On Thu, Jan 25, 2018 at 9:42 AM, David Laight David.Laight@aculab.com wrote:
From: Dmitry Vyukov [mailto:dvyukov@google.com]
Sent: 25 January 2018 08:33
On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
On Wed, Jan 24, 2018 at 12:54 AM, Rasmus Villemoes rasmus.villemoes@prevas.dk 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.
Was the first byte at least kmalloc-ed? That's what KASAN checks, it does not care about "written". KMSAN can detect uses of uninit data, but that's another story.