linux-kernel - Re: [tip:x86/tdx 8/12] vmlinux.o: warning: objtool: __tdx_hypercall+0x128: __tdx_hypercall_failed() is missing a _

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Date:   Thu, 14 Sep 2023 10:59:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        lkp <lkp@...el.com>,
        "oe-kbuild-all@...ts.linux.dev" <oe-kbuild-all@...ts.linux.dev>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [tip:x86/tdx 8/12] vmlinux.o: warning: objtool:
 __tdx_hypercall+0x128: __tdx_hypercall_failed() is missing a __noreturn
 annotation

On Thu, Sep 14, 2023 at 07:54:10AM +0000, Huang, Kai wrote:

> > The point of noreturn is that the caller should know to stop generating
> > code. For that the declaration needs the attribute, because call sites
> > typically do not have access to the function definition in C.
> 
> Ah that makes perfect sense.  Thanks!
> 
> Then I assume we don't need to annotate __noreturn in the function body, but
> only in the declaration?  Because the compiler must already have seen the
> declaration when it generates the code for the function body.

I think so, I'm sure it'll tell you if it disagrees :-)

> Btw, I happened to notice that the objtool documentation suggests that we should
> also add the the function to tools/objtool/noreturns.h:
> 
> 3. file.o: warning: objtool: foo+0x48c: bar() is missing a __noreturn annotation
> 
>    The call from foo() to bar() doesn't return, but bar() is missing the
>    __noreturn annotation.  NOTE: In addition to annotating the function
>    with __noreturn, please also add it to tools/objtool/noreturns.h.
> 
> Is it a behaviour that we still need to follow?

Yes. objtool has some heuristics to detect noreturn, but is is very
difficult. Sadly noreturn is not something that is reflected in the ELF
object file so we have to guess.

For now manually adding it to the objtool list is required, we're trying
to get to the point where it is generated/validated by the compiler,
perhaps with a plugin, but we're not there yet.