Fix pthread_getattr_np for the main thread.

On most architectures the kernel subtracts a random offset to the stack
pointer in create_elf_tables by calling arch_align_stack before writing
the auxval table and so on. On all but x86 this doesn't cause a problem
because the random offset is less than a page, but on x86 it's up to two
pages. This means that our old technique of rounding the stack pointer
doesn't work. (Our old implementation of that technique was wrong too.)

It's also incorrect to assume that the main thread's stack base and size
are constant. Likewise to assume that the main thread has a guard page.
The main thread is not like other threads.

This patch switches to reading /proc/self/maps (and checking RLIMIT_STACK)
whenever we're asked.

Bug: 17111575
Signed-off-by: Fengwei Yin <fengwei.yin@intel.com>

(cherry picked from commit 57b7a6110e7e8b446fc23cce4765ff625ee0a105)

Change-Id: I87e679ee1c0db8092f2d1221c8e7c1461545c5a4

2 files changed, 87 insertions, 3 deletions

diff --git a/tests/dlfcn_test.cpp b/tests/dlfcn_test.cpp
index c2c9286..457fcd5 100644
--- a/tests/dlfcn_test.cpp
+++ b/tests/dlfcn_test.cpp
@@ -188,10 +188,8 @@ TEST(dlfcn, dladdr) {
   // Look in /proc/pid/maps to find out what address we were loaded at.
   // TODO: factor /proc/pid/maps parsing out into a class and reuse all over bionic.
   void* base_address = NULL;
-  char path[PATH_MAX];
-  snprintf(path, sizeof(path), "/proc/%d/maps", getpid());
   char line[BUFSIZ];
-  FILE* fp = fopen(path, "r");
+  FILE* fp = fopen("/proc/self/maps", "r");
   ASSERT_TRUE(fp != NULL);
   while (fgets(line, sizeof(line), fp) != NULL) {
     uintptr_t start = strtoul(line, 0, 16);
diff --git a/tests/pthread_test.cpp b/tests/pthread_test.cpp
index 5328e48..5f74e38 100644
--- a/tests/pthread_test.cpp
+++ b/tests/pthread_test.cpp
@@ -23,6 +23,7 @@
 #include <pthread.h>
 #include <signal.h>
 #include <sys/mman.h>
+#include <sys/syscall.h>
 #include <time.h>
 #include <unistd.h>
 
@@ -816,3 +817,88 @@ TEST(pthread, pthread_mutex_timedlock) {
   ASSERT_EQ(0, pthread_mutex_unlock(&m));
   ASSERT_EQ(0, pthread_mutex_destroy(&m));
 }
+
+TEST(pthread, pthread_attr_getstack__main_thread) {
+  // This test is only meaningful for the main thread, so make sure we're running on it!
+  ASSERT_EQ(getpid(), syscall(__NR_gettid));
+
+  // Get the main thread's attributes.
+  pthread_attr_t attributes;
+  ASSERT_EQ(0, pthread_getattr_np(pthread_self(), &attributes));
+
+  // Check that we correctly report that the main thread has no guard page.
+  size_t guard_size;
+  ASSERT_EQ(0, pthread_attr_getguardsize(&attributes, &guard_size));
+  ASSERT_EQ(0U, guard_size); // The main thread has no guard page.
+
+  // Get the stack base and the stack size (both ways).
+  void* stack_base;
+  size_t stack_size;
+  ASSERT_EQ(0, pthread_attr_getstack(&attributes, &stack_base, &stack_size));
+  size_t stack_size2;
+  ASSERT_EQ(0, pthread_attr_getstacksize(&attributes, &stack_size2));
+
+  // The two methods of asking for the stack size should agree.
+  EXPECT_EQ(stack_size, stack_size2);
+
+  // What does /proc/self/maps' [stack] line say?
+  void* maps_stack_base = NULL;
+  size_t maps_stack_size = 0;
+  FILE* fp = fopen("/proc/self/maps", "r");
+  ASSERT_TRUE(fp != NULL);
+  char line[BUFSIZ];
+  while (fgets(line, sizeof(line), fp) != NULL) {
+    uintptr_t lo, hi;
+    char name[10];
+    sscanf(line, "%" PRIxPTR "-%" PRIxPTR " %*4s %*x %*x:%*x %*d %10s", &lo, &hi, name);
+    if (strcmp(name, "[stack]") == 0) {
+      maps_stack_base = reinterpret_cast<void*>(lo);
+      maps_stack_size = hi - lo;
+      break;
+    }
+  }
+  fclose(fp);
+
+#if defined(__BIONIC__)
+  // bionic thinks that the stack base and size should correspond to the mapped region.
+  EXPECT_EQ(maps_stack_base, stack_base);
+  EXPECT_EQ(maps_stack_size, stack_size);
+#else
+  // glibc doesn't give the true extent for some reason.
+#endif
+
+  // Both bionic and glibc agree that the high address you can compute from the returned
+  // values should match what /proc/self/maps says.
+  void* stack_end = reinterpret_cast<uint8_t*>(stack_base) + stack_size;
+  void* maps_stack_end = reinterpret_cast<uint8_t*>(maps_stack_base) + maps_stack_size;
+  EXPECT_EQ(maps_stack_end, stack_end);
+
+  //
+  // What if the rlimit is smaller than the stack's current extent?
+  //
+  rlimit rl;
+  rl.rlim_cur = rl.rlim_max = 1024; // 1KiB. We know the stack must be at least a page already.
+  rl.rlim_max = RLIM_INFINITY;
+  ASSERT_EQ(0, setrlimit(RLIMIT_STACK, &rl));
+
+  ASSERT_EQ(0, pthread_getattr_np(pthread_self(), &attributes));
+  ASSERT_EQ(0, pthread_attr_getstack(&attributes, &stack_base, &stack_size));
+  ASSERT_EQ(0, pthread_attr_getstacksize(&attributes, &stack_size2));
+
+  EXPECT_EQ(stack_size, stack_size2);
+  ASSERT_EQ(1024U, stack_size);
+
+  //
+  // What if the rlimit isn't a whole number of pages?
+  //
+  rl.rlim_cur = rl.rlim_max = 6666; // Not a whole number of pages.
+  rl.rlim_max = RLIM_INFINITY;
+  ASSERT_EQ(0, setrlimit(RLIMIT_STACK, &rl));
+
+  ASSERT_EQ(0, pthread_getattr_np(pthread_self(), &attributes));
+  ASSERT_EQ(0, pthread_attr_getstack(&attributes, &stack_base, &stack_size));
+  ASSERT_EQ(0, pthread_attr_getstacksize(&attributes, &stack_size2));
+
+  EXPECT_EQ(stack_size, stack_size2);
+  ASSERT_EQ(6666U, stack_size);
+}