Re: [PATCH] ubsan: Implement __ubsan_handle_alignment_assumption

[Nathan Chancellor]

On Tue, Jan 12, 2021 at 01:53:30PM -0800, Nick Desaulniers wrote:
> On Tue, Jan 12, 2021 at 1:37 PM Nathan Chancellor
> <natechancellor@xxxxxxxxx> wrote:
> >
> > > if real_ptr is an unsigned long, do we want to use `__ffs(real_ptr) +
> > > 1` here rather than ffs which takes an int?  It seems the kernel is
> > > missing a definition of ffsl. :(
> >
> > Why the + 1? I think if we use __ffs (which it seems like we should), I
> > think that needs to become
> 
> This came up recently in an internal code review; ffs and __ffs differ
> in output by one.  See also the definition of ffs for alpha in
> arch/alpha/include/asm/bitops.h.

Interesting, thanks for bringing it up! Looks like ffs returns 1-32 and
__ffs returns 0-31. I think that we want __ffs here because we are
shifting (1UL << 32 overflows on 32-bit architectures) and the code in
LLVM appears to agree. LeastSignificantSetBitIndex evaluates to
__builtin_ctzl, which is the asm-generic implementation of __ffs.

Cheers,
NAthan