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To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux@googlegroups.com, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org
From:   Nathan Chancellor <nathan@kernel.org>
Subject: objtool warning in cfg80211_edmg_chandef_valid() with ThinLTO
Message-ID: <5913cdf4-9c8e-38f8-8914-d3b8a3565d73@kernel.org>
Date:   Tue, 24 Aug 2021 13:08:58 -0700
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Hi Josh and Peter,

An in-review LLVM patch [1] introduces an optimization around switch 
cases, marking the default case unreachable when all known values are 
covered. This ended up introducing a boot regression when building a 
kernel with ThinLTO, which Sami reported initially [2] and I reported it 
upstream, generating some discussion starting from [3].

There is an objtool warning about the function that the author of the 
patch tracked down to be problematic:

vmlinux.o: warning: objtool: cfg80211_edmg_chandef_valid()+0x169: can't 
find jump dest instruction at .text.cfg80211_edmg_chandef_valid+0x17b

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'."

I can confirm that adding '-mllvm -trap-unreachable' to 
KBUILD_{C,LD}FLAGS does resolve the issue but I am curious if that is an 
adequate solution (as Nick has frowned upon the presence of '-mllvm' 
flags) or if there is something that could be done on the objtool side. 
I do have a small reproducer on Phabricator [4] and I can provide any 
binary files that would be helpful for seeing what is going on here.

[1] https://reviews.llvm.org/D106056
[2] https://github.com/ClangBuiltLinux/linux/issues/1440
[3] https://reviews.llvm.org/D106056#2953104
[4] https://reviews.llvm.org/D106056#2961136

Cheers,
Nathan