Re: [PATCH V2 5/9] objtool: Add support for intra-function calls

[Julien Thierry]

On 4/8/20 3:06 PM, Alexandre Chartre wrote:
> On 4/7/20 3:28 PM, Alexandre Chartre wrote:
> > On 4/7/20 3:07 PM, Peter Zijlstra wrote:
> > > On Tue, Apr 07, 2020 at 09:31:38AM +0200, Alexandre Chartre wrote:
> > >
> > > > index a62e032863a8..7ee1561bf7ad 100644
> > > > --- a/tools/objtool/arch/x86/decode.c
> > > > +++ b/tools/objtool/arch/x86/decode.c
> > > > @@ -497,3 +497,15 @@ void arch_initial_func_cfi_state(struct 
> > > > cfi_state *state)
> > > > ÂÂÂÂÂ state->regs[16].base = CFI_CFA;
> > > > ÂÂÂÂÂ state->regs[16].offset = -8;
> > > > Â }
> > > > +
> > > > +
> > > > +void arch_configure_intra_function_call(struct stack_op *op)
> > > > +{
> > > > +ÂÂÂ /*
> > > > +ÂÂÂÂ * For the impact on the stack, make an intra-function
> > > > +ÂÂÂÂ * call behaves like a push of an immediate value (the
> > > > +ÂÂÂÂ * return address).
> > > > +ÂÂÂÂ */
> > > > +ÂÂÂ op->src.type = OP_SRC_CONST;
> > > > +ÂÂÂ op->dest.type = OP_DEST_PUSH;
> > > > +}
> > >
> > > An alternative is to always set up stack ops for CALL/RET on decode, but
> > > conditionally run update_insn_state() for them.
> > >
> > > Not sure that makes more logical sense, but the patch would be simpler I
> > > think.
> >
> > Right, this would avoid adding a new arch dependent function and the 
> > patch
> > will be simpler. This probably makes sense as the stack impact is the 
> > same
> > for all calls (but objtool will use it only for intra-function calls).
>
> Actually the processing of the ret instruction is more complicated than I
> anticipated with intra-function calls, and so my implementation is not
> complete at the moment.
>
> The issue is to correctly handle how the ret is going to behave 
> depending how
> the stack (or register on arm) is modified before the ret. Adjusting the 
> stack
> offset makes the stack state correct, but objtool still needs to correctly
> figure out where the ret is going to return and where the code flow 
> continues.

A hint indicating the target "jump" address could be useful. It could be 
used to add the information on some call/jump dynamic that aren't 
associated with jump tables. Currently when objtool finds a jump 
dynamic, if no branches were added to it, it will just return.

Having such a hint could help make additional links (at least on arm64). 
I don't know what Peter and Josh would think of that (if that helps in 
your case of course).

--
Julien Thierry