linux-kernel - Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

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Date: Wed, 15 May 2024 01:10:03 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "olsajiri@...il.com" <olsajiri@...il.com>
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On Mon, 2024-05-13 at 15:23 -0600, Jiri Olsa wrote:
> so at the moment the patch 6 changes shadow stack for
> 
> 1) current uretprobe which are not working at the moment and we change
>    the top value of shadow stack with shstk_push_frame
> 2) optimized uretprobe which needs to push new frame on shadow stack
>    with shstk_update_last_frame
> 
> I think we should do 1) and have current uretprobe working with shadow
> stack, which is broken at the moment
> 
> I'm ok with not using optimized uretprobe when shadow stack is detected
> as enabled and we go with current uretprobe in that case
> 
> would this work for you?

Sorry for the delay. It seems reasonable to me due to 1 being at a fixed address
where 2 was arbitrary address. But Peterz might have felt the opposite earlier.
Not sure.

I'd also love to get some second opinions from broonie (arm shadow stack) and
Deepak (riscv shadow stack).

Deepak, even if riscv has a special instruction that pushes to the shadow stack,
will it be ok if there is a callable operation that does the same thing? Like,
aren't you relying on endbranches or the compiler or something such that
arbitrary data can't be pushed via that instruction?

BTW Jiri, thanks for considering shadow stack in your work.