Re: [PATCH v4 net-next 01/13] bpf/verifier: rework value tracking

From: Edward Cree
Date: Mon Aug 07 2017 - 08:39:25 EST

On 07/08/17 00:35, Daniel Borkmann wrote:
> On 08/03/2017 06:11 PM, Edward Cree wrote:
>> Unifies adjusted and unadjusted register value types (e.g. FRAME_POINTER is
>> now just a PTR_TO_STACK with zero offset).
>> Tracks value alignment by means of tracking known & unknown bits. This
>> also replaces the 'reg->imm' (leading zero bits) calculations for (what
>> were) UNKNOWN_VALUEs.
>> If pointer leaks are allowed, and adjust_ptr_min_max_vals returns -EACCES,
>> treat the pointer as an unknown scalar and try again, because we might be
>> able to conclude something about the result (e.g. pointer & 0x40 is either
>> 0 or 0x40).
>> Signed-off-by: Edward Cree <ecree@xxxxxxxxxxxxxx>
> [...]
>> - dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
>> + if (BPF_CLASS(insn->code) != BPF_ALU64) {
>> + /* 32-bit ALU ops are (32,32)->64 */
>> + coerce_reg_to_32(dst_reg);
>> + coerce_reg_to_32(src_reg);
>> }
> Looks like the same check was added twice here right after
> the first one?
Yes, it must've gotten duplicated when I rebased. Thanks for spotting it!
> Shouldn't we just temporarily coerce the src
> reg to 32 bit here given in the actual op the src reg is not
> being modified?
You're quite right, I need to make a copy of the src_reg state and use
that, at least in the case where it's a real register. Probably the
place to do it is at the call sites in adjust_reg_min_max_vals().
I'll sprinkle a few consts around as well, to catch that sort of thing.