[asan] Move AsanCheckDynamicRTPrereqs check under flag.

[m.ostapenko]

Summary

The patch addresses https://github.com/google/sanitizers/issues/786. Currently AsanCheckDynamicRTPrereqs prevents ASan from running in some important environments e.g. cowbuilder and fakeroot that may also work with interposition. Let's allow users to switch off the check given that they know what they do.

[ygribov]

lib/asan/asan_flags.inc

151

ignore_shlib_order? Although let's see what maintainers say.

152

"Do not check position of ASan runtime in library list (useful when other library has to be preloaded system-wide)"?

[m.ostapenko]

lib/asan/asan_flags.inc

151

Hm, perhaps check_asan_comes_first? ignore_shlib_order doesn't look cleaner than check_dynamic_rt_prereqs imho.

[eugenis]

How about "verify_asan_link_order"? Naming is hard.

[m.ostapenko]

Changed flag name.

[eugenis]

LGTM

lib/asan/asan_flags.inc

ASAN_FLAG(bool, dump_registers, true,
           "If true, dump values of CPU registers when SEGV happens. Only "
           "available on OS X for now.")
 ASAN_FLAG(const char *, suppressions, "", "Suppressions file name.")
 ASAN_FLAG(bool, halt_on_error, true,
           "Crash the program after printing the first error report "
           "(WARNING: USE AT YOUR OWN RISK!)")
 ASAN_FLAG(bool, use_odr_indicator, false,
           "Use special ODR indicator symbol for ODR violation detection")
+ASAN_FLAG(bool, check_dynamic_rt_prereqs, true,
+          "Check that dynamic ASan runtime comes first in initial library list")

lib/asan/asan_linux.cc

 }
 
 static void ReportIncompatibleRT() {
   Report("Your application is linked against incompatible ASan runtimes.\n");
   Die();
 }
 
 void AsanCheckDynamicRTPrereqs() {
-  if (!ASAN_DYNAMIC)
+  if (!ASAN_DYNAMIC || !flags()->check_dynamic_rt_prereqs)
     return;
 
   // Ensure that dynamic RT is the first DSO in the list
   const char *first_dso_name = nullptr;
   dl_iterate_phdr(FindFirstDSOCallback, &first_dso_name);
   if (first_dso_name && !IsDynamicRTName(first_dso_name)) {
     Report("ASan runtime does not come first in initial library list; "
            "you should either link runtime to your application or "

test/asan/TestCases/Linux/asan_dlopen_test.cc

 // Test that dlopen of dynamic runtime is prohibited.
 //
 // RUN: %clangxx %s -DRT=\"%shared_libasan\" -o %t -ldl
 // RUN: not %run %t 2>&1 | FileCheck %s
+// RUN: %env_asan_opts=check_dynamic_rt_prereqs=true not %run %t 2>&1 | FileCheck %s
+// RUN: %env_asan_opts=check_dynamic_rt_prereqs=false %run %t 2>&1
 // REQUIRES: asan-dynamic-runtime
 // XFAIL: android
 
 #include <dlfcn.h>
 
 int main(int argc, char **argv) {
   dlopen(RT, RTLD_LAZY);
   return 0;
 }
 
 // CHECK: ASan runtime does not come first in initial library list