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

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Message-ID: <20220321122259.28146a7a@gandalf.local.home>
Date:   Mon, 21 Mar 2022 12:22:59 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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, 21 Mar 2022 12:15:49 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> And I just thought of another issue, where even my solution wont fix it.
> What happens if we trace funcA but not funcB? How do we get to trace the
> end of funcA?

The only solution I can think of to handle all these cases is if you enable
-mfexit, you have to disable tail calls completely. That's going to cause
a performance impact.

Perhaps we need need compiler help to give us a way to hijack the return
address. But is there a way to do this and still not give up the security
that CET SHSTK gives us?

Or maybe another solution is:

funcA:
	[..]
	jmp funcB
	call __fexit__
	ret

And if funcA is being traced, we change jmp to a call.

	[..]
	call funcB
	call __fexit__
	ret

Such that we only remove the tail calls if we enable tracing on the
function with the tail call.

-- Steve