Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 16 Jan 2020 09:44:08 +0100 | Subject | Re: [RFC PATCH] UML: add support for KASAN under x86_64 |
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On Wed, Jan 15, 2020 at 7:28 PM Patricia Alfonso <trishalfonso@google.com> wrote: > +config KASAN_SHADOW_OFFSET > + hex > + depends on KASAN > + default 0x100000000000 > + help > + This is the offset at which the ~2.25TB of shadow memory is > + initialized and used by KASAN for memory debugging. The default > + is 0x100000000000.
What are restrictions on this value? In user-space we use 0x7fff8000 as a base (just below 2GB) and it's extremely profitable wrt codegen since it fits into immediate of most instructions. We can load and add the base with a short instruction: 2d8c: 48 81 c2 00 80 ff 7f add $0x7fff8000,%rdx Or even add base, load shadow and check it with a single 7-byte instruction: 1e4: 80 b8 00 80 ff 7f 00 cmpb $0x0,0x7fff8000(%rax)
While with the large base, it takes 10 bytes just to load the const into a register (current x86 KASAN codegen): ffffffff81001571: 48 b8 00 00 00 00 00 fc ff df movabs $0xdffffc0000000000,%rax Most instructions don't have 8-byte immediates, so then we separately need to add/load/check.
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