Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

[Julien Thierry]

On 4/2/20 7:41 AM, Julien Thierry wrote:
> On 4/1/20 6:09 PM, Peter Zijlstra wrote:
> > On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
> > > > +ÂÂÂÂÂÂÂ return true;
> > > > +
> > > > +ÂÂÂ if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
> > > > ÂÂÂÂÂÂÂÂÂÂ return true;
> > > >
> > > > -ÂÂÂ for (i = 0; i < CFI_NUM_REGS; i++)
> > > > +ÂÂÂ for (i = 0; i < CFI_NUM_REGS; i++) {
> > > > ÂÂÂÂÂÂÂÂÂÂ if (state->regs[i].base != initial_func_cfi.regs[i].base ||
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->regs[i].offset != 
> > > > initial_func_cfi.regs[i].offset)
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return true;
> > > > +ÂÂÂ }
> > > >
> > > > ÂÂÂÂÂÂ return false;
> > > > ÂÂ }
> >
> > > > @@ -2185,6 +2148,13 @@ static int validate_branch(struct objtoo
> > > >
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > >
> > > > +ÂÂÂÂÂÂÂ case INSN_EXCEPTION_RETURN:
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ if (func) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state.stack_size -= arch_exception_frame_size;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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.
>
> Right, I see.. However I'm not completely convinced by this. I must 
> admit I haven't followed the whole conversation, but what was the issue 
> with the HINT_IRET_SELF? It seemed more elegant, but I might be missing 
> some context.
>
> Otherwise, it might be worth having a comment in the code to point that 
> this only handles the sync_core() case.
>
>
> Also, instead of adding a special "arch_exception_frame_size", I could 
> suggest:
> - Picking this patch [1] from a completely arbitrary source
> - Getting rid of INSN_STACK type, any instruction could then include 
> stack ops on top of their existing semantics, they can just have an 
> empty list if they don't touch SP/BP
> - x86 decoder adds a stack_op to the iret to modify the stack pointer by 
> the right amount

And the x86 decode could also lookup the symbol containing an IRET and 
chose whether its type should be INSN_CONTEXT_SWITCH or INSN_OTHER 
depending on whether the symbol is a function or not.

This would avoid having the arch specific pattern detected the generic 
stack validation part of objtool.

--
Julien Thierry