linux-kernel - Re: [RFC][PATCH] objtool,ftrace: Implement UNWIND_HINT_RET

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Message-ID: <20200331211755.pb7f3wa6oxzjnswc@treble>
Date:   Tue, 31 Mar 2020 16:17:55 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

On Tue, Mar 31, 2020 at 10:40:47PM +0200, Peter Zijlstra wrote:
> > I now understand what you're trying to do with the RET_TAIL thing, and I
> > guess it's ok for the ftrace case.  But I'd rather an UNWIND_HINT_IGNORE
> > before the tail cail, which would tell objtool to just silence the tail
> > call warning.  It's simpler for the user to understand, it's simpler
> > logic in objtool, and I think an "ignore warnings for the next insn"
> > hint would be more generally applicable anyway.
> 
> I like how this is specific on how far the stack can be off, as opposed
> so say 'ignore any warning on this instruction'.
> 
> Because by saying this RET should be +8, we'll still get a warning when
> this is not the case (and in fact I should strengthen the patch to
> implement that).
> 
> Also, you don't want to suppress any other valid warning at that
> instruction.

Ok, I guess I'm convinced :-)  As we continue to add new warnings to
objtool, it is true that "ignore all warnings at this insn" is probably
too broad.

/me stops writing patch

BTW, if we're in agreement that this hint doesn't belong for
sync_core(), will sp_offset always be +8?  Just wondering if we can
hard-code that assumption.

> > I know you said it's like an indirect tail call with a bigger frame, but
> > that's kind of stretching it because the function frame is still there.
> > 
> > And objtool doesn't treat it like a tail call at all.  In fact, it
> > handles it *completely* differently from the normal ret-tail-call case.
> > Instead of silencing a tail call warning, it adjusts the stack offset
> > and continues the code path.
> > 
> > This basically adds *two* new hint types, while trying to call them the
> > same thing.  There's no overlapping functionality between them in
> > objtool, other than the use of the same insn->ret_offset variable.  But
> > it's two distinct functionalities, depending on the context (return/tail
> > vs IRETQ).
> 
> I'm not against adding a second/separate hint for this. In fact, I
> almost considered teaching objtool how to interpret the whole IRET frame
> so that we can do it without hints. It's just that that's too much code
> for this one case.
> 
> HINT_IRET_SELF ?

Despite my earlier complaint about stack size knowledge, we could just
forget the hint and make "iretq in C code" equivalent to "reduce stack
size by arch_exception_stack_size()" and keep going.  There's
file->c_file which tells you it's a C file.

-- 
Josh