[PATCH] D39393: [asan] Use dynamic shadow on 32-bit Android.

[Evgenii Stepanov via Phabricator via llvm-commits]

eugenis created this revision.
Herald added subscribers: hiraditya, kristof.beyls, kubamracek, srhines, aemerson.

The following kernel change has moved ET_DYN base to 0x4000000 on arm32:
https://marc.info/?l=linux-kernel&m=149825162606848&w=2

Switch to dynamic shadow base to avoid such conflicts in the future.

In my limited testing, this is causing 1.3% binary size increase, and
from 1% to 6% CPU overhead, depending on the benchmark.


https://reviews.llvm.org/D39393

Files:
  compiler-rt/lib/asan/asan_linux.cc
  compiler-rt/lib/asan/asan_mapping.h
  llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp


Index: llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -479,10 +479,8 @@
   ShadowMapping Mapping;
 
   if (LongSize == 32) {
-    // Android is always PIE, which means that the beginning of the address
-    // space is always available.
     if (IsAndroid)
-      Mapping.Offset = 0;
+      Mapping.Offset = kDynamicShadowSentinel;
     else if (IsMIPS32)
       Mapping.Offset = kMIPS32_ShadowOffset32;
     else if (IsFreeBSD)
Index: compiler-rt/lib/asan/asan_mapping.h
===================================================================
--- compiler-rt/lib/asan/asan_mapping.h
+++ compiler-rt/lib/asan/asan_mapping.h
@@ -156,7 +156,7 @@
 #  define SHADOW_OFFSET (0)
 #elif SANITIZER_WORDSIZE == 32
 #  if SANITIZER_ANDROID
-#    define SHADOW_OFFSET (0)
+#    define SHADOW_OFFSET __asan_shadow_memory_dynamic_address
 #  elif defined(__mips__)
 #    define SHADOW_OFFSET kMIPS32_ShadowOffset32
 #  elif SANITIZER_FREEBSD
Index: compiler-rt/lib/asan/asan_linux.cc
===================================================================
--- compiler-rt/lib/asan/asan_linux.cc
+++ compiler-rt/lib/asan/asan_linux.cc
@@ -82,8 +82,22 @@
 }
 
 uptr FindDynamicShadowStart() {
-  UNREACHABLE("FindDynamicShadowStart is not available");
-  return 0;
+  uptr granularity = GetMmapGranularity();
+  uptr alignment = granularity * 8;
+  uptr left_padding = granularity;
+  uptr shadow_size = kHighShadowEnd + left_padding;
+  uptr map_size = shadow_size + alignment;
+
+  uptr map_start = (uptr)MmapNoAccess(map_size);
+  CHECK_NE(map_start, ~(uptr)0);
+
+  uptr shadow_start = RoundUpTo(map_start, alignment);
+  UnmapOrDie((void *)map_start, map_size);
+  // MmapNoAccess does not touch TotalMmap, but UnmapOrDie decreases it.
+  // Compensate.
+  IncreaseTotalMmap(map_size);
+
+  return shadow_start;
 }
 
 void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {


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