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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