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SubjectRe: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
On Wed, Apr 01, 2020 at 07:09:10PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
>
> > > +static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
> > > {
> > > + u8 ret_offset = insn->ret_offset;
> > > int i;
> > >
> > > - if (state->cfa.base != initial_func_cfi.cfa.base ||
> > > - state->cfa.offset != initial_func_cfi.cfa.offset ||
> > > - state->stack_size != initial_func_cfi.cfa.offset ||
> > > - state->drap)
> > > + if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
> > > + return true;
> > > +
> > > + if (state->cfa.offset != initial_func_cfi.cfa.offset &&
> > > + !(ret_offset && state->cfa.offset == initial_func_cfi.cfa.offset + ret_offset))
> >
> > Isn't that the same thing as "state->cfa.offset !=
> > initial_func_cfi.cfa.offset + ret_offset" ?
>
> I'm confused on what cfa.offset is, sometimes it increase with
> stack_size, sometimes it doesn't.
>
> ISTR that for the ftrace case it was indeed cfa.offset + 8, but for the
> IRET case below (where it is now not used anymore) it was cfa.offset
> (not cfa.offset + 40, which I was expecting).

It depends on the value of cfa.base. If cfa.base is CFI_SP, then
cfa.offset changes with stack_size. If cfa.base is CFI_BP (i.e. if the
function uses a frame pointer), then cfa.offset is constant (the
distance between RBP on the stack and the previous frame).

--
Josh

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