linux-kernel - Re: objtool warning in cfg80211_edmg_chandef

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Date:   Tue, 24 Aug 2021 14:19:07 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 8/24/2021 2:05 PM, Peter Zijlstra wrote:
> 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?

Fair enough. It does make me wonder why LLVM does that.

> Emitting them is a waste of space, so I'd say it's a compiler bug :-))

Isn't it always? :)

Turns out Nick brought up an issue very similar to this (unreachable 
conditions with switches) on LLVM's issue tracker 
(https://bugs.llvm.org/show_bug.cgi?id=50080) with the same workaround 
suggestion ('-mllvm -trap-unreachable') and there was no follow up after 
that so maybe that is one thing to look into once Nick is back online.

> It's been brought up before; but perhaps we should look at an 'informal'
> ABI for jump-tables ?
Not a bad idea, especially if this has come up before.

Cheers,
Nathan