Date: Wed, 14 Dec 2016 12:56:12 -0500 (EST) From: David Miller <davem@...emloft.net> To: Jason@...c4.com Cc: David.Laight@...lab.com, netdev@...r.kernel.org, kernel-hardening@...ts.openwall.com, ak@...ux.intel.com, linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org Subject: Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform From: "Jason A. Donenfeld" <Jason@...c4.com> Date: Wed, 14 Dec 2016 13:53:10 +0100 > In all current uses of __packed in the code, I think the impact is > precisely zero, because all structures have members in descending > order of size, with each member being a perfect multiple of the one > below it. The __packed is therefore just there for safety, in case > somebody comes in and screws everything up by sticking a u8 in > between. Just marking the structure __packed, whether necessary or not, makes the compiler assume that the members are not aligned and causes byte-by-byte accesses to be performed for words. Never, _ever_, use __packed unless absolutely necessary, it pessimizes the code on cpus that require proper alignment of types.
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