On Thu, Aug 29, 2019 at 03:26:30PM -0700, Linus Torvalds wrote: > On Thu, Aug 29, 2019 at 1:22 PM Arnd Bergmann <a...@arndb.de> wrote: > > > > Maybe we can just pass -fno-builtin-memcpy -fno-builtin-memset > > for clang when CONFIG_KASAN is set and hope for the best? > > I really hate how that disables conversions both ways, which is kind > of pointless and wrong. It's really just "we don't want surprising > memcpy calls for single writes". > > Disabling all the *good* "optimize memset/memcpy" cases is really sad. > > We actually have a lot of small structures in the kernel on purpose > (often for type safety), and I bet we use memcpy on them on purpose at > times. I'd hate to see that become a function call rather than "copy > two words by hand". > > Even for KASAN. > > And I guess that when the compiler sees 20+ "set to zero" it's quite > reasonable to say "just turn it into a memset".
For KASAN, the Clang threshold for inserting memset() is *2* consecutive writes instead of 17. Isn't that likely to cause tearing-related surprises? I suppose people don't run KASAN in production, but we don't want tearing-related bugs showing up there because somebody's going to end up having to debug them regardless. -- Josh
