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Message-ID: <20110823064101.GA3780@albatros> Date: Tue, 23 Aug 2011 10:41:01 +0400 From: Vasiliy Kulikov <segoon@...nwall.com> To: "H. Peter Anvin" <hpa@...or.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, kernel-hardening@...ts.openwall.com, Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [RFC] x86, mm: start mmap allocation for libs from low addresses On Mon, Aug 22, 2011 at 13:17 -0700, H. Peter Anvin wrote: > On 08/22/2011 01:14 PM, Vasiliy Kulikov wrote: > > > >> Code-wise: > >> > >> The code is horrific; it is full of open-coded magic numbers; > > > > Agreed, the magic needs macro definition and comments. > > > >> it also > >> puts a function called arch_get_unmapped_exec_area() in a generic file, > >> which could best be described as "WTF" -- the arch_ prefix we use > >> specifically to denote a per-architecture hook function. > > > > Agreed. But I'd want to leave it in mm/mmap.c as it's likely be used by > > other archs - the changes are bitness specific, not arch specific. Is > > it OK if I do this? > > > > #ifndef HAVE_ARCH_UNMAPPED_EXEC_AREA > > void *arch_get_unmapped_exec_area(...) > > { > > ... > > } > > #endif > > > > Only if this is really an architecture-specific function overridden in > specific architectures. I'm not so sure that applies here. It is a more or less generic allocator. Arch specific constants will be moved to arch headers, so it will be a 32-bit specific function, not arch specific (64 bit architectures don't need ASCII shield at all as mmap addresses already contain a zero byte). It will not be overriden by x86 as it is "enough generic" for x86. I've defined it as arch_* looking at other allocator implementations. All of them are arch_* and are located in mm/mmap.c with the ability to override them in architecture specific files. Probably nobody will override it, but I tried to make it consistent with the existing code. If this HAVE_ARCH_*/arch_* logic is not suitable for exec_area, I'll remove arch_ prefix. > Furthermore, I'm not even all that sure what this function *does*. This is a bottom-up allocator, which tries to reuse all holes in the ASCII-protected region. It differs from arch_get_unmapped_area() in the priority of the first 16 Mb - arch_get_unmapped_area() tries to walk through all vmas in the whole VM space, arch_get_unmapped_exec_area() tries to reuse all memory from the first 16 Mb and only then allocating arbitrary addressed by fallbacking to the default allocator (top down in case of x86). I'll add the comment for the allocator. Thank you, -- Vasiliy Kulikov http://www.openwall.com - bringing security into open computing environments
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