linux-kernel - Re: [RFC PATCH v4 22/37] arm64: kernel: Skip validation of kuser32.o

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Date:   Thu, 5 May 2022 11:56:45 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Chen Zhongjin <chenzhongjin@...wei.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arch@...r.kernel.org, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org, masahiroy@...nel.org,
        jpoimboe@...hat.com, ycote@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, ardb@...nel.org, maz@...nel.org,
        tglx@...utronix.de, luc.vanoostenryck@...il.com
Subject: Re: [RFC PATCH v4 22/37] arm64: kernel: Skip validation of kuser32.o

On Thu, May 05, 2022 at 11:24:48AM +0200, Peter Zijlstra wrote:
> On Thu, May 05, 2022 at 11:36:12AM +0800, Chen Zhongjin wrote:
> > Hi Peter,
> > 
> > IIRC now the blacklist mechanisms all run on check stage, which after
> > decoding, but the problem of kuser32.S happens in decoding stage. Other
> > than that the assembly symbols in kuser32 is STT_NOTYPE and
> > STACK_FRAME_NON_STANDARD will throw an error for this.
> > 
> > OBJECT_FILES_NON_STANDARD works for the single file but as you said
> > after LTO it's invalid. However STACK_FRAME_NON_STANDARD doesn't work
> > for kuser32 case at all.
> > 
> > Now my strategy for undecodable instructions is: show an error message
> > and mark insn->ignore = true, but do not stop anything so decoding work
> > can going on.
> > 
> > To totally solve this my idea is that applying blacklist before decode.
> > However for this part objtool doesn't have any insn or func info, so we
> > should add a new blacklist just for this case...
> 
> OK, so Mark explained that this is 32bit userspace (VDSO) code.
> 
> And as such there's really no point in running objtool on it. Does all
> that live in it's own section? Should it?

It's placed in .rodata by a linker script:

* The 32-bit vdso + kuser code is placed in .rodata, between the `vdso32_start`
  and `vdso32_end` symbols, as raw bytes (via .incbin).
  See arch/arm64/kernel/vdso32-wrap.S.

* The 64-bit vdso code is placed in .rodata, between the `vdso_start`
  and `vdso32` symbols, as raw bytes (via .incbin).
  See arch/arm64/kernel/vdso-wrap.S.

The objects under arch/arm64/kernel/{vdso,vdso32}/ are all userspace objects,
and from userspace's PoV the existing secrtions within those objects are
correct, so I don't think those should change.

How does x86 deal with its vdso objects?

Thanks,
Mark.