Re: stack validation warning on lttng-modules bytecode interpreter

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Wed, Jun 15, 2016 at 07:13:39PM +0000, Mathieu Desnoyers wrote:
> ----- On Jun 15, 2016, at 2:18 PM, Josh Poimboeuf jpoimboe@redhat.com wrote:
> 
> > On Wed, Jun 15, 2016 at 04:55:16PM +0000, Mathieu Desnoyers wrote:
> >> Hi Josh,
> >> 
> >> I notice that with gcc 6.1.1, kernel 4.6, with
> >> CONFIG_STACK_VALIDATION=y, building lttng-modules master
> >> at commit 6c09dd94 gives this warning:
> >> 
> >> lttng-modules/lttng-filter-interpreter.o: warning: objtool:
> >> lttng_filter_interpret_bytecode()+0x58: sibling call from callable instruction
> >> with changed frame pointer
> >> 
> >> this object implements a bytecode interpreter using an explicit
> >> jump table (see
> >> https://github.com/lttng/lttng-modules/blob/master/lttng-filter-interpreter.c)
> >> 
> >> If I define "INTERPRETER_USE_SWITCH" at the top of the file,
> >> thus using the switch-case fallback implementation, the
> >> warning vanishes.
> >> 
> >> We use an explicit jump table rather than a switch case whenever
> >> possible for performance reasons.
> >> 
> >> I notice that tools/objtool/builtin-check.c needs to be aware of
> >> switch-cases transformed into jump tables by the compiler. Are
> >> explicit jump tables supported by the stack validator ? Do we
> >> need to add annotation to our code ?
> > 
> > Hi Mathieu,
> > 
> > Unfortunately objtool doesn't know how to validate this type of jump
> > table.  So to avoid the warning you'll need to add an annotation to tell
> > objtool to ignore it:
> > 
> >  STACK_FRAME_NON_STANDARD(lttng_filter_interpret_bytecode);
> > 
> > We had to annotate __bpf_prog_run() in the kernel for the same reason.
> 
> Thanks for the tip! Unfortunately it does not seem to work.
> 
> objdump -t lttng/lttng-filter-interpreter.o output gives:
> 
> 0000000000000000 l    d  __func_stack_frame_non_standard        0000000000000000 __func_stack_frame_non_standard
> 0000000000000000 l     O __func_stack_frame_non_standard        0000000000000008 __func_stack_frame_non_standard_lttng_filter_interpret_bytecode
> 
> Running objtool check (built in O0) in gdb on lttng-filter-interpreter.o
> built with the STACK_FRAME_NON_STANDARD define, it appears that the
> following function:
> 
> static bool ignore_func(struct objtool_file *file, struct symbol *func)
> {
>         struct rela *rela;
>         struct instruction *insn;
> 
>         /* check for STACK_FRAME_NON_STANDARD */
>         if (file->whitelist && file->whitelist->rela)
>                 list_for_each_entry(rela, &file->whitelist->rela->rela_list, list)
>                         if (rela->sym->sec == func->sec &&
>                             rela->addend == func->offset)
>                                 return true;
> 
>         /* check if it has a context switching instruction */
>         func_for_each_insn(file, func, insn)
>                 if (insn->type == INSN_CONTEXT_SWITCH)
>                         return true;
> 
>         return false;
> }
> 
> For lttng_filter_interpret_bytecode, while in the first list
> iteration:
> 
> (gdb) print rela->sym->sec
> $18 = (struct section *) 0x7ffff7e20010
> (gdb) print func->sec
> $19 = (struct section *) 0x7ffff7e20010
> 
> But
> 
> (gdb) print rela->addend
> $20 = 0
> (gdb) print func->offset
> $21 = 928
> 
> So for some reason it never match the ignore_func.
> This happens both when I build lttng-modules as a kernel module,
> and when I build it into the kernel image.
> 
> Any idea why ?

Hm, no idea.  Can you send me the object file?

-- 
Josh