Have pthread_attr_getstack for the main thread report RLIMIT_STACK...

...rather than just what's already mapped in. This seems somewhat
contrary to POSIX's "All pages within the stack described by stackaddr
and stacksize shall be both readable and writable by the thread", but
it's what glibc does.

Bug: 17111575

(cherry picked from commit 9e4ffa70)

Change-Id: I73f219a569917b2e4546c09436d7ef5231facc07

libc/bionic/pthread_attr.cpp

@@ -116,6 +116,16 @@ int pthread_attr_setstack(pthread_attr_t* attr, void* stack_base, size_t stack_s
 static int __pthread_attr_getstack_main_thread(void** stack_base, size_t* stack_size) {
   ErrnoRestorer errno_restorer;
 
+  rlimit stack_limit;
+  if (getrlimit(RLIMIT_STACK, &stack_limit) == -1) {
+    return errno;
+  }
+
+  // If the current RLIMIT_STACK is RLIM_INFINITY, only admit to an 8MiB stack for sanity's sake.
+  if (stack_limit.rlim_cur == RLIM_INFINITY) {
+    stack_limit.rlim_cur = 8 * 1024 * 1024;
+  }
+
   // It doesn't matter which thread we are; we're just looking for "[stack]".
   FILE* fp = fopen("/proc/self/maps", "re");
   if (fp == NULL) {
@@ -126,17 +136,8 @@ static int __pthread_attr_getstack_main_thread(void** stack_base, size_t* stack_
     if (ends_with(line, " [stack]\n")) {
       uintptr_t lo, hi;
       if (sscanf(line, "%" SCNxPTR "-%" SCNxPTR, &lo, &hi) == 2) {
-        *stack_base = reinterpret_cast<void*>(lo);
-        *stack_size = hi - lo;
-
-        // Does our current RLIMIT_STACK mean we won't actually get everything /proc/maps promises?
-        rlimit stack_limit;
-        if (getrlimit(RLIMIT_STACK, &stack_limit) != -1) {
-          if (*stack_size > stack_limit.rlim_cur) {
-            *stack_size = stack_limit.rlim_cur;
-          }
-        }
-
+        *stack_size = stack_limit.rlim_cur;
+        *stack_base = reinterpret_cast<void*>(hi - *stack_size);
         fclose(fp);
         return 0;
       }

tests/pthread_test.cpp

@@ -842,8 +842,7 @@ TEST(pthread, pthread_attr_getstack__main_thread) {
   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;
+  void* maps_stack_hi = NULL;
   FILE* fp = fopen("/proc/self/maps", "r");
   ASSERT_TRUE(fp != NULL);
   char line[BUFSIZ];
@@ -852,31 +851,25 @@ TEST(pthread, pthread_attr_getstack__main_thread) {
     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;
+      maps_stack_hi = reinterpret_cast<void*>(hi);
       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
+  // The stack size should correspond to RLIMIT_STACK.
+  rlimit rl;
+  ASSERT_EQ(0, getrlimit(RLIMIT_STACK, &rl));
+  EXPECT_EQ(rl.rlim_cur, stack_size);
 
-  // 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);
+  // The high address of the /proc/self/maps [stack] region should equal stack_base + stack_size.
+  // Remember that the stack grows down (and is mapped in on demand), so the low address of the
+  // region isn't very interesting.
+  EXPECT_EQ(maps_stack_hi, reinterpret_cast<uint8_t*>(stack_base) + stack_size);
 
   //
-  // What if the rlimit is smaller than the stack's current extent?
+  // What if RLIMIT_STACK 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));
@@ -889,7 +882,7 @@ TEST(pthread, pthread_attr_getstack__main_thread) {
   ASSERT_EQ(1024U, stack_size);
 
   //
-  // What if the rlimit isn't a whole number of pages?
+  // What if RLIMIT_STACK 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;