On Fri, May 29, 2020 at 11:07 PM Syed Nayyar Waris <syednwaris@xxxxxxxxx> wrote: > On Sat, May 30, 2020 at 12:08 AM Andy Shevchenko > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote: > > On Fri, May 29, 2020 at 11:38:18PM +0530, Syed Nayyar Waris wrote: > > > On Sun, May 24, 2020 at 8:15 PM kbuild test robot <lkp@xxxxxxxxx> wrote: ... > > > Taking the example statement (in my patch) where compilation warning > > > is getting reported: > > > return (map[index] >> offset) & GENMASK(nbits - 1, 0); > > > > > > 'nbits' is of type 'unsigned long'. > > > In above, the sanity check is comparing '0' with unsigned value. And > > > unsigned value can't be less than '0' ever, hence the warning. > > > But this warning will occur whenever there will be '0' as one of the > > > 'argument' and an unsigned variable as another 'argument' for GENMASK. > > Proper fix is to fix GENMASK(), but allowed workaround is to use > > (BIT(nbits) - 1) > > instead. > When I used BIT macro (earlier), I had faced a problem. I want to tell > you about that. > > Inside functions 'bitmap_set_value' and 'bitmap_get_value' when nbits (or > clump size) is BITS_PER_LONG, unexpected calculation happens. > > Explanation: > Actually when nbits (clump size) is 64 (BITS_PER_LONG is 64 on my computer), > (BIT(nbits) - 1) > gives a value of zero and when this zero is ANDed with any value, it > makes it full zero. This is unexpected and incorrect calculation happening. > > What actually happens is in the macro expansion of BIT(64), that is 1 > << 64, the '1' overflows from leftmost bit position (most significant > bit) and re-enters at the rightmost bit position (least significant > bit), therefore 1 << 64 becomes '0x1', and when another '1' is > subtracted from this, the final result becomes 0. > > Since this macro is being used in both bitmap_get_value and > bitmap_set_value functions, it will give unexpected results when nbits or clump > size is BITS_PER_LONG (32 or 64 depending on arch). I see, something like https://elixir.bootlin.com/linux/latest/source/include/linux/dma-mapping.h#L139 should be done. But yes, let's try to fix GENMASK(). So, if we modify the following #define GENMASK_INPUT_CHECK(h, l) \ (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ __builtin_constant_p((l) > (h)), (l) > (h), 0))) to be #define GENMASK_INPUT_CHECK(h, l) \ (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ __builtin_constant_p((l) > (h)), (l) ? (l) > (h) : 0, 0))) would it work? > William also knows about this issue: > "This is undefined behavior in the C standard (section 6.5.7 in the N1124)" I think it is about 6.5.7.3 here, 1U << 31 (or 63) is okay. -- With Best Regards, Andy Shevchenko
