linux-kernel - Re: [PATCH v2] ARM: kprobes: rewrite test-[arm|thumb].c in UAL

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Message-ID: <CAKwvOdneTwx8LwKyAA+iMELEBWBxu2nkr9dVuQ=+hgsZROu-tw@mail.gmail.com>
Date:   Thu, 28 Jan 2021 16:21:55 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] ARM: kprobes: rewrite test-[arm|thumb].c in UAL

> On Thu, 28 Jan 2021 at 20:34, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > +       TEST_RX("tbh    [pc, r",7, (9f-(1f+4))>>1,", lsl #1]",
> >
> On Thu, Jan 28, 2021 at 1:03 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> Why is this change needed? Are the resulting opcodes equivalent? Does
> GAS infer the lsl #1 but Clang doesn't?

Yes; it seems if you serialize/deserialize this using GNU `as` and
objdump, that's the canonical form (GNU objdump seems to print in UAL
form, IIUC).  I didn't see anything specifically about `tbh` in
https://developer.arm.com/documentation/dui0473/c/writing-arm-assembly-language/assembly-language-changes-after-rvctv2-1?lang=en
but it's what GNU objdump produces and what clang's integrated
assembler accepts.

> >
> >  #define _DATA_PROCESSING32_DNM(op,s,val)                                       \
> > -       TEST_RR(op s".w r0,  r",1, VAL1,", r",2, val, "")                       \
> > +       TEST_RR(op s"   r0,  r",1, VAL1,", r",2, val, "")                       \
>
> What is wrong with these .w suffixes? Shouldn't the assembler accept
> these even on instructions that only exist in a wide encoding?

Yeah, I'm not sure these have anything to do with UAL.  Looking at
LLVM's sources and IIRC, LLVM has "InstAlias"es it uses for .w
suffixes. I think I need to fix those in LLVM for a couple
instructions, rather than modify these in kernel sources.  I'll split
off the arm-test.c and thumb-test.c into separate patches, fix LLVM,
and drop the .w suffix changes to thumb-test.c.

-- 
Thanks,
~Nick Desaulniers