netdev - Re: bpf jit PPC64 (BE) test_verifier PTR_TO

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Date:   Fri, 15 Mar 2019 18:46:45 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>
Cc:     bpf@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        Jiri Olsa <jolsa@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>, netdev@...r.kernel.org,
        Sandipan Das <sandipan@...ux.ibm.com>
Subject: Re: bpf jit PPC64 (BE) test_verifier PTR_TO_STACK store/load failure

Segher Boessenkool wrote:
> Hi!
> 
> On Wed, Mar 13, 2019 at 12:54:16PM +0200, Yauheni Kaliuta wrote:
>> This is because of the handling of the +2 offset.
> 
> The low two bits of instructions with primary opcodes 58 and 62 are part
> of the opcode, not the offset.  These instructions can not have offsets
> with the low two bits non-zero.
> 
>> For stores it is:
>> #define PPC_STD(r, base, i)	EMIT(PPC_INST_STD | ___PPC_RS(r) |	      \
>> 				     ___PPC_RA(base) | ((i) & 0xfffc))
>> 
>> and for loads
>> #define PPC_LD(r, base, i)	EMIT(PPC_INST_LD | ___PPC_RT(r) |	      \
>> 				     ___PPC_RA(base) | IMM_L(i))
>> #define IMM_L(i)		((uintptr_t)(i) & 0xffff)
>> 
>> So, in the load case the offset +2 (immediate value) is not
>> masked and turns the instruction to lwa instead of ld.
>> 
>> Would it be correct to & 0xfffc the immediate value as well?
> 
> That is only part of it.  The other thing is you have to make sure those
> low bits are zero *already* (and then you do not need the mask anymore).
> For example, if the low two bits are not zero load the offset into a
> register instead (and then do ldx or lwax).

Thanks for pointing that out, Segher. That is a detail that is easily 
missed.

- Naveen