On 03/20/2015 02:12 PM, Andrey Ryabinin wrote:
Usually ELF_ET_DYN_BASE is 2/3 of TASK_SIZE. With 3G/1G user/kernel
split this is not so, because 2*TASK_SIZE overflows 32 bits,
so the actual value of ELF_ET_DYN_BASE is:
        (2 * TASK_SIZE / 3) = 0x2a000000

AFAIK on most platforms (e.g. Intel) that's (TASK_SIZE / 3 * 2) so ARM is kind of special here.


When ASLR is disabled PIE binaries will load at ELF_ET_DYN_BASE address.
On 32bit platforms AddressSanitzer uses addresses [0x20000000 - 0x40000000]
for shadow memory [1]. So ASan doesn't work for PIE binaries when ASLR disabled
as it fails to map shadow memory.
Also after Kees's 'split ET_DYN ASLR from mmap ASLR' patchset PIE binaries
has a high chance of loading somewhere in between [0x2a000000 - 0x40000000]
even if ASLR enabled. This makes ASan with PIE absolutely incompatible.

Fix overflow by dividing TASK_SIZE prior to multiplying.
After this patch ELF_ET_DYN_BASE equals to (for CONFIG_VMSPLIT_3G=y):
        (TASK_SIZE / 3 * 2) = 0x7f555554

[1] 
https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerAlgorithm#Mapping

Perhaps we should fix other platforms as well?

-Y
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