Messages in this thread | | | Date | Wed, 1 Apr 2020 19:09:10 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET |
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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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