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

From: David Laight
Date: Thu Oct 25 2018 - 12:03:37 EST

From: Colin King
> Sent: 25 October 2018 15:32
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> 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.

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.


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