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

Alexei Starovoitov
2018-01-10T01:57:14Z

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.