Revert "bionic: Allocate a shadow call stack for each thread."

This reverts commit 65c84848fd4666e4163bf741660f8c6a83ef618e.

Causes mmap/malloc failure after creating many threads.

Test: Play music change tracks, then attempt Camera video record.
Bug: 118593766
Change-Id: I6287752d35dd31124c21708e94fddce9facdbf6d
(cherry picked from commit 75302dbcb743695834495e8ee2ee25b0b16e51a1)
diff --git a/libc/bionic/__libc_init_main_thread.cpp b/libc/bionic/__libc_init_main_thread.cpp
index be9d32e..758b295 100644
--- a/libc/bionic/__libc_init_main_thread.cpp
+++ b/libc/bionic/__libc_init_main_thread.cpp
@@ -101,5 +101,5 @@
 
   __init_thread(&main_thread);
 
-  __init_additional_stacks(&main_thread);
+  __init_alternate_signal_stack(&main_thread);
 }
diff --git a/libc/bionic/pthread_create.cpp b/libc/bionic/pthread_create.cpp
index 3ba787b..543fdc5 100644
--- a/libc/bionic/pthread_create.cpp
+++ b/libc/bionic/pthread_create.cpp
@@ -32,7 +32,6 @@
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/prctl.h>
-#include <sys/random.h>
 #include <unistd.h>
 
 #include "pthread_internal.h"
@@ -87,7 +86,7 @@
   thread->tls[TLS_SLOT_STACK_GUARD] = reinterpret_cast<void*>(__stack_chk_guard);
 }
 
-static void __init_alternate_signal_stack(pthread_internal_t* thread) {
+void __init_alternate_signal_stack(pthread_internal_t* thread) {
   // Create and set an alternate signal stack.
   void* stack_base = mmap(nullptr, SIGNAL_STACK_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
   if (stack_base != MAP_FAILED) {
@@ -110,32 +109,6 @@
   }
 }
 
-static void __init_shadow_call_stack(pthread_internal_t* thread) {
-  (void)thread;
-#ifdef __aarch64__
-  char* scs_guard_region = reinterpret_cast<char*>(
-      mmap(nullptr, SCS_GUARD_REGION_SIZE, 0, MAP_PRIVATE | MAP_ANON, -1, 0));
-  thread->shadow_call_stack_guard_region = scs_guard_region;
-
-  // We need to page align scs_offset and ensure that [scs_offset,scs_offset+SCS_SIZE) is in the
-  // guard region. We can't use arc4random_uniform in init because /dev/urandom might not have
-  // been created yet.
-  size_t scs_offset =
-      (getpid() == 1) ? 0 : (arc4random_uniform(SCS_GUARD_REGION_SIZE / SCS_SIZE) * SCS_SIZE);
-
-  // Allocate the stack and store its address in register x18. This is deliberately the only place
-  // where the address is stored.
-  __asm__ __volatile__(
-      "mov x18, %0" ::"r"(mmap(scs_guard_region + scs_offset, SCS_SIZE, PROT_READ | PROT_WRITE,
-                               MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0)));
-#endif
-}
-
-void __init_additional_stacks(pthread_internal_t* thread) {
-  __init_alternate_signal_stack(thread);
-  __init_shadow_call_stack(thread);
-}
-
 int __init_thread(pthread_internal_t* thread) {
   thread->cleanup_stack = nullptr;
 
@@ -279,7 +252,7 @@
   // accesses previously made by the creating thread are visible to us.
   thread->startup_handshake_lock.lock();
 
-  __init_additional_stacks(thread);
+  __init_alternate_signal_stack(thread);
 
   void* result = thread->start_routine(thread->start_routine_arg);
   pthread_exit(result);
diff --git a/libc/bionic/pthread_exit.cpp b/libc/bionic/pthread_exit.cpp
index 010cc06..220f7a0 100644
--- a/libc/bionic/pthread_exit.cpp
+++ b/libc/bionic/pthread_exit.cpp
@@ -103,11 +103,6 @@
     thread->alternate_signal_stack = nullptr;
   }
 
-#ifdef __aarch64__
-  // Free the shadow call stack and guard pages.
-  munmap(thread->shadow_call_stack_guard_region, SCS_GUARD_REGION_SIZE);
-#endif
-
   ThreadJoinState old_state = THREAD_NOT_JOINED;
   while (old_state == THREAD_NOT_JOINED &&
          !atomic_compare_exchange_weak(&thread->join_state, &old_state, THREAD_EXITED_NOT_JOINED)) {
diff --git a/libc/bionic/pthread_internal.h b/libc/bionic/pthread_internal.h
index 9d55eba..b68cb94 100644
--- a/libc/bionic/pthread_internal.h
+++ b/libc/bionic/pthread_internal.h
@@ -106,29 +106,6 @@
 
   void* alternate_signal_stack;
 
-  // The start address of the shadow call stack's guard region (arm64 only).
-  // This address is only used to deallocate the shadow call stack on thread
-  // exit; the address of the stack itself is stored only in the x18 register.
-  // Because the protection offered by SCS relies on the secrecy of the stack
-  // address, storing the address here weakens the protection, but only
-  // slightly, because it is relatively easy for an attacker to discover the
-  // address of the guard region anyway (e.g. it can be discovered by reference
-  // to other allocations), but not the stack itself, which is <0.1% of the size
-  // of the guard region.
-  //
-  // There are at least two other options for discovering the start address of
-  // the guard region on thread exit, but they are not as simple as storing in
-  // TLS.
-  // 1) Derive it from the value of the x18 register. This is only possible in
-  //    processes that do not contain legacy code that might clobber x18,
-  //    therefore each process must declare early during process startup whether
-  //    it might load legacy code.
-  // 2) Mark the guard region as such using prctl(PR_SET_VMA_ANON_NAME) and
-  //    discover its address by reading /proc/self/maps. One issue with this is
-  //    that reading /proc/self/maps can race with allocations, so we may need
-  //    code to handle retries