[lttng-dev] stack validation warning on lttng-modules bytecode interpreter

[Mathieu Desnoyers]

----- On Jun 15, 2016, at 3:38 PM, Josh Poimboeuf jpoimboe at redhat.com wrote:

> On Wed, Jun 15, 2016 at 07:13:39PM +0000, Mathieu Desnoyers wrote:
>> ----- On Jun 15, 2016, at 2:18 PM, Josh Poimboeuf jpoimboe at 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?

Sure. See attached. Built from lttng-modules master branch
commit 63155590 against a 4.6 kernel tree, with lttng-modules
"built-in.sh" script executed targeting the kernel tree. Kernel
config attached too.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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