On 16/10/17 17:30, Jakub Kicinski wrote:
> On Mon, 16 Oct 2017 17:16:24 +0100, Edward Cree wrote:
>> On 16/10/17 16:45, Jakub Kicinski wrote:
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index 8b8d6ba39e23..8499759d0c7a 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -1116,7 +1116,12 @@ static int check_mem_access(struct bpf_verifier_env
>>> *env, int insn_idx, u32 regn
>>> /* ctx accesses must be at a fixed offset, so that we can
>>> * determine what type of data were returned.
>>> */
>>> - if (!tnum_is_const(reg->var_off)) {
>>> + if (reg->off) {
>>> + verbose("derefence of modified ctx ptr R%d off=%d+%d,
>>> ctx+const is allowed, ctx+const+const is not\n",
>> This is slightly unclear, it's not that two adds is bad (e.g. r1 += 8;
>> r0 = *(u32 *)r1 is bad too), it's that the offset must be in the load,
>> not the register; your message might be accurate for some compilers but
>> not in full generality (especially for assemblers without compiling).
> I'm happy to hear better suggestions :) I've spent quite a bit of time
> scratching my head thinking how to phrase this best. The first
> part of the message is general enough IMHO, the second is targeted
> mostly at C developers.
Hmm, what really bugs me is that if e.g. the compiler turned
*(ctx + 4 + 4)
or
ctx[4 + 4]
or even
ctx->arraymemb[4]
into this kind of arithmetic on ctx, arguably that would be a bug in the
compiler — if it's doing proper constexpr folding on its IR (or something
along those lines) it should be able to turn them all into good LDX. The
same even goes for if (ctx + 4) got stored in a local, because there's no
reason that has to map to a register.
So it's not even that "your C source breaks the rules", it's that "your C
compiler did something silly that we don't handle".
Maybe the message should be "compiler maybe mishandled ctx+const+const"?
-Ed
