Commit d6c763af authored Nov 17, 2014 by Yann Droneaud Committed by Will Deacon Nov 18, 2014

arm64/mm: Remove hack in mmap randomize layout



Since commit 8a0a9bd4 ('random: make get_random_int() more
random'), get_random_int() returns a random value for each call,
so comment and hack introduced in mmap_rnd() as part of commit
1d18c47c ('arm64: MMU fault handling and page table management')
are incorrects.

Commit 1d18c47c seems to use the same hack introduced by
commit a5adc91a ('powerpc: Ensure random space between stack
and mmaps'), latter copied in commit 5a0efea0 ('sparc64: Sharpen
address space randomization calculations.').

But both architectures were cleaned up as part of commit
fa8cbaaf ('powerpc+sparc64/mm: Remove hack in mmap randomize
layout') as hack is no more needed since commit 8a0a9bd4.

So the present patch removes the comment and the hack around
get_random_int() on AArch64's mmap_rnd().

Cc: David S. Miller <davem@davemloft.net>
Cc: Anton Blanchard <anton@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

parent 7d57511d

arch/arm64/mm/mmap.c

+2 −10

Original line number	Diff line number	Diff line
		@@ -47,22 +47,14 @@ static int mmap_is_legacy(void)
		return sysctl_legacy_va_layout;
		}

		/*
		* Since get_random_int() returns the same value within a 1 jiffy window, we
		* will almost always get the same randomisation for the stack and mmap
		* region. This will mean the relative distance between stack and mmap will be
		* the same.
		*
		* To avoid this we can shift the randomness by 1 bit.
		*/
		static unsigned long mmap_rnd(void)
		{
		unsigned long rnd = 0;

		if (current->flags & PF_RANDOMIZE)
		rnd = (long)get_random_int() & (STACK_RND_MASK >> 1);
		rnd = (long)get_random_int() & STACK_RND_MASK;

		return rnd << (PAGE_SHIFT + 1);
		return rnd << PAGE_SHIFT;
		}

		static unsigned long mmap_base(void)

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