netdev - Re: [PATCH bpf] bpf, riscv: clear target register high 32-bits for and/or/xor on ALU32

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Date:   Tue, 21 May 2019 16:12:12 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 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.

> If so, could you follow-up with alu32 test cases for it, so other
> JITs can be tracked for these kind of issue as well. We should
> probably have one for every alu32 alu op to make sure it's not
> forgotten anywhere.
>

I'll hack a test_verifier test right away.

Thanks,
Björn


> Thanks,
> Daniel