linux-kernel - Re: objtool warning in cfg80211_edmg_chandef

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Message-ID: <20210824210507.GC17784@worktop.programming.kicks-ass.net>
Date:   Tue, 24 Aug 2021 23:05:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux@...glegroups.com, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: objtool warning in cfg80211_edmg_chandef_valid() with ThinLTO

On Tue, Aug 24, 2021 at 01:08:58PM -0700, Nathan Chancellor wrote:

> The LLVM developers are under the impression that this is an issue with
> objtool; specifically quoting Eli Friedman:
> 
> "The backend can, in general, create basic blocks that don't contain any
> instructions, and don't fall through to another block. A jump table entry
> can refer to such a block. I guess certain tools could be confused by this.
> 
> If that's the issue, it should be possible to work around it using '-mllvm
> -trap-unreachable'."

So jump-tables are a weak point; ARM64 was having worse problems than
x86 there, they can't even locate them.

As to having a jump-table entry to an empty block and not falling
through; how are we supposed to know? Emitting them is a waste of space,
so I'd say it's a compiler bug :-))

It's been brought up before; but perhaps we should look at an 'informal'
ABI for jump-tables ?