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

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Date:   Mon, 23 Oct 2023 14:45:05 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Kees Cook <kees@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, 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 Sat, Oct 21, 2023 at 04:36:19PM -0700, Kees Cook wrote:
> 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.

I'm sorry, but I can't see how this is not an error prone approach.
If it's a timing issue then the arbitrary object change may help and it doesn't
prove anything. As earlier I tried to comment out the error message, and it
worked with GCC as well. The difference is so little (according to Linus) that
it may not be suspectible. Maybe I am missing the point...

-- 
With Best Regards,
Andy Shevchenko