linux-kernel - odd objtool 'unreachable instruction' warning

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wi6goUT36sR8GE47_P-aVrd5g38=VTRHpktWARbyE-0ow@mail.gmail.com>
Date: Tue, 28 Oct 2025 12:29:11 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: odd objtool 'unreachable instruction' warning

Josh, Peter,
 due to another entirely unrelated discussion, I ended up resurrecting
my "make asm readable" patch that I have had in my local tree when I
want to look at the actual generated code for user accesses.

That is a local hack that just removes the alternative noise for the
common ops, so that I actually see the fences and clac/stac
instructions as such, instead of seeing them as nops in the object
file or as horrible noise in the assembler output.

So that patch is not something I'd ever commit in general, but it's
really useful for checking code generation - but it results in odd
objdump warnings these days (I say "these days", because I've used
that patch locally over the years, and the objdump warning hasn't
always been there).

It's a pretty odd warning, because the code looks fine to me, but I
might be missing something obvious.

Anyway, this is clearly not a big and urgent problem, but I'd love for
you to take a look. I'm attaching the patch I use so  you can see what
I mean.. Any ideas what triggers that warning? Because I'd love to
keep this patch in my local tree without having objtool be upset with
me....

                   Linus

View attachment "0001-Avoid-alternative-assembler-noise-for-common-ops.patch" of type "text/x-patch" (3214 bytes)