On Wed, May 29, 2019 at 12:57:15PM +0200, Dmitry Vyukov wrote: > On Wed, May 29, 2019 at 12:30 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Wed, May 29, 2019 at 12:16:31PM +0200, Marco Elver wrote: > > > On Wed, 29 May 2019 at 12:01, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > > > On Wed, May 29, 2019 at 11:20:17AM +0200, Marco Elver wrote: > > > > > For the default, we decided to err on the conservative side for now, > > > > > since it seems that e.g. x86 operates only on the byte the bit is on. > > > > > > > > This is not correct, see for instance set_bit(): > > > > > > > > static __always_inline void > > > > set_bit(long nr, volatile unsigned long *addr) > > > > { > > > > if (IS_IMMEDIATE(nr)) { > > > > asm volatile(LOCK_PREFIX "orb %1,%0" > > > > : CONST_MASK_ADDR(nr, addr) > > > > : "iq" ((u8)CONST_MASK(nr)) > > > > : "memory"); > > > > } else { > > > > asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0" > > > > : : RLONG_ADDR(addr), "Ir" (nr) : "memory"); > > > > } > > > > } > > > > > > > > That results in: > > > > > > > > LOCK BTSQ nr, (addr) > > > > > > > > when @nr is not an immediate. > > > > > > Thanks for the clarification. Given that arm64 already instruments > > > bitops access to whole words, and x86 may also do so for some bitops, > > > it seems fine to instrument word-sized accesses by default. Is that > > > reasonable? > > > > Eminently -- the API is defined such; for bonus points KASAN should also > > do alignment checks on atomic ops. Future hardware will #AC on unaligned > > [*] LOCK prefix instructions. > > > > (*) not entirely accurate, it will only trap when crossing a line. > > https://lkml.kernel.org/r/1556134382-58814-1-git-send-email-fenghua.yu@xxxxxxxxx > > Interesting. Does an address passed to bitops also should be aligned, > or alignment is supposed to be handled by bitops themselves? > > This probably should be done as a separate config as not related to > KASAN per se. But obviously via the same > {atomicops,bitops}-instrumented.h hooks which will make it > significantly easier. Makes sense to me -- that should be easy to hack into gen_param_check() in gen-atomic-instrumented.sh, something like: ---- diff --git a/scripts/atomic/gen-atomic-instrumented.sh b/scripts/atomic/gen-atomic-instrumented.sh index e09812372b17..2f6b8f521e57 100755 --- a/scripts/atomic/gen-atomic-instrumented.sh +++ b/scripts/atomic/gen-atomic-instrumented.sh @@ -21,6 +21,13 @@ gen_param_check() [ ${type#c} != ${type} ] && rw="read" printf "\tkasan_check_${rw}(${name}, sizeof(*${name}));\n" + + [ "${type#c}" = "v" ] || return + +cat <<EOF + if (IS_ENABLED(CONFIG_PETERZ)) + WARN_ON(!IS_ALIGNED(${name}, sizeof(*${name}))); +EOF } #gen_param_check(arg...) ---- On arm64 our atomic instructions always perform an alignment check, so we'd only miss if an atomic op bailed out after a plain READ_ONCE() of an unaligned atomic variable. Thanks, Mark.