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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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