linux-kernel - Re: [PATCH] riscv, kprobes: Stricter c.jr/c.jalr decoding

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Date:   Tue, 3 Jan 2023 10:33:20 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Björn Töpel <bjorn@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...nel.org>,
        Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv, kprobes: Stricter c.jr/c.jalr decoding

On Tue, Jan 03, 2023 at 07:44:49AM +0100, Björn Töpel wrote:
> Conor Dooley <conor@...nel.org> writes:
> > On Mon, Jan 02, 2023 at 05:07:48PM +0100, Björn Töpel wrote:
> >> From: Björn Töpel <bjorn@...osinc.com>
> >> diff --git a/arch/riscv/kernel/probes/simulate-insn.h b/arch/riscv/kernel/probes/simulate-insn.h
> >> index cb6ff7dccb92..de8474146a9b 100644
> >> --- a/arch/riscv/kernel/probes/simulate-insn.h
> >> +++ b/arch/riscv/kernel/probes/simulate-insn.h
> >> @@ -31,9 +31,9 @@ __RISCV_INSN_FUNCS(fence,	0x7f, 0x0f);
> >>  	} while (0)
> >>  
> >>  __RISCV_INSN_FUNCS(c_j,		0xe003, 0xa001);
> >> -__RISCV_INSN_FUNCS(c_jr,	0xf007, 0x8002);
> >
> > Hmm, I wonder where the mask originally came from!
> 
> I think it's just a simple bug -- missing that "rs2" must be zero.
> 
> > I had a look at the compressed spec, of which the version google gave to
> > me was v1.9 [1], and Table 1.6 in that (Instruction listing for RVC,
> > Quadrant 2) seems to list them all together.

> > [1] -
> > https://riscv.org/wp-content/uploads/2015/11/riscv-compressed-spec-v1.9.pdf
> 
> C-ext is part of the unpriv spec:
> https://github.com/riscv/riscv-isa-manual/releases

Yah, I was trying to see if there was some period piece which was
misleading that would have explained the mask. I looked again & the v1.7
spec doesn't have that table, but also has no reason to suggest the
current mask either. Guess it was just a mistake :)

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