Messages in this thread | | | Date | Wed, 15 Jun 2016 14:38:46 -0500 | From | Josh Poimboeuf <> | Subject | Re: stack validation warning on lttng-modules bytecode interpreter |
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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
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