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Re: [PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont
- From: Zack Weinberg <zackw at panix dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 11 Dec 2015 15:52:40 -0500
- Subject: Re: [PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont
- Authentication-results: sourceware.org; auth=none
- References: <20151211143706 dot GA7868 at intel dot com> <alpine dot DEB dot 2 dot 10 dot 1512111539300 dot 17023 at digraph dot polyomino dot org dot uk> <CAMe9rOqbqyFw3CMa35vwOEefdFq1xK2Q9hX8GXoGMKVZ-A2y0g at mail dot gmail dot com> <566AF894 dot 4060300 at linaro dot org> <CAMe9rOr-LypZXvq4Y4uwE_JybYoTXctZXMLjo4TH517NnC6omg at mail dot gmail dot com> <566B01BE dot 1070703 at linaro dot org> <CAKCAbMhMArQ9wsXhw2y+Fvv+_3O5i4g8pdDQdWo6_1YxqfVxkQ at mail dot gmail dot com> <CAMe9rOrVjSnhp-EzmAnVBg10wbqk9U4n+hL-3xF5=DPZP5co1A at mail dot gmail dot com> <CAKCAbMhk69hUBbrQ=0j0NDYjRT6R-EK1+F43+Mmi9FwS7epexQ at mail dot gmail dot com> <CAKCAbMhA6x4r6Bhw8cnAavoWzjWsm6WM8JPzrnCsrqxbEswS_g at mail dot gmail dot com>
On Fri, Dec 11, 2015 at 3:44 PM, Zack Weinberg <zackw@panix.com> wrote:
> Digging into the kernel a little, it appears that MAP_32BIT (in
> 4.3) selects a page-aligned address at random from within a 1GB (not
> 2GB) space
Correction: the page-aligned address is selected at random from, if
I'm reading this correctly (and I'm not sure I am), the range
0x4000_0000 to 0x4200_0000. That is *not* a 1GB space; it is only a
32MB space. However, 'thirteen bits of randomness' is accurate. I'm
looking at
http://lxr.free-electrons.com/source/arch/x86/kernel/sys_x86_64.c#L100
and I cannot tell whether this means each mmap() gets its own 13 bits
of randomness or what.
zw