[compiler-rt] r244364 - [asan] Try different values of the shadow gap start.

[Evgeniy Stepanov via llvm-commits]

Author: eugenis
Date: Fri Aug  7 17:38:44 2015
New Revision: 244364

URL: http://llvm.org/viewvc/llvm-project?rev=244364&view=rev
Log:
[asan] Try different values of the shadow gap start.

The number of unaccessible pages at the beginning of the address
space can differ between processes on the same machine. Try different
values at runtime to protect as much memory as possible.

Modified:
    compiler-rt/trunk/lib/asan/asan_mapping.h
    compiler-rt/trunk/lib/asan/asan_rtl.cc

Modified: compiler-rt/trunk/lib/asan/asan_mapping.h
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/asan/asan_mapping.h?rev=244364&r1=244363&r2=244364&view=diff
==============================================================================
--- compiler-rt/trunk/lib/asan/asan_mapping.h (original)
+++ compiler-rt/trunk/lib/asan/asan_mapping.h Fri Aug  7 17:38:44 2015
@@ -178,7 +178,8 @@ static const u64 kWindowsShadowOffset32
 
 // With the zero shadow base we can not actually map pages starting from 0.
 // This constant is somewhat arbitrary.
-#define kZeroBaseShadowStart 4096
+#define kZeroBaseShadowStart 0
+#define kZeroBaseMaxShadowStart (1 << 18)
 
 #define kShadowGapBeg   (kLowShadowEnd ? kLowShadowEnd + 1 \
                                        : kZeroBaseShadowStart)

Modified: compiler-rt/trunk/lib/asan/asan_rtl.cc
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/asan/asan_rtl.cc?rev=244364&r1=244363&r2=244364&view=diff
==============================================================================
--- compiler-rt/trunk/lib/asan/asan_rtl.cc (original)
+++ compiler-rt/trunk/lib/asan/asan_rtl.cc Fri Aug  7 17:38:44 2015
@@ -301,6 +301,20 @@ static void ProtectGap(uptr addr, uptr s
   void *res = MmapNoAccess(addr, size, "shadow gap");
   if (addr == (uptr)res)
     return;
+  // A few pages at the start of the address space can not be protected.
+  // But we really want to protect as much as possible, to prevent this memory
+  // being returned as a result of a non-FIXED mmap().
+  if (addr == kZeroBaseShadowStart) {
+    uptr step = GetPageSizeCached();
+    while (size > step && addr < kZeroBaseMaxShadowStart) {
+      addr += step;
+      size -= step;
+      void *res = MmapNoAccess(addr, size, "shadow gap");
+      if (addr == (uptr)res)
+        return;
+    }
+  }
+
   Report("ERROR: Failed to protect the shadow gap. "
          "ASan cannot proceed correctly. ABORTING.\n");
   DumpProcessMap();