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[217.70.183.201]) by mx.google.com with ESMTPS id s58si11811841edm.24.2019.07.26.04.48.13 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 26 Jul 2019 04:48:13 -0700 (PDT) Received-SPF: neutral (google.com: 217.70.183.201 is neither permitted nor denied by best guess record for domain of alex@ghiti.fr) client-ip=217.70.183.201; Authentication-Results: mx.google.com; spf=neutral (google.com: 217.70.183.201 is neither permitted nor denied by best guess record for domain of alex@ghiti.fr) smtp.mailfrom=alex@ghiti.fr X-Originating-IP: 81.250.144.103 Received: from [10.30.1.20] (lneuilly-657-1-5-103.w81-250.abo.wanadoo.fr [81.250.144.103]) (Authenticated sender: alex@ghiti.fr) by relay8-d.mail.gandi.net (Postfix) with ESMTPSA id 3CAE81BF207; Fri, 26 Jul 2019 11:48:08 +0000 (UTC) Subject: Re: [PATCH REBASE v4 14/14] riscv: Make mmap allocation top-down by default To: Paul Walmsley Cc: linux-arm-kernel@lists.infradead.org, Albert Ou , Kees Cook , Catalin Marinas , Palmer Dabbelt , Will Deacon , Russell King , Ralf Baechle , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Paul Burton , Alexander Viro , James Hogan , linux-fsdevel@vger.kernel.org, Andrew Morton , linux-mips@vger.kernel.org, Christoph Hellwig , linux-riscv@lists.infradead.org, Daniel Cashman , Luis Chamberlain References: <20190724055850.6232-1-alex@ghiti.fr> <20190724055850.6232-15-alex@ghiti.fr> From: Alexandre Ghiti Message-ID: <6b2b45a5-0ac4-db73-8f50-ab182a0cb621@ghiti.fr> Date: Fri, 26 Jul 2019 13:48:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: fr 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 7/26/19 2:20 AM, Paul Walmsley wrote: > Hi Alexandre, > > I have a few questions about this patch. Sorry to be dense here ... > > On Wed, 24 Jul 2019, Alexandre Ghiti wrote: > >> In order to avoid wasting user address space by using bottom-up mmap >> allocation scheme, prefer top-down scheme when possible. >> >> Before: >> root@qemuriscv64:~# cat /proc/self/maps >> 00010000-00016000 r-xp 00000000 fe:00 6389 /bin/cat.coreutils >> 00016000-00017000 r--p 00005000 fe:00 6389 /bin/cat.coreutils >> 00017000-00018000 rw-p 00006000 fe:00 6389 /bin/cat.coreutils >> 00018000-00039000 rw-p 00000000 00:00 0 [heap] >> 1555556000-155556d000 r-xp 00000000 fe:00 7193 /lib/ld-2.28.so >> 155556d000-155556e000 r--p 00016000 fe:00 7193 /lib/ld-2.28.so >> 155556e000-155556f000 rw-p 00017000 fe:00 7193 /lib/ld-2.28.so >> 155556f000-1555570000 rw-p 00000000 00:00 0 >> 1555570000-1555572000 r-xp 00000000 00:00 0 [vdso] >> 1555574000-1555576000 rw-p 00000000 00:00 0 >> 1555576000-1555674000 r-xp 00000000 fe:00 7187 /lib/libc-2.28.so >> 1555674000-1555678000 r--p 000fd000 fe:00 7187 /lib/libc-2.28.so >> 1555678000-155567a000 rw-p 00101000 fe:00 7187 /lib/libc-2.28.so >> 155567a000-15556a0000 rw-p 00000000 00:00 0 >> 3fffb90000-3fffbb1000 rw-p 00000000 00:00 0 [stack] >> >> After: >> root@qemuriscv64:~# cat /proc/self/maps >> 00010000-00016000 r-xp 00000000 fe:00 6389 /bin/cat.coreutils >> 00016000-00017000 r--p 00005000 fe:00 6389 /bin/cat.coreutils >> 00017000-00018000 rw-p 00006000 