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

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Date: Wed, 15 May 2024 17:26:09 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Deepak Gupta <debug@...osinc.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"songliubraving@...com" <songliubraving@...com>,
	"luto@...nel.org" <luto@...nel.org>,
	"mhiramat@...nel.org" <mhiramat@...nel.org>,
	"andrii@...nel.org" <andrii@...nel.org>,
	"john.fastabend@...il.com" <john.fastabend@...il.com>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"ast@...nel.org" <ast@...nel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
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	"linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>,
	"bp@...en8.de" <bp@...en8.de>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On 05/15, Jiri Olsa wrote:
>
> On Wed, May 15, 2024 at 01:19:20PM +0200, Oleg Nesterov wrote:
> > Let me ask a couple of really stupid questions. What if the shadow stack
> > is "shorter" than the normal stack? I mean,
> >
> > 	enable_shstk()
> > 	{
> > 		prctl(ARCH_SHSTK_SHSTK);

I meant ARCH_SHSTK_ENABLE, of course

> > 	}
> >
> > what happens when enable_shstk() returns?
>
> I think it will crash, there's explanation in the comment in
> tools/testing/selftests/x86/test_shadow_stack.c test

OK, thanks...

But test_shadow_stack.c doesn't do ARCH_PRCTL(ARCH_SHSTK_DISABLE) if
all the tests succeed ? Confused but nevermind.

> > And what is the purpose of fpregs_lock_and_load() ? Why do we need to
> > fpregs_restore_userregs() in shstk_setup() and other places?
> >
> > Oleg.
> >
>