Re: [PATCH 16/18] net: mpls: prevent bounds-check bypass via speculative execution

[Alexei Starovoitov]

On Tue, Jan 09, 2018 at 04:48:24PM -0800, Dan Williams wrote:
> 
> #define __nospec_array_ptr(base, idx, sz)                               \
> ({                                                                      \
>         union { typeof(&base[0]) _ptr; unsigned long _bit; } __u;       \
>         unsigned long _i = (idx);                                       \
>         unsigned long _s = (sz);                                        \
>         unsigned long _v = (long)(_i | _s - 1 - _i)                     \
>                                         >> BITS_PER_LONG - 1;           \
>         unsigned long _mask = _v * ~0UL;                                 \
>         OPTIMIZER_HIDE_VAR(_mask);                                      \
>         __u._ptr = &base[_i & _mask];                                   \
>         __u._bit &= _mask;                                              \
>         __u._ptr;                                                       \
> })

_v * ~0UL doesn't seem right and non intuitive.
What's wrong with:
  unsigned long _mask = ~(long)(_i | _s - 1 - _i) >> BITS_PER_LONG - 1;

and why OPTIMIZER_HIDE_VAR ?
Could you remove '&' ?
since in doesn't work for:
struct {
  int fd[4];
  ...
} *fdt;
it cannot be used as array_acces(fdt->fd, ...);

Could you please drop nospec_ prefix since it is misleading ?
This macro doesn't prevent speculation.
I think array_access() was the best name so far.