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).