[asan] Don't print rows of shadow bytes outside shadow memory

[rnk]

Summary

They might not be mapped on some platforms such as Win64. In
particular, this happens if the user address is null. There will not be
any shadow memory 5*16 bytes before the user address.

[rnk]

• longer name
• format

[vitalybuka]

compiler-rt/lib/asan/asan_errors.cc

429

What do users see now, without your patch?

[rnk]

compiler-rt/lib/asan/asan_errors.cc

429

A crash, the shadow below is unmapped.

[vitalybuka]

compiler-rt/lib/asan/asan_errors.cc

429

Maybe some placeholder would be nice, to show that shadow is missing.
Still it's improvement as is.

compiler-rt/lib/asan/asan_errors.cc

 static void PrintShadowMemoryForAddress(uptr addr) {
   if (!AddrIsInMem(addr)) return;
   uptr shadow_addr = MemToShadow(addr);
   const uptr n_bytes_per_row = 16;
   uptr aligned_shadow = shadow_addr & ~(n_bytes_per_row - 1);
   InternalScopedString str(4096 * 8);
   str.append("Shadow bytes around the buggy address:\n");
   for (int i = -5; i <= 5; i++) {
+    uptr row_addr = aligned_shadow + i * n_bytes_per_row;
+    // Skip rows that would be outside the shadow range. This can happen when
+    // the user address is near the bottom, top, or shadow gap of the address
+    // space.
+    if (!AddrIsInShadow(row_addr))
+      continue;
     const char *prefix = (i == 0) ? "=>" : "  ";
-    PrintShadowBytes(&str, prefix, (u8 *)(aligned_shadow + i * n_bytes_per_row),
-                     (u8 *)shadow_addr, n_bytes_per_row);
+    PrintShadowBytes(&str, prefix, (u8 *)row_addr, (u8 *)shadow_addr,
+                     n_bytes_per_row);
   }
   if (flags()->print_legend) PrintLegend(&str);
   Printf("%s", str.data());
 }
 
 void ErrorGeneric::Print() {
   Decorator d;
   Printf("%s", d.Warning());