[asan] Add an offset for the kernel address sanitizer on FreeBSD

[markj]

Summary

This is based on a WIP port of the sanitizer runtime to the FreeBSD
kernel.

Test Plan:

[dim]

LGTM

[markj]

Is there anything I can do to get this committed? I'd like to import KASAN support to FreeBSD but of course this change and D298286 are prerequisites.

[arichardson]

In D98285#2682952, @markj wrote:

Is there anything I can do to get this committed? I'd like to import KASAN support to FreeBSD but of course this change and D298286 are prerequisites.

I can commit this and D98286 for you if you would like?

[markj]

In D98285#2691767, @arichardson wrote:

In D98285#2682952, @markj wrote:

Is there anything I can do to get this committed? I'd like to import KASAN support to FreeBSD but of course this change and D298286 are prerequisites.

I can commit this and D98286 for you if you would like?

That would be appreciated, thanks.

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

 static const uint64_t kPPC64_ShadowOffset64 = 1ULL << 44;
 static const uint64_t kSystemZ_ShadowOffset64 = 1ULL << 52;
 static const uint64_t kMIPS32_ShadowOffset32 = 0x0aaa0000;
 static const uint64_t kMIPS64_ShadowOffset64 = 1ULL << 37;
 static const uint64_t kAArch64_ShadowOffset64 = 1ULL << 36;
 static const uint64_t kRISCV64_ShadowOffset64 = 0xd55550000;
 static const uint64_t kFreeBSD_ShadowOffset32 = 1ULL << 30;
 static const uint64_t kFreeBSD_ShadowOffset64 = 1ULL << 46;
+static const uint64_t kFreeBSDKasan_ShadowOffset64 = 0xdffff7c000000000;
 static const uint64_t kNetBSD_ShadowOffset32 = 1ULL << 30;
 static const uint64_t kNetBSD_ShadowOffset64 = 1ULL << 46;
 static const uint64_t kNetBSDKasan_ShadowOffset64 = 0xdfff900000000000;
 static const uint64_t kPS4CPU_ShadowOffset64 = 1ULL << 40;
 static const uint64_t kWindowsShadowOffset32 = 3ULL << 28;
 static const uint64_t kEmscriptenShadowOffset = 0;
 
 static const uint64_t kMyriadShadowScale = 5;
if (LongSize == 32) {
     // Fuchsia is always PIE, which means that the beginning of the address
     // space is always available.
     if (IsFuchsia)
       Mapping.Offset = 0;
     else if (IsPPC64)
       Mapping.Offset = kPPC64_ShadowOffset64;
     else if (IsSystemZ)
       Mapping.Offset = kSystemZ_ShadowOffset64;
-    else if (IsFreeBSD && !IsMIPS64)
+    else if (IsFreeBSD && !IsMIPS64) {
+      if (IsKasan)
+        Mapping.Offset = kFreeBSDKasan_ShadowOffset64;
+      else
         Mapping.Offset = kFreeBSD_ShadowOffset64;
-    else if (IsNetBSD) {
+    } else if (IsNetBSD) {
       if (IsKasan)
         Mapping.Offset = kNetBSDKasan_ShadowOffset64;
       else
         Mapping.Offset = kNetBSD_ShadowOffset64;
     } else if (IsPS4CPU)
       Mapping.Offset = kPS4CPU_ShadowOffset64;
     else if (IsLinux && IsX86_64) {
       if (IsKasan)