Messages in this thread | | | Date | Mon, 29 Aug 2022 21:46:27 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] x86/sev: Mark snp_abort() noreturn |
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On Wed, Aug 24, 2022 at 10:45:12PM +0200, Peter Zijlstra wrote: > On Wed, Aug 24, 2022 at 12:29:29PM -0500, Segher Boessenkool wrote: > > Hi! > > > > On Wed, Aug 24, 2022 at 05:24:20PM +0200, Borislav Petkov wrote: > > > Mark both the function prototype and definition as noreturn in order to > > > prevent the compiler from doing transformations which confuse objtool > > > like so: > > > > > > vmlinux.o: warning: objtool: sme_enable+0x71: unreachable instruction > > > > Would -Wmissing-noreturn have caught this? It sounds like you need this > > (and then fix all resulting warnings) to not upset objtool? > > > > It is nice to have this anyway (if there aren't a zillion false > > positives), but it seems objtool is very fragile. > > Well, just like gcc has noreturn heuristics so has objtool, it just > turns into pain when they don't agree with one another. > > Ideally noreturn would be reflected in the object file so we don't have > to guess at it. STT_FUNC_NORETURN would do I suppose, except then all > the tools will need to be taught how to deal with that, which is also > very painful. > > Another options is something like .symtab.noreturn which is another > symbol table explicitly listing the noreturn functions. Since it's an > extra section tools that don't know about it can freely ignore it and > carry on as usual.
We're planning to talk about this at the LPC toolchains microconference.
My proposal is similar to yours except I called it .annotate.noreturn. It would be enabled with a --annotate=noreturn compiler option. It would report both explicit noreturns (with the "noreturn" function attribute) and implicit noreturns (static functions which only call other noreturn functions).
There would also be an --annotate=jump_table option which creates an .annotate.jump_table to describe switch statement jump table flow.
Those are the two biggest challenges for objtool.
-- Josh
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