Messages in this thread | | | Date | Wed, 15 Jun 2016 13:18:17 -0500 | From | Josh Poimboeuf <> | Subject | Re: stack validation warning on lttng-modules bytecode interpreter |
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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.
-- Josh
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