[Asan] Eliminate SHADOW_TO_MEM() macro

[kosarev]

Summary

This patch simplifies the Asan shadow memory interface and makes it possible to lookup for stack frame accesses efficiently when we are in the (coming-soon) software memory manager mode.

[eugenis]

LGTM

lib/asan/asan_mapping.h

 #   define SHADOW_OFFSET __asan_shadow_memory_dynamic_address
 #  else
 #   define SHADOW_OFFSET kDefaultShort64bitShadowOffset
 #  endif
 #endif
 
 #define SHADOW_GRANULARITY (1ULL << SHADOW_SCALE)
 #define MEM_TO_SHADOW(mem) (((mem) >> SHADOW_SCALE) + (SHADOW_OFFSET))
-#define SHADOW_TO_MEM(shadow) (((shadow) - SHADOW_OFFSET) << SHADOW_SCALE)
 
 #define kLowMemBeg      0
 #define kLowMemEnd      (SHADOW_OFFSET ? SHADOW_OFFSET - 1 : 0)
 
 #define kLowShadowBeg   SHADOW_OFFSET
 #define kLowShadowEnd   MEM_TO_SHADOW(kLowMemEnd)
 
 #define kHighMemBeg     (MEM_TO_SHADOW(kHighMemEnd) + 1)

lib/asan/asan_thread.cc

if (AddrIsInStack(addr)) {
     bottom = fake_stack()->AddrIsInFakeStack(addr);
     CHECK(bottom);
     access->offset = addr - bottom;
     access->frame_pc = ((uptr*)bottom)[2];
     access->frame_descr = (const char *)((uptr*)bottom)[1];
     return true;
   }
   uptr aligned_addr = addr & ~(SANITIZER_WORDSIZE/8 - 1);  // align addr.
+  uptr mem_ptr = RoundDownTo(aligned_addr, SHADOW_GRANULARITY);
   u8 *shadow_ptr = (u8*)MemToShadow(aligned_addr);
   u8 *shadow_bottom = (u8*)MemToShadow(bottom);
 
   while (shadow_ptr >= shadow_bottom &&
          *shadow_ptr != kAsanStackLeftRedzoneMagic) {
     shadow_ptr--;
+    mem_ptr -= SHADOW_GRANULARITY;
   }
 
   while (shadow_ptr >= shadow_bottom &&
          *shadow_ptr == kAsanStackLeftRedzoneMagic) {
     shadow_ptr--;
+    mem_ptr -= SHADOW_GRANULARITY;
   }
 
   if (shadow_ptr < shadow_bottom) {
     return false;
   }
 
-  uptr* ptr = (uptr*)SHADOW_TO_MEM((uptr)(shadow_ptr + 1));
+  uptr* ptr = (uptr*)(mem_ptr + SHADOW_GRANULARITY);
   CHECK(ptr[0] == kCurrentStackFrameMagic);
   access->offset = addr - (uptr)ptr;
   access->frame_pc = ptr[2];
   access->frame_descr = (const char*)ptr[1];
   return true;
 }
 
 bool AsanThread::AddrIsInStack(uptr addr) {