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Date: Wed, 7 Sep 2011 14:16:08 +0400
From: Solar Designer <solar@...nwall.com>
To: kernel-hardening@...ts.openwall.com
Subject: Re: [RFC] x86, mm: start mmap allocation for libs from low addresses
On Wed, Sep 07, 2011 at 01:55:08PM +0400, Vasiliy Kulikov wrote:
> OK, fully agree. But why 100 KB? Probably 0x10000 (64 KB)? It looks
> nicer and not so magic.
Well, on Owl we have mmap_min_addr at 96 KB, which is sufficient e.g. in
case we have a struct field offset not larger than 32 KB and the field
itself is an array indexed by a 16-bit value. Or if the field offset is
not larger than 64 KB and the index is a signed 16-bit value.
100 KB is a very cheap enhancement of the above, also allowing for two
levels of indirection (up to one 16-bit signed and one 16-bit unsigned)
relative to a fixed offset that fits in 4 KB.
Maybe we should move from 96 KB to 100 KB for Owl's mmap_min_addr
default. Or maybe we should use 132 KB (4+64+64).
Oh, this assumes arrays of char, or our 16-bit variable being byte
offset rather than index. 132 KB would also support arrays of 16-bit
words, and even 16-bit signed indexes into arrays of 32-bit words.
OK, maybe I am imagining these possibilities, but to me these values
feel a little bit more reasonable than a mere 64 KB, which might be
just insufficient e.g. if we have a 16-bit unsigned byte offset variable
and the array itself is a struct field. Even 68 KB would be a lot more
likely to help then.
Alexander
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