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

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Date: Tue, 21 May 2024 12:11:57 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On Tue, May 21, 2024 at 01:31:53AM +0000, Edgecombe, Rick P wrote:
> On Mon, 2024-05-20 at 00:18 +0200, Jiri Olsa wrote:
> > anyway I think we can fix that in another way by using the optimized
> > trampoline,
> > but returning to the user space through iret when shadow stack is detected
> > (as I did in the first version, before you adjusted it to the sysret path).
> > 
> > we need to update the return address on stack only when returning through the
> > trampoline, but we can jump to original return address directly from syscall
> > through iret.. which is slower, but with shadow stack we don't care
> > 
> > basically the only change is adding the shstk_is_enabled check to the
> > following condition in SYSCALL_DEFINE0(uretprobe):
> > 
> >         if (regs->sp != sp || shstk_is_enabled())
> >                 return regs->ax;
> 
> On the surface it sounds reasonable. Thanks.
> 
> And then I guess if tradeoffs are seen differently in the future, and we want to
> enable the fast path for shadow stack we can go with your other solution. So
> this just simply fixes things functionally without much code.

yes, if we want to enable the fast path for shadow stack in future
we'll need to remove that shstk_is_enabled and push extra frame on
shadow stack

jirka