[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.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@244364 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 16 insertions, 1 deletions

diff --git a/lib/asan/asan_mapping.h b/lib/asan/asan_mapping.h
index 8b4857e6d..937c57b28 100644
--- a/lib/asan/asan_mapping.h
+++ b/lib/asan/asan_mapping.h
@@ -178,7 +178,8 @@ static const u64 kWindowsShadowOffset32 = 3ULL << 28;  // 0x30000000
 
 // 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)
diff --git a/lib/asan/asan_rtl.cc b/lib/asan/asan_rtl.cc
index 4fa8ea2f0..7ff657bc5 100644
--- a/lib/asan/asan_rtl.cc
+++ b/lib/asan/asan_rtl.cc
@@ -301,6 +301,20 @@ static void ProtectGap(uptr addr, uptr size) {
   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();