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Date:   Thu, 23 May 2019 15:58:00 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Netdev <netdev@...r.kernel.org>, linux-riscv@...ts.infradead.org,
        bpf <bpf@...r.kernel.org>, Jiong Wang <jiong.wang@...ronome.com>
Subject: Re: [PATCH bpf] bpf, riscv: clear target register high 32-bits for
 and/or/xor on ALU32

On 05/21/2019 04:12 PM, Björn Töpel wrote:
> On Tue, 21 May 2019 at 16:02, Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 05/21/2019 03:46 PM, Björn Töpel wrote:
>>> When using 32-bit subregisters (ALU32), the RISC-V JIT would not clear
>>> the high 32-bits of the target register and therefore generate
>>> incorrect code.
>>>
>>> E.g., in the following code:
>>>
>>>   $ cat test.c
>>>   unsigned int f(unsigned long long a,
>>>              unsigned int b)
>>>   {
>>>       return (unsigned int)a & b;
>>>   }
>>>
>>>   $ clang-9 -target bpf -O2 -emit-llvm -S test.c -o - | \
>>>       llc-9 -mattr=+alu32 -mcpu=v3
>>>       .text
>>>       .file   "test.c"
>>>       .globl  f
>>>       .p2align        3
>>>       .type   f,@function
>>>   f:
>>>       r0 = r1
>>>       w0 &= w2
>>>       exit
>>>   .Lfunc_end0:
>>>       .size   f, .Lfunc_end0-f
>>>
>>> The JIT would not clear the high 32-bits of r0 after the
>>> and-operation, which in this case might give an incorrect return
>>> value.
>>>
>>> After this patch, that is not the case, and the upper 32-bits are
>>> cleared.
>>>
>>> Reported-by: Jiong Wang <jiong.wang@...ronome.com>
>>> Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G")
>>> Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
>>
>> Was this missed because test_verifier did not have test coverage?
> 
> Yup, and Jiong noted it.

Applied, thanks!

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