[NFC][asan] Update shadow mapping comments for AArch64 non-Android Linux

[thurstond]

This adds commentary to explain why ASan does not work for AArch64 non-Android Linux with 39-bit and 42-bit VMAs (e.g., #145259).

Additionally, it updates the 42-bit VMA shadow map comment, which has been outdated for the last 10 years
(18b2258 changed 39-bit and 42-bit to use the same offset), and adds a comment for the 48-bit VMA shadow map.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Thurston Dang (thurstond)

Changes

This adds commentary to explain why ASan does not work for AArch64 non-Android Linux (*) with 39-bit and 42-bit VMAs (e.g., #145259).

Additionally, it updates the 42-bit VMA shadow map comment, which has been outdated for the last 10 years
(18b2258 changed 39-bit and 42-bit to use the same offset).

(*) Thankfully, ASan on AArch64 Android still works with 39-bit and 42-bit VMAs, in the absence of something more robust such as MTE.

---

Full diff: https://github.com/llvm/llvm-project/pull/152412.diff

1 Files Affected:

• (modified) compiler-rt/lib/asan/asan_mapping.h (+21-10)

diff --git a/compiler-rt/lib/asan/asan_mapping.h b/compiler-rt/lib/asan/asan_mapping.h
index 91fe60db6329a..bddae9a074056 100644
--- a/compiler-rt/lib/asan/asan_mapping.h
+++ b/compiler-rt/lib/asan/asan_mapping.h
@@ -83,18 +83,29 @@
 // || `[0x0000000000, 0x0d5554ffff]` || LowMem     ||
 //
 // Default Linux/AArch64 (39-bit VMA) mapping:
-// || `[0x2000000000, 0x7fffffffff]` || highmem    ||
-// || `[0x1400000000, 0x1fffffffff]` || highshadow ||
-// || `[0x1200000000, 0x13ffffffff]` || shadowgap  ||
-// || `[0x1000000000, 0x11ffffffff]` || lowshadow  ||
-// || `[0x0000000000, 0x0fffffffff]` || lowmem     ||
+// TODO: this mapping is ok, but the allocator size is too large on non-Android
+//       AArch64 platforms (see asan_allocator.h)
+// || `[0x2000000000, 0x7fffffffff]` || highmem    || 384GB
+// || `[0x1400000000, 0x1fffffffff]` || highshadow || 48GB
+// || `[0x1200000000, 0x13ffffffff]` || shadowgap  || 8GB
+// || `[0x1000000000, 0x11ffffffff]` || lowshadow  || 8GB
+// || `[0x0000000000, 0x0fffffffff]` || lowmem     || 64GB
 //
 // Default Linux/AArch64 (42-bit VMA) mapping:
-// || `[0x10000000000, 0x3ffffffffff]` || highmem    ||
-// || `[0x0a000000000, 0x0ffffffffff]` || highshadow ||
-// || `[0x09000000000, 0x09fffffffff]` || shadowgap  ||
-// || `[0x08000000000, 0x08fffffffff]` || lowshadow  ||
-// || `[0x00000000000, 0x07fffffffff]` || lowmem     ||
+// TODO: this mapping is ok, but the allocator size is too large on non-Android
+//       AArch64 platforms (see asan_allocator.h)
+// || `[0x09000000000, 0x03ffffffffff]` || highmem    || 3520GB
+// || `[0x02200000000, 0x008fffffffff]` || highshadow || 440GB
+// || `[0x01200000000, 0x0021ffffffff]` || shadowgap  || 64GB
+// || `[0x01000000000, 0x0011ffffffff]` || lowshadow  || 8GB
+// || `[0x00000000000, 0x000fffffffff]` || lowmem     || 64GB
+//
+// Default Linux/AArch64 (48-bit VMA) mapping:
+// || `[0x201000000000, 0xffffffffffff]` || HighMem    || 229312GB
+// || `[0x041200000000, 0x200fffffffff]` || HighShadow || 28664GB
+// || `[0x001200000000, 0x0411ffffffff]` || ShadowGap  || 4096GB
+// || `[0x001000000000, 0x0011ffffffff]` || LowShadow  || 8GB
+// || `[0x000000000000, 0x000fffffffff]` || LowMem     || 64GB
 //
 // Default Linux/S390 mapping:
 // || `[0x30000000, 0x7fffffff]` || HighMem    ||

[fmayer]

(*) Thankfully, ASan on AArch64 Android still works with 39-bit and 42-bit VMAs, in the absence of something more robust such as MTE.

Remove this. ASan is deprecated on AArch64 Android in favor of HWASan 1.

[thurstond]

(*) Thankfully, ASan on AArch64 Android still works with 39-bit and 42-bit VMAs, in the absence of something more robust such as MTE.

Remove this. ASan is deprecated on AArch64 Android in favor of HWASan 1.

Big Byte Ignore is masking the truth