fe:00 6389 /bin/cat.coreutils >> 2de81000-2dea2000 rw-p 00000000 00:00 0 [heap] >> 3ff7eb6000-3ff7ed8000 rw-p 00000000 00:00 0 >> 3ff7ed8000-3ff7fd6000 r-xp 00000000 fe:00 7187 /lib/libc-2.28.so >> 3ff7fd6000-3ff7fda000 r--p 000fd000 fe:00 7187 /lib/libc-2.28.so >> 3ff7fda000-3ff7fdc000 rw-p 00101000 fe:00 7187 /lib/libc-2.28.so >> 3ff7fdc000-3ff7fe2000 rw-p 00000000 00:00 0 >> 3ff7fe4000-3ff7fe6000 r-xp 00000000 00:00 0 [vdso] >> 3ff7fe6000-3ff7ffd000 r-xp 00000000 fe:00 7193 /lib/ld-2.28.so >> 3ff7ffd000-3ff7ffe000 r--p 00016000 fe:00 7193 /lib/ld-2.28.so >> 3ff7ffe000-3ff7fff000 rw-p 00017000 fe:00 7193 /lib/ld-2.28.so >> 3ff7fff000-3ff8000000 rw-p 00000000 00:00 0 >> 3fff888000-3fff8a9000 rw-p 00000000 00:00 0 [stack] >> >> Signed-off-by: Alexandre Ghiti >> Reviewed-by: Christoph Hellwig >> Reviewed-by: Kees Cook >> --- >> arch/riscv/Kconfig | 11 +++++++++++ >> 1 file changed, 11 insertions(+) >> >> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig >> index 59a4727ecd6c..6a63973873fd 100644 >> --- a/arch/riscv/Kconfig >> +++ b/arch/riscv/Kconfig >> @@ -54,6 +54,17 @@ config RISCV >> select EDAC_SUPPORT >> select ARCH_HAS_GIGANTIC_PAGE >> select ARCH_WANT_HUGE_PMD_SHARE if 64BIT >> + select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU >> + select HAVE_ARCH_MMAP_RND_BITS >> + >> +config ARCH_MMAP_RND_BITS_MIN >> + default 18 > Could you help me understand the rationale behind this constant? Indeed, I took that from arm64 code and I did not think enough about it: that's great you spotted this because that's a way too large value for 32 bits as it would, at minimum, make mmap random offset go up to 1GB (18 + 12), which is a big hole for this small address space :) arm and mips propose 8 as default value for 32bits systems which is 1MB offset at minimum. > >> + >> +# max bits determined by the following formula: >> +# VA_BITS - PAGE_SHIFT - 3 > I realize that these lines are probably copied from arch/arm64/Kconfig. > But the rationale behind the "- 3" is not immediately obvious. This > apparently originates from commit 8f0d3aa9de57 ("arm64: mm: support > ARCH_MMAP_RND_BITS"). Can you provide any additional context here? The formula comes from commit d07e22597d1d ("mm: mmap: add new /proc tunable for mmap_base ASLR"), where the author states that "generally a 3-4 bits less than the number of bits in the user-space accessible virtual address space [allows to] give the greatest flexibility without generating an invalid mmap_base address". In practice, that limits the mmap random offset to at maximum 1/8 (for - 3) of the total address space. > >> +config ARCH_MMAP_RND_BITS_MAX >> + default 33 if 64BIT # SV48 based > The rationale here is clear for Sv48, per the above formula: > > (48 - 12 - 3) = 33 > >> + default 18 > However, here it is less clear to me. For Sv39, shouldn't this be > > (39 - 12 - 3) = 24 > > ? And what about Sv32? You're right. Is there a way to distinguish between sv39 and sv48 here ? Thanks Paul, Alex > > > - Paul > > _______________________________________________ > linux-riscv mailing list > linux-riscv@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-riscv