[sanitizer] Use AT_EXECFN in ReExec() if available

execve("/proc/self/exe") will not work if the binary relies on
$EXEC_ORIGIN in an rpath. Query AT_EXECFN instead, which will give the
same string that the current binary was exec'd with.

Differential Revision: https://reviews.llvm.org/D54113

llvm-svn: 346215

compiler-rt/lib/sanitizer_common/sanitizer_linux.cc

@@ -640,6 +640,10 @@ void ReExec() {
 #elif SANITIZER_SOLARIS
   pathname = getexecname();
   CHECK_NE(pathname, NULL);
+#elif SANITIZER_USE_GETAUXVAL
+  // Calling execve with /proc/self/exe sets that as $EXEC_ORIGIN. Binaries that
+  // rely on that will fail to load shared libraries. Query AT_EXECFN instead.
+  pathname = reinterpret_cast<const char *>(getauxval(AT_EXECFN));
 #endif
 
   GetArgsAndEnv(&argv, &envp);

compiler-rt/test/msan/Linux/reexec_unlimited_stack.cc

@@ -0,0 +1,23 @@
+// MSAN re-execs on unlimited stacks. We use that to verify ReExec() uses the
+// right path.
+// RUN: %clangxx_msan -O0 %s -o %t && ulimit -s unlimited && %run %t | FileCheck %s
+
+#include <stdio.h>
+
+#if !defined(__GLIBC_PREREQ)
+#define __GLIBC_PREREQ(a, b) 0
+#endif
+
+#if __GLIBC_PREREQ(2, 16)
+#include <sys/auxv.h>
+#endif
+
+int main() {
+#if __GLIBC_PREREQ(2, 16)
+  // Make sure AT_EXECFN didn't get overwritten by re-exec.
+  puts(reinterpret_cast<const char *>(getauxval(AT_EXECFN)));
+#else
+  puts("No getauxval");
+#endif
+  // CHECK-NOT: /proc/self/exe
+}