linux-kernel - Re: [RFC PATCH v1 0/9] arm64: livepatch: Use DWARF Call Frame Information for frame pointer validation

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Message-ID: <YlAlK+94gBlVlv2J@hirez.programming.kicks-ass.net>
Date:   Fri, 8 Apr 2022 14:06:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     madvenka@...ux.microsoft.com
Cc:     mark.rutland@....com, broonie@...nel.org, jpoimboe@...hat.com,
        ardb@...nel.org, nobuta.keiya@...itsu.com,
        sjitindarsingh@...il.com, catalin.marinas@....com, will@...nel.org,
        jmorris@...ei.org, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/9] arm64: livepatch: Use DWARF Call Frame
 Information for frame pointer validation

On Thu, Apr 07, 2022 at 03:25:09PM -0500, madvenka@...ux.microsoft.com wrote:
> The solution
> ============
> 
> The goal here is to use the absolute minimum CFI needed to compute the FP at
> every instruction address. The unwinder can compute the FP in each frame,
> compare the actual FP with the computed one and validate the actual FP.
> 
> Objtool is enhanced to parse the CFI, extract just the rules required,
> encode them in compact data structures and create special sections for
> the rules. The unwinder uses the special sections to find the rules for
> a given instruction address and compute the FP.
> 
> Objtool can be invoked as follows:
> 
> 	objtool dwarf generate <object-file>
> 
> The version of the DWARF standard supported in this work is version 4. The
> official documentation for this version is here:
> 
> 	https://dwarfstd.org/doc/DWARF4.pdf
> 
> Section 6.4 contains the description of the CFI.

The problem is of course that DWARF is only available for compiler
generated code and doesn't cover assembly code, of which is there is
always lots.

I suppose you can go add DWARF annotations to all the assembly, but IIRC
those are pretty terrible. We were *REALLY* happy to delete all that
nasty from the x86 code.

On top of that, AFAIK compilers don't generally consider DWARF
generation to be a correctness issue. For them it's debug info and
having it be correct is nice but not required. So using it as input for
something that's required to be correct, seems unfortunate.