[LLVMdev] LLVM-based address sanity checker

[Kostya Serebryany]

On Fri, Jun 17, 2011 at 12:04 PM, Renato Golin <rengolin at systemcall.org>wrote:

> On 17 June 2011 08:55, Kostya Serebryany <kcc at google.com> wrote:
> > I am rather reluctant to add 'generic' code that handles unknown/untested
> > platforms because the memory mapping is very platform specific anyway.
>
> Indeed, but the point of that is more for helping writing
> platform-specific versions than actually using it as a general-purpose
> routine. Kinda documentation of your intentions on C, which is better
> than ASM.
>
> If you're lucky, it might even work... ;)
>

Maybe the fallback code should just use a function call. Much simpler for
documentation purposes.


>
>
>
> > For extreme performance ASAN uses the fixed memory-to-shadow mapping
> > (addr>>3)+offset. I guess there are plenty of platforms where this
> mapping
> > won't work or will require different offset.
>
> I got a bit confused there, TBH. How do you guarantee that that part
> of memory will be allocated?


On 32-bit, the shadow region is:
[0x28000000, 0x3fffffff]HighShadow[0x24000000, 0x27ffffff]ShadowGap[0x20000000,
0x23ffffff]LowShadow

This is 0.5G total. So, I mmap all these three shadow subregions and
'mprotect' the ShadowGap.
This is done at startup. If the mmap fails, an assert will fire.

On 64-bit, the shadow looks like this:
[0x0000140000000000, 0x00001fffffffffff]HighShadow[0x0000120000000000,
0x000013ffffffffff]ShadowGap[0x0000100000000000, 0x000011ffffffffff]
LowShadow

This is quite a lot, I can not mmap/mprotect this thing.
So, I basically *hope* that it won't be used by anyone but the ASAN run time
(of course, there are asserts here and there to check it).
When some part of the shadow region is being written to (when we poison
memory), SEGV happens and the SEGV handler mmaps the required region.



> If I'm not mistaken, the Native Client
> guys have done a memory place-holder, with enough space pre- and
> post-code, is it similar of what you're doing?


Not very similar. 64-bit NaCl mmaps 88G of address space. On 64-bit ASAN I
need 16384G of RAM, which is a bit too much to mmap.


> Or are you using a big
> BSS region?
>
> Depending on how you did it, it might just work on other platforms...
>
> cheers,
> --renato
>

--kcc
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