ART: Change main-thread thread paging scheme
Modify the code that ensures we can install a stack guard page into
the main thread.
A recent kernel change means that our previous approach of using a
free pointer does not work. It is important to actually extend the
stack properly. For portability, use a function with a large stack
frame (suggested by and adapted from hboehm).
(cherry picked from commit e43dfd6498545ab54121bf330941aef82d9bf339)
Bug: 62952017
Test: m
Test: m test-art-host
Test: Device boots (x86_64 emulator)
Test: Device boots (bullhead)
Merged-In: Ic2a0c3d6d05a1ea9f655329d147b46949e1b9db3
Change-Id: Ic2a0c3d6d05a1ea9f655329d147b46949e1b9db3
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 5274f9e..b0651e7 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -337,9 +337,6 @@
return stack_size;
}
-// Global variable to prevent the compiler optimizing away the page reads for the stack.
-uint8_t dont_optimize_this;
-
// Install a protected region in the stack. This is used to trigger a SIGSEGV if a stack
// overflow is detected. It is located right below the stack_begin_.
//
@@ -357,20 +354,47 @@
uint8_t* stack_top = reinterpret_cast<uint8_t*>(reinterpret_cast<uintptr_t>(&stack_himem) &
~(kPageSize - 1)); // Page containing current top of stack.
- // First remove the protection on the protected region as will want to read and
- // write it. This may fail (on the first attempt when the stack is not mapped)
- // but we ignore that.
+ // There is a little complexity here that deserves a special mention. On some
+ // architectures, the stack is created using a VM_GROWSDOWN flag
+ // to prevent memory being allocated when it's not needed. This flag makes the
+ // kernel only allocate memory for the stack by growing down in memory. Because we
+ // want to put an mprotected region far away from that at the stack top, we need
+ // to make sure the pages for the stack are mapped in before we call mprotect.
+ //
+ // The failed mprotect in UnprotectStack is an indication of a thread with VM_GROWSDOWN
+ // with a non-mapped stack (usually only the main thread).
+ //
+ // We map in the stack by reading every page from the stack bottom (highest address)
+ // to the stack top. (We then madvise this away.) This must be done by reading from the
+ // current stack pointer downwards.
+
+ // (Defensively) first remove the protection on the protected region as we'll want to read
+ // and write it. Ignore errors.
UnprotectStack();
- // Map in the stack. This must be done by reading from the
- // current stack pointer downwards as the stack may be mapped using VM_GROWSDOWN
- // in the kernel. Any access more than a page below the current SP might cause
- // a segv.
+ VLOG(threads) << "Need to map in stack for thread at " << std::hex <<
+ static_cast<void*>(pregion);
- // Read every page from the high address to the low.
- for (uint8_t* p = stack_top; p >= pregion; p -= kPageSize) {
- dont_optimize_this = *p;
- }
+ struct RecurseDownStack {
+ // This function has an intentionally large stack size.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wframe-larger-than="
+ NO_INLINE
+ static void Touch(uintptr_t target) {
+ volatile size_t zero = 0;
+ // Use a large local volatile array to ensure a large frame size. Do not use anything close
+ // to a full page for ASAN. It would be nice to ensure the frame size is at most a page, but
+ // there is no pragma support for this.
+ volatile char space[kPageSize - 256];
+ char sink ATTRIBUTE_UNUSED = space[zero];
+ if (reinterpret_cast<uintptr_t>(space) >= target + kPageSize) {
+ Touch(target);
+ }
+ zero *= 2; // Try to avoid tail recursion.
+ }
+#pragma GCC diagnostic pop
+ };
+ RecurseDownStack::Touch(reinterpret_cast<uintptr_t>(pregion));
VLOG(threads) << "installing stack protected region at " << std::hex <<
static_cast<void*>(pregion) << " to " <<
