Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

On Wed, Aug 09, 2023 at 04:34:38PM +0100, Mark Brown wrote:

On Wed, Aug 09, 2023 at 03:24:14PM +0100, Catalin Marinas wrote:

quoted

On Mon, Aug 07, 2023 at 11:00:08PM +0100, Mark Brown wrote:

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+* When set PR_SHADOW_STACK_ENABLE flag allocates a Guarded Control Stack for

The 'for' at the end of the line above is not needed.

quoted

+  and enables GCS for the thread, enabling the functionality controlled by

I find it a little clearer that it's a per thread stack here but sure.

If it reads better for you, feel free to keep it as is.

quoted

quoted

+3.  Allocation of Guarded Control Stacks
+----------------------------------------

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+* When GCS is enabled for a thread a new Guarded Control Stack will be
+  allocated for it of size RLIMIT_STACK / 2 or 2 gigabytes, whichever is
+  smaller.

quoted

Is this number based on the fact that a function call would only push
the LR to GCS while standard function prologue pushes at least two
registers?

It's actually based on bitrot that I'd initially chosen a smaller value
since it's likely that functions will push at least something as you
suggest, the patches now just use RLIMIT_STACK.  I'll fix.

A related question - it may have been discussed intensively on the x86
thread (I may read it sometime) - why not have the libc map the shadow
stack and pass the pointer/size to clone3()? It saves us from having to
guess what the right size we'd need. struct clone_args is extensible.

(I plan to get back next week to this series, I'll need to read a bit
more on the spec)

-- 
Catalin

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