linux-kernel - Re: [PATCH 3/3] objtool: check: Make SP memory operation match PUSH/POP semantics

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Date:   Mon, 21 Sep 2020 10:12:26 -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 3/3] objtool: check: Make SP memory operation match
 PUSH/POP semantics

On Mon, Sep 21, 2020 at 11:31:28AM +0100, Julien Thierry wrote:
> > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > > index f45991c2db41..7ff87fa3caec 100644
> > > --- a/tools/objtool/check.c
> > > +++ b/tools/objtool/check.c
> > > @@ -2005,6 +2005,13 @@ static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,
> > >   			break;
> > >   		case OP_SRC_REG_INDIRECT:
> > > +			if (!cfi->drap && op->dest.reg == cfa->base) {
> > 
> > && op->dest.reg == CFI_BP ?
> > 
> 
> Does it matter? My unstandig was that the register used to point to the CFA
> is getting overwritten, so we need to fallback to something known which is
> the offset from the stack pointer.
> 
> Was that not the case?

Maybe.  I was wondering if it would be possible to overwrite the stack
pointer, like 'mov disp(%rsp), %rsp', which could be possible in asm.

Though I suppose the below code would be harmless, since the CFA
base/offset would already be CFI_SP/cfi->stack_size respectively.

Still, no harm in making the condition more precise.

> > > +
> > > +				/* mov disp(%rsp), %rbp */
> > > +				cfa->base = CFI_SP;
> > > +				cfa->offset = cfi->stack_size;
> > > +			}

-- 
Josh