Message-ID: <20250926192919.349578-1-cmirabil@redhat.com>
Date: Fri, 26 Sep 2025 15:29:19 -0400
From: Charles Mirabile <cmirabil@...hat.com>
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Subject: Re: [PATCH v19 00/27] riscv control-flow integrity for usermode

Hi - 

Hoping that I got everything right with git-send-email so that this is
delivered alright...

Wanted to jump in to head off a potential talking past one another / 
miscommunication situation I see here.

On Wed, Sep 24, 2025 at 08:36:11AM -0600, Paul Walmsley wrote:
> Hi,
> 
> On Thu, 31 Jul 2025, Deepak Gupta wrote:
> 
> [ ... ]
> 
> > vDSO related Opens (in the flux)
> > =================================
> > 
> > I am listing these opens for laying out plan and what to expect in future
> > patch sets. And of course for the sake of discussion.
> > 
> 
> [ ... ]
> 
> > How many vDSOs
> > ---------------
> > Shadow stack instructions are carved out of zimop (may be operations) and if CPU
> > doesn't implement zimop, they're illegal instructions. Kernel could be running on
> > a CPU which may or may not implement zimop. And thus kernel will have to carry 2
> > different vDSOs and expose the appropriate one depending on whether CPU implements
> > zimop or not.
> 
> If we merge this series without this, then when CFI is enabled in the 
> Kconfig, we'll wind up with a non-portable kernel that won't run on older 
> hardware.  We go to great lengths to enable kernel binary portability 
> across the presence or absence of other RISC-V extensions, and I think 
> these CFI extensions should be no different.

That is not true, this series does not contain the VDSO changes so it can
be merged as is.

> 
> So before considering this for merging, I'd like to see at least an 
> attempt to implement the dual-vDSO approach (or something equivalent) 
> where the same kernel binary with CFI enabled can run on both pre-Zimop 
> and post-Zimop hardware, with the existing userspaces that are common 
> today.

I agree that when the VDSO patches are submitted for inclusion they should
be written in a way that avoids limiting the entire kernel to either
pre-Zimop or post-Zimop hardware based on the config, but I think it
should be quite possible to perform e.g. runtime patching of the VDSO
to replace the Zimop instructions with nops if the config is enabled but
the hardware does not support Zimop.

However, that concern should not hold up this patch series. Raise it again
when the VDSO patches are posted.

> 
> thanks Deepak,
> 
> - Paul

Best - Charlie

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