On 2018-01-09 17:37, Andrey Ryabinin wrote: > strscpy() performs the word-at-a-time optimistic reads. So it may > may access the memory past the end of the object, which is perfectly fine > since strscpy() doesn't use that (past-the-end) data and makes sure the > optimistic read won't cross a page boundary. > > But KASAN doesn't know anything about that so it will complain. > There are several possible ways to address this issue, but none > are perfect. See https://lkml.kernel.org/r/9f0a9cf6-51f7-cd1f-5dc6-6d510a7b8ec4@xxxxxxxxxxxxx > > It seems the best solution is to simply disable word-at-a-time > optimization. My trivial testing shows that byte-at-a-time > could be up to x4.3 times slower than word-at-a-time. > It may seems like a lot, but it's actually ~1.2e-10 sec per symbol vs > ~4.8e-10 sec per symbol on modern hardware. And we don't use strscpy() > in a performance critical paths to copy large amounts of data, > so it shouldn't matter anyway. > > Fixes: 30035e45753b7 ("string: provide strscpy()") > Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Acked-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Your microbenchmark even favours word-at-a-time slightly, since in practice I think at least one of src or dst will be unaligned a lot of the time, and while x86 may HAVE_EFFICIENT_UNALIGNED_ACCESS, it's still a little more expensive than doing aligned access. And since strscpy is not called that often, I expect some of the ~300 bytes of instruction cache it occupies can be put to better use elsewhere. Rasmus