christian Schoenebeck wrote on Wed, May 04, 2022 at 08:33:36PM +0200:
> On Dienstag, 3. Mai 2022 12:21:23 CEST asmadeus@codewreck.org wrote:
> >  - add some complex code to track the exact byte range that got updated
> > in some conditions e.g. WRONLY or read fails?
> > That'd still be useful depending on how the backend tracks file mode,
> > qemu as user with security_model=mapped-file keeps files 600 but with
> > passthrough or none qemu wouldn't be able to read the file regardless of
> > what we do on client...
> > Christian, if you still have an old kernel around did that use to work?
> 
> Sorry, what was the question, i.e. what should I test / look for precisely? :)

I was curious if older kernel does not issue read at all, or issues read
on writeback fid correctly opened as root/RDRW

You can try either the append.c I pasted a few mails back or the dd
commands, as regular user.

$ dd if=/dev/zero of=test bs=1M count=1
$ chmod 400 test
# drop cache or remount
$ dd if=/dev/urandom of=test bs=102 seek=2 count=1 conv=notrunc
dd: error writing 'test': Bad file descriptor

... But honestly I should just find the time to do it myself, this has
been dragging on for too long...
-- 
Dominique