RE: [PATCH] lib/strscpy: remove word-at-a-time optimization.

[David Laight]

From: Dmitry Vyukov [mailto:dvyukov@xxxxxxxxxx]
> Sent: 25 January 2018 08:33
> 
> On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Jan 24, 2018 at 12:54 AM, Rasmus Villemoes
> > <rasmus.villemoes@xxxxxxxxx> wrote:
> >>
> >> I see something similar, but at the 30->31 transition, and the
> >> branch-misses remain at 1-3% for higher values, until 42 where it drops
> >> back to 0%. Anyway, I highly doubt we do a lot of string copies of
> >> strings longer then 32.
> >
> > So I really dislike that microbenchmark, because it just has the same
> > length all the time. Which is very wrong, and makes the benchmark
> > pointless. A big part of this all is branch mispredicts, you shouldn't
> > just hand it the pattern on a plate.
> >
> > Anyway, the reason I really dislike the patch is not because I think
> > strscpy() is all that important, but I *do* think that the
> > word-at-a-time thing is conceptually something we do care about, and I
> > hate removing it just because of KASAN not understanding it.
> >
> > So I'd *much* rather have some way to tell KASAN that word-at-a-time
> > is going on. Because that approach definitely makes a difference in
> > other places.
> 
> 
> The other option was to use READ_ONCE_NOCHECK(). Not sure if the "read
> once" part will affect codegen here, though.
> But if word-at-a-time thing is conceptually something we do care
> about, we could also introduce something like READ_PARTIALLY_VALID(),
> which would check that at least first byte of the read is valid and
> that it does not cross heap block boundary (but outside of KASAN is a
> normal read).

The first byte might not have been written either.
For example, doing a strlen() on a misaligned string you might read
the aligned word containing the first byte and adjust the value so
that the initial byte(s) are not zero.
After scanning for a zero byte the length would be corrected.

	David