linux-kernel - Re: linux-next: build warnings after merge of the tip tree

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Message-ID: <YjiiDFHIQg78QwSb@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Mar 2022 17:04:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        mhiramat@...nel.org, ast@...nel.org, hjl.tools@...il.com,
        rick.p.edgecombe@...el.com, rppt@...nel.org,
        linux-toolchains@...r.kernel.org, Andrew.Cooper3@...rix.com,
        ndesaulniers@...gle.com
Subject: Re: linux-next: build warnings after merge of the tip tree

On Mon, Mar 21, 2022 at 11:28:05AM -0400, Steven Rostedt wrote:
> On Mon, 21 Mar 2022 14:04:05 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:

> > Also, folks, I'm thinking we should start to move to __fexit__, if CET
> > SHSTK ever wants to come to kernel land return trampolines will
> > insta-stop working.
> > 
> > Hjl, do you think we could get -mfexit to go along with -mfentry ?

> int funcA () {
> 	[..]
> 	return funcB();
> }

> This currently works with function graph and kretprobe tracing because of
> the shadow stack. Let's say we traced both funcA and funcB
> 
> funcA:
> 	call __fentry__
			push funcA on trace-stack
> 
> 	[..]
> 	jmp funcB
> 
> funcB:
> 	call __fentry__
			push funcB on trace-stack
> 
> 	[..]
	call __fexit__
			pop trace-stack until empty
			  'exit funcB'
			  'exit funcA'

> 	ret

> 
> That is, the current algorithm traces the end of both funcA and funcB
> without issue, because of how the shadow stack works.

And it all works, no? Or what am I missing?