linux-kernel - Re: [PATCH v2 1/3] objtool: check: Fully validate the stack frame

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Message-ID: <20201012153500.owcyvgjv4m3t3nh3@treble>
Date:   Mon, 12 Oct 2020 10:35:00 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Julien Thierry <jthierry@...hat.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, mbenes@...e.cz,
        raphael.gault@....com, benh@...nel.crashing.org
Subject: Re: [PATCH v2 1/3] objtool: check: Fully validate the stack frame

On Mon, Oct 12, 2020 at 11:21:49AM +0100, Julien Thierry wrote:
> On 9/29/20 8:18 PM, Josh Poimboeuf wrote:
> > "Stack frame" has more than one meaning now, I suppose.  i.e. it could
> > also include the callee-saved registers and any other stack space
> > allocated by the function.
> > 
> > Would "call frame" be clearer?
> > 
> >    CALL_FRAME_BP_OFFSET
> >    CALL_FRAME_RA_OFFSET
> > 
> > ?
> 
> I would've thought that the call-frame could include the stackframe + other
> callee saved regs.

Hm, probably so.

> Whereas stackframe tends to used for the caller's frame pointer +
> return address (i.e. what allows unwinding). Unless I'm getting lost
> with things.

I've always seen "stack frame" used to indicate the function's entire
stack.

> And if call frame is associated with the region starting from the stack
> pointer at the parent call point (since this is what CFA is), then it
> shouldn't be associated with the framepointer + return address structure
> since this could be anywhere on the call frame (not at a fixed offset) as
> long as the new frame pointer points to the structure.

I suppose "call frame" and "stack frame" probably mean the same thing,
in which case neither is appropriate here...

In fact, maybe we could forget the concept of a frame (or even a struct)
here.

If cfa.base is CFI_BP, then is regs[CFI_BP].offset always the same as
-cfa.offset?  i.e. could the BP checks could it just be a simple

  regs[CFI_BP].offset == -cfa.offset

check?

And then is RA at regs[CFI_BP].offset + 8?

-- 
Josh