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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH v2] xfs: fix an undefined behaviour in _da3_path_shift
Date:   Tue, 25 Feb 2020 16:55:56 -0500
Message-Id: <F151ED18-55CF-482E-98BE-45A5A4D9A565@lca.pw>
References: <20200225214045.GA14399@infradead.org>
Cc:     darrick.wong@oracle.com, linux-xfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <20200225214045.GA14399@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Feb 25, 2020, at 4:40 PM, Christoph Hellwig <hch@infradead.org> wrote:
> 
> In xfs_da3_path_shift() blk can be assigned to state->path.blk[-1] if
> state->path.active is 1 (which is a valid state) when it tries to add an
> entry > to a single dir leaf block and then to shift forward to see if
> there's a sibling block that would be a better place to put the new
> entry.  This causes a KASAN warning given

s/KASAN/UBSAN/

> negative array indices are
> undefined behavior in C.  In practice the warning is entirely harmless
> given that blk is never dereference in this case, but it is still better
> to fix up the warning and slightly improve the code.

Agree. This is better.

Darrick, do you need me to send a v3 for it or you could squash this in?