[asan] Fix an OS X startup crash when an empty section is present

[kubamracek]

Summary

On OS X, when the main instrumented binary contains a custom section with zero length, ASan will crash (assert failure) early in the initialization. This can be reproduced with a linker option -sectcreate mysegment mysection /dev/null. Running such a binary with ASan produces just:

$ ./a.out
==38355==AddressSanitizer CHECK failed: .../llvm/projects/compiler-rt/lib/sanitizer_common/sanitizer_posix.cc:258 "((start1 <= end1)) != (0)" (0x0, 0x0)

There seems to be an off-by-one bug in MemoryRangeIsAvailable, this patch fixes it and adds a regression test.

[samsonov]

lib/sanitizer_common/sanitizer_posix.cc

275

This is incorrect. IntervalsAreSeparate and MemoryRangeIsAvailable assume that start and end are inclusive (just audit its code and uses).
You need a special case for zero-size interval.

[kubamracek]

Second take on the fix.

[samsonov]

LGTM

[kubamracek]

Landed in r241474.

[glider]

LGTM

lib/sanitizer_common/sanitizer_posix.cc

 // several worker threads on Mac, which aren't expected to map big chunks of
 // memory).
 bool MemoryRangeIsAvailable(uptr range_start, uptr range_end) {
   MemoryMappingLayout proc_maps(/*cache_enabled*/true);
   uptr start, end;
   while (proc_maps.Next(&start, &end,
                         /*offset*/0, /*filename*/0, /*filename_size*/0,
                         /*protection*/0)) {
+    if (start == end) continue;  // Empty range.
     CHECK_NE(0, end);
     if (!IntervalsAreSeparate(start, end - 1, range_start, range_end))
       return false;
   }
   return true;
 }
 
 void DumpProcessMap() {
   MemoryMappingLayout proc_maps(/*cache_enabled*/true);
   uptr start, end;

test/asan/TestCases/Darwin/empty-section.cc

+// Regression test with an empty (length = 0) custom section.
+
+// RUN: %clangxx_asan -g -O0 %s -c -o %t.o
+// RUN: %clangxx_asan -g -O0 %t.o -o %t -sectcreate mysegment mysection /dev/null
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include <stdio.h>
+
+int main() {
+  printf("Hello, world!\n");
+  // CHECK: Hello, world!
+}