Re: BUG: mmapfile/writev spurious zero bytes (x86_64/not i386,bisected, reproducable)

[Bron Gondwana]

On Tue, Jun 17, 2008 at 02:20:49PM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 17 Jun 2008, Linus Torvalds wrote:
> > 
> > Hmm. Something like this *may* salvage it.
> > 
> > Untested, so far (I'll reboot and test soon enough), but even if it fixes 
> > things, it's not really very good. 
> 
> Ok, so I just rebooted with this, and it does indeed fix the bug.
> 
> I'd be happier with a more complete fix (ie being byte-accurate and 
> actually doing the partial copy when it hits a fault in the middle), but 
> this seems to be the minimal fix, and at least fixes the totally bogus 
> return values from the x86-64 __copy_user*() functions.
> 
> Not that I checked that I got _all_ cases correct (and maybe there are 
> other versions of __copy_user that I missed entirely), but Bron's 
> test-case at least seems to work properly for me now.
> 
> Bron? If you have a more complete test-suite (ie the real-world case that 
> made you find this), it would be good to verify the whole thing. 

I have a real world test case using "cyr_dbtool" from Cyrus 2.3.12 on a
known-bug-inducing piece of data (the key and value sizes in the example
code were taken from that.  Indeed, the example code started off
building byte-for-byte identical files, then I changed it to write only
a single character across the entire key and value so the hexdump was
shorter)

I'll give it a go.

Bron.
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