stack validation warning on lttng-modules bytecode interpreter

Mathieu Desnoyers
2016-06-15T16:55:25Z

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 ?

Thanks,