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[66.111.4.227]) by smtp.gmail.com with ESMTPSA id w81sm803656ilk.38.2020.11.06.02.19.36 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 06 Nov 2020 02:19:36 -0800 (PST) Received: from compute5.internal (compute5.nyi.internal [10.202.2.45]) by mailauth.nyi.internal (Postfix) with ESMTP id 1E4FB27C005A; Fri, 6 Nov 2020 05:19:36 -0500 (EST) Received: from mailfrontend2 ([10.202.2.163]) by compute5.internal (MEProxy); Fri, 06 Nov 2020 05:19:36 -0500 X-ME-Sender: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedujedruddtledgudehucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepfffhvffukfhfgggtuggjsehttdertddttddvnecuhfhrohhmpeeuohhquhhn ucfhvghnghcuoegsohhquhhnrdhfvghnghesghhmrghilhdrtghomheqnecuggftrfgrth htvghrnhepvdelieegudfggeevjefhjeevueevieetjeeikedvgfejfeduheefhffggedv geejnecukfhppedufedurddutdejrddugeejrdduvdeinecuvehluhhsthgvrhfuihiivg epudenucfrrghrrghmpehmrghilhhfrhhomhepsghoqhhunhdomhgvshhmthhprghuthhh phgvrhhsohhnrghlihhthidqieelvdeghedtieegqddujeejkeehheehvddqsghoqhhunh drfhgvnhhgpeepghhmrghilhdrtghomhesfhhigihmvgdrnhgrmhgv X-ME-Proxy: Received: from localhost (unknown [131.107.147.126]) by mail.messagingengine.com (Postfix) with ESMTPA id AC58C306005F; Fri, 6 Nov 2020 05:19:35 -0500 (EST) Date: Fri, 6 Nov 2020 18:19:32 +0800 From: Boqun Feng To: Marco Elver Cc: paulmck@kernel.org, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, kernel-team@fb.com, mingo@kernel.org, andreyknvl@google.com, glider@google.com, dvyukov@google.com, cai@lca.pw Subject: Re: [PATCH v2] kcsan: Fix encoding masks and regain address bit Message-ID: <20201106101932.GD3025@boqun-archlinux> References: <20201105220302.GA15733@paulmck-ThinkPad-P72> <20201105220324.15808-3-paulmck@kernel.org> <20201106093456.GB2851373@elver.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201106093456.GB2851373@elver.google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 06, 2020 at 10:34:56AM +0100, Marco Elver wrote: > The watchpoint encoding masks for size and address were off-by-one bit > each, with the size mask using 1 unnecessary bit and the address mask > missing 1 bit. However, due to the way the size is shifted into the > encoded watchpoint, we were effectively wasting and never using the > extra bit. > > For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write > bit, 14 bits for the size bits, and then 49 bits left for the address. > Prior to this fix we would end up with this usage: > > [ write<1> | size<14> | wasted<1> | address<48> ] > > Fix it by subtracting 1 bit from the GENMASK() end and start ranges of > size and address respectively. The added static_assert()s verify that > the masks are as expected. With the fixed version, we get the expected > usage: > > [ write<1> | size<14> | address<49> ] > > Functionally no change is expected, since that extra address bit is > insignificant for enabled architectures. > > Signed-off-by: Marco Elver Acked-by: Boqun Feng Regards, Boqun > --- > v2: > * Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 - > WATCHPOINT_SIZE_BITS" per Boqun's suggestion. > --- > kernel/kcsan/encoding.h | 14 ++++++-------- > 1 file changed, 6 insertions(+), 8 deletions(-) > > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h > index 4f73db6d1407..7ee405524904 100644 > --- a/kernel/kcsan/encoding.h > +++ b/kernel/kcsan/encoding.h > @@ -37,14 +37,12 @@ > */ > #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS) > > -/* > - * Masks to set/retrieve the encoded data. > - */ > -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1) > -#define WATCHPOINT_SIZE_MASK \ > - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS) > -#define WATCHPOINT_ADDR_MASK \ > - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0) > +/* Bitmasks for the encoded watchpoint access information. */ > +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1) > +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS) > +#define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0) > +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1); > +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL); > > static inline bool check_encodable(unsigned long addr, size_t size) > { > -- > 2.29.2.222.g5d2a92d10f8-goog >