On 5/29/19 1:57 PM, 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? > It should be aligned. This even documented in Documentation/core-api/atomic_ops.rst: Native atomic bit operations are defined to operate on objects aligned to the size of an "unsigned long" C data type, and are least of that size. The endianness of the bits within each "unsigned long" are the native endianness of the cpu. > 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. > Agreed.