Re: [Qemu-devel] [PATCH 2/2] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP

[Kevin Wolf]

Am 13.11.2014 um 00:25 hat Eric Blake geschrieben:
> On 11/12/2014 01:27 PM, Markus Armbruster wrote:
> > +    /* in hole, end not yet known */
> > +    offs = lseek(s->fd, start, SEEK_DATA);
> > +    if (offs < 0) {
> > +        /* no idea where the hole ends, give up (unlikely to happen) */
> > +        goto dunno;
> > +    }
> > +    assert(offs >= start);
> > +    *hole = start;
> > +    *data = offs;
> 
> This assertion feels like an off-by-one.  The same offset cannot be both
> a hole and data (except in some racy situation where some other process
> is writing data to that offset in between our two lseek calls, but
> that's already in no-man's land because no one else should be writing
> the file while qemu has it open).  Is it worth using 'assert(offs >
> start)' instead?

As soon as you say "except", it's wrong to assert this at all. We can't
guarantee that the condition is true and it's not a programming error
in qemu if it's false. Sounds to me as if it should be a normal error
check rather than an assertion.

Also, what happens after EOF? I haven't read the patch yet, maybe it
handles the situation already earlier, but if it doesn't, won't we get
offset == start then?

Kevin

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