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[209.85.220.65]) by mx.google.com with SMTPS id b1sor17678156plx.42.2019.02.17.13.55.59 for (Google Transport Security); Sun, 17 Feb 2019 13:56:00 -0800 (PST) Received-SPF: pass (google.com: domain of bsingharora@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=bXiPb+jj; spf=pass (google.com: domain of bsingharora@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=bsingharora@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=0wmxiqqk7vAqmoD3PZT4D63Pg1Meu3+th20mrox0DOk=; b=bXiPb+jjfszHalIcoiGQpr79ma8nHUzFESYIQEBjgvau7dlK3K1YyqqgkSGMsyuTAN oi91PYriZRkfrrF/V8hoEH4ml69BOQMHy86EvXVDzpK9wlww8T5kstXI0AMa8MqsFZ8z 3fG41To4sZ5ucp8bstPbknWBHxLatVJjmDIUOxEoyMVB0DeNxDf6UGbs1vZoXgD8zalt e3smdskO9nQgH11fmjyY+g0kVftd5WfwBF4+keK2y5lFUDra1xiQJRWIFzHKCkLDIkDh lSKxg/2+KIrGF33aPuTrHYpSBVlfKgeEfJ0cPEw/EYfT2hvYkkv1+NJtoq8qq+rwXG8Y Se8A== X-Google-Smtp-Source: AHgI3IbqXw5YEK0XINbUyF5eYSMqG3seFTkN2WP0zeb+/N+xaXLutf2s82q5LgbDabqLwxuFqUbxag== X-Received: by 2002:a17:902:28e9:: with SMTP id f96mr21903651plb.169.1550440559472; Sun, 17 Feb 2019 13:55:59 -0800 (PST) Received: from localhost ([203.219.252.113]) by smtp.gmail.com with ESMTPSA id k74sm5381900pfb.172.2019.02.17.13.55.58 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 17 Feb 2019 13:55:58 -0800 (PST) Date: Mon, 18 Feb 2019 08:55:56 +1100 From: Balbir Singh To: Michael Ellerman Cc: Segher Boessenkool , erhard_f@mailbox.org, jack@suse.cz, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, aneesh.kumar@linux.vnet.ibm.com Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present() Message-ID: <20190217215556.GH31125@350D> References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> <20190216142206.GE14180@gate.crashing.org> <20190217062333.GC31125@350D> <87ef86dd9v.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87ef86dd9v.fsf@concordia.ellerman.id.au> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, Feb 17, 2019 at 07:34:20PM +1100, Michael Ellerman wrote: > Balbir Singh writes: > > On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote: > >> Hi all, > >> > >> On Sat, Feb 16, 2019 at 09:55:11PM +1100, Balbir Singh wrote: > >> > On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote: > >> > > In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT > >> > > rather than just checking that the value is non-zero, e.g.: > >> > > > >> > > static inline int pgd_present(pgd_t pgd) > >> > > { > >> > > - return !pgd_none(pgd); > >> > > + return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT)); > >> > > } > >> > > > >> > > Unfortunately this is broken on big endian, as the result of the > >> > > bitwise && is truncated to int, which is always zero because > >> > >> (Bitwise "&" of course). > >> > >> > Not sure why that should happen, why is the result an int? What > >> > causes the casting of pgd_t & be64 to be truncated to an int. > >> > >> Yes, it's not obvious as written... It's simply that the return type of > >> pgd_present is int. So it is truncated _after_ the bitwise and. > >> > > > > Thanks, I am surprised the compiler does not complain about the truncation > > of bits. I wonder if we are missing -Wconversion > > Good luck with that :) > > What I should start doing is building with it enabled and then comparing > the output before and after commits to make sure we're not introducing > new cases. > Fair enough, my point was that the compiler can help out. I'll see what -Wconversion finds on my local build :) Balbir Singh.