From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964922AbeB1XTb (ORCPT ); Wed, 28 Feb 2018 18:19:31 -0500 Received: from mail-wm0-f65.google.com ([74.125.82.65]:54332 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964908AbeB1XT2 (ORCPT ); Wed, 28 Feb 2018 18:19:28 -0500 X-Google-Smtp-Source: AG47ELvtiRRDXl3TbDW41NczJAGmc2WvRfUWoAoP5bNVgjWMPJNJSmlh6UNVGjfzpJ527vBeuhrFlg== From: Rasmus Villemoes To: Alexander Viro Cc: Rasmus Villemoes , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] vfs: make sure struct filename->iname is word-aligned Date: Thu, 1 Mar 2018 00:19:21 +0100 Message-Id: <20180228231922.15796-1-linux@rasmusvillemoes.dk> X-Mailer: git-send-email 2.15.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I noticed that offsetof(struct filename, iname) is actually 28 on 64 bit platforms, so we always pass an unaligned pointer to strncpy_from_user. This is mostly a problem for those 64 bit platforms without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned accesses carry a penalty. A user-space microbenchmark doing nothing but strncpy_from_user from the same (aligned) source string runs about 5% faster when the destination is aligned. That number increases to 20% when the string is long enough (~32 bytes) that we cross a cache line boundary - that's for example the case for about half the files a "git status" in a kernel tree ends up stat'ing. This won't make any real-life workloads 5%, or even 1%, faster, but path lookup is common enough that cutting even a few cycles should be worthwhile. So ensure we always pass an aligned destination pointer to strncpy_from_user. Instead of explicit padding, simply swap the refcnt and aname members, as suggested by Al Viro. Signed-off-by: Rasmus Villemoes --- fs/namei.c | 2 ++ include/linux/fs.h | 2 +- 2 files changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/namei.c b/fs/namei.c index 921ae32dbc80..5a66e7ca5d60 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -39,6 +39,7 @@ #include #include #include +#include #include "internal.h" #include "mount.h" @@ -130,6 +131,7 @@ getname_flags(const char __user *filename, int flags, int *empty) struct filename *result; char *kname; int len; + BUILD_BUG_ON(offsetof(struct filename, iname) % sizeof(long) != 0); result = audit_reusename(filename); if (result) diff --git a/include/linux/fs.h b/include/linux/fs.h index 2a815560fda0..d7b2caadb292 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2380,8 +2380,8 @@ struct audit_names; struct filename { const char *name; /* pointer to actual string */ const __user char *uptr; /* original userland pointer */ - struct audit_names *aname; int refcnt; + struct audit_names *aname; const char iname[]; }; -- 2.15.1