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    SubjectRe: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
    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).

    > > + return true;
    > > +
    > > + if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
    > > return true;
    > >
    > > - for (i = 0; i < CFI_NUM_REGS; i++)
    > > + for (i = 0; i < CFI_NUM_REGS; i++) {
    > > if (state->regs[i].base != initial_func_cfi.regs[i].base ||
    > > state->regs[i].offset != initial_func_cfi.regs[i].offset)
    > > return true;
    > > + }
    > >
    > > return false;
    > > }

    > > @@ -2185,6 +2148,13 @@ static int validate_branch(struct objtoo
    > >
    > > break;
    > >
    > > + case INSN_EXCEPTION_RETURN:
    > > + if (func) {
    > > + state.stack_size -= arch_exception_frame_size;
    > > + break;
    >
    > Why break instead of returning? Shouldn't an exception return mark the end
    > of a branch (whether inside or outside a function) ?
    >
    > Here it seems it will continue to the next instruction which might have been
    > unreachable.

    The code in question (x86's sync_core()), is an exception return to
    self. It pushes an exception frame that points to right after the
    exception return instruction.

    This is the only usage of IRET in STT_FUNC symbols.

    So rather than teaching objtool how to interpret the whole
    push;push;push;push;push;iret sequence, teach it how big the frame is
    (arch_exception_frame_size) and let it continue.

    All the other (real) IRETs are in STT_NOTYPE in the entry assembly.

    > > + }
    > > +
    > > + /* fallthrough */
    >
    > What is the purpose of the fallthrough here? If the exception return was in
    > a function, it carried on to the next instruction, so it won't use the
    > WARN_FUNC(). So, if I'm looking at the right version of the code only the
    > "return 0;" will be used. And, unless my previous comment is wrong, I'd
    > argue that we should return both for func and !func.

    That came from the fact that we split it out of INSN_CONTEXT_SWITCH.
    You're right that it has now reduced to just return 0.

    > > case INSN_CONTEXT_SWITCH:
    > > if (func && (!next_insn || !next_insn->hint)) {
    > > WARN_FUNC("unsupported instruction in callable function",



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