On Tue, Apr 2, 2019 at 1:44 PM David Laight <david.lai...@aculab.com> wrote: > > From: Alexander Potapenko > > Sent: 02 April 2019 12:28 > > > > 1. Use memory clobber in bitops that touch arbitrary memory > > > > Certain bit operations that read/write bits take a base pointer and an > > arbitrarily large offset to address the bit relative to that base. > > Although x86_64 can use a signed 64bit bit number, looking at arm and arm64 > they use 'int nr' throughout as do the generic functions. > Maybe x86 ought to be consistent here. > I doubt negative bit numbers are expected to work? I don't have a strong opinion on this, but the corresponding Intel instructions do accept 64-bit operands.
> Did you try telling gcc that a big buffer (250MB is the limit for 32bit) > from the pointer might be changed? Yes, I did, see https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1966993.html This still isn't a silver bullet, e.g. I saw an example where touching a function parameter cast to a big buffer in the assembly resulted in clobbering a global. Moreover, one can imagine a situation where such a trick may be harmful, e.g.: void foo(int size) { struct arr { long val[1U<<28]; }; long *bitmap = malloc(size); asm("#do something" : "+m"(*(struct arr*)bitmap); if (size < 1024) process(bitmap[size]); } If a (smart enough) compiler knows that malloc(size) returns a pointer to |size| bytes in memory, it may assume that |size| is at least 1U<<28 (because otherwise it's incorrect to treat |bitmap| as a pointer to a big array) and delete the size check. This is of course a synthetic example, but not a completely impossible one. > That ought to be softer than a full 'memory' clobber as it should > only affect memory that could be accessed through the pointer. > > .... > > -#define BITOP_ADDR(x) "+m" (*(volatile long *) (x)) > > +#define RLONG_ADDR(x) "m" (*(volatile long *) (x)) > > +#define WBYTE_ADDR(x) "+m" (*(volatile char *) (x)) > > > > -#define ADDR BITOP_ADDR(addr) > > +#define ADDR RLONG_ADDR(addr) > > Is it worth just killing ADDR ? > (as a different patch) Agreed. > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 > 1PT, UK > Registration No: 1397386 (Wales) -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
