[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 ↗

(On Diff #93236)

ignore_shlib_order? Although let's see what maintainers say.

152 ↗

(On Diff #93236)

"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 ↗

(On Diff #93236)

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

compiler-rt/trunk/lib/asan/asan_flags.inc

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, allocator_frees_and_returns_null_on_realloc_zero, true,
           "realloc(p, 0) is equivalent to free(p) by default (Same as the "
           "POSIX standard). If set to false, realloc(p, 0) will return a "
           "pointer to an allocated space which can not be used.")
+ASAN_FLAG(bool, verify_asan_link_order, true,
+          "Check position of ASan runtime in library list (needs to be disabled"
+          " when other library has to be preloaded system-wide)")

compiler-rt/trunk/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()->verify_asan_link_order)
     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 "

compiler-rt/trunk/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=verify_asan_link_order=true not %run %t 2>&1 | FileCheck %s
+// RUN: %env_asan_opts=verify_asan_link_order=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