linux-kernel - Re: [PATCH] ext4: mballoc: avoid garbage value from err

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Message-ID: <20230724205721.GA2504240@mit.edu>
Date:   Mon, 24 Jul 2023 16:57:21 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Su Hui <suhui@...china.com>, adilger.kernel@...ger.ca,
        nathan@...nel.org, trix@...hat.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] ext4: mballoc: avoid garbage value from err

On Mon, Jul 24, 2023 at 10:19:02AM -0700, Nick Desaulniers wrote:
> >
> > err is uninitialized and will be judged when it enters the
> > loop first time and the condition "!ext4_sb_block_valid()"
> > is true. Although this can't make problems now, it's better
> > to correct it.
> >
> > Signed-off-by: Su Hui <suhui@...china.com>
> 
> Hi Su,
> Thanks for the patch!  I see what the warning is getting at;
> 
> If `len <= 0` then `err` is never initialized, then is used at line
> 4178 (that is UB).
> 
> Would you mind sending a v2 with the commit message updated to reflect
> the above points?  I'd be happy to sign-off on that.

Fortunately, as near as I can tell, ext4_mb_mark_bb() should never be
called with len <= 0.  It might be possible to trick ext4 via a
corrupted file system --- I'd have to take a closer look at that, but
fortunately, in the case where len <= 0, bitmap_bh will be NULL, so
regardless of whether err is 0, or some garbage non-zero value,
brelse(NULL) is a no-op.

So while it's good to avoid the clang warning, but the fact that it
might be possible for err to be a "garbage value" shouldn't be causing
any problem.

Cheers,

						- Ted