linux-ext4 - Re: [GIT PULL] ext2, quota, and udf fixes for 6.6-rc1

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Message-ID: <BF6761C0-B813-4C98-9563-8323C208F67D@kernel.org>
Date:   Sat, 21 Oct 2023 16:36:19 -0700
From:   Kees Cook <kees@...nel.org>
To:     andy.shevchenko@...il.com, Jan Kara <jack@...e.cz>
CC:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        Baokun Li <libaokun1@...wei.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ferry Toth <ftoth@...londelft.nl>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [GIT PULL] ext2, quota, and udf fixes for 6.6-rc1



On October 20, 2023 1:36:36 PM PDT, andy.shevchenko@...il.com wrote:
>That said, if you or anyone has ideas how to debug futher, I'm all ears!

I don't think this has been tried yet:

When I've had these kind of hard-to-find glitches I've used manual built-binary bisection. Assuming you have a source tree that works when built with Clang and not with GCC:
- build the tree with Clang with, say, O=build-clang
- build the tree with GCC, O=build-gcc
- make a new tree for testing: cp -a build-clang build-test
- pick a suspect .o file (or files) to copy from build-gcc into build-test
- perform a relink: "make O=build-test" should DTRT since the copied-in .o files should be newer than the .a and other targets
- test for failure, repeat

Once you've isolated it to (hopefully) a single .o file, then comes the byte-by-byte analysis or something similar...

I hope that helps! These kinds of bugs are super frustrating.

-Kees

-- 
Kees Cook