linux-kernel - RE: [PATCH] RDMA/cxgb3: Fix unintended sign extension

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Message-ID: <25e5b2a5255b42ca92add0177fa4832d@AcuMS.aculab.com>
Date:   Thu, 25 Oct 2018 16:03:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Colin King' <colin.king@...onical.com>,
        Steve Wise <swise@...lsio.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
CC:     "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] RDMA/cxgb3: Fix unintended sign extension

From: Colin King
> Sent: 25 October 2018 15:32
> 
> From: Colin Ian King <colin.king@...onical.com>
> 
> In the expression "utx_len << 28", utx_len starts as u8, but is promoted
> to a signed int, then sign-extended to u64.  If utx_len is 0xf8 or greater
> then the sign extension will set all the upper bits of utx_cmd which is
> probably not what was intended.  Cast to utx_len to u64  to avoid the sign
> extension.

RTFC...
utx_len is only ever 1, 2 or 3.
The 'problem' would arise if utx_len << 28 set the high bit.
This can only happen if utx_len is more than 7 (NFI where 0xf8 came from).

In any case the best fix is to use 'unsigned int' for wr_len and utx_len.
There is no point making local variable (or functions parameters/results)
smaller that 'int' unless you explicitly want the arithmetic to wrap.

	David

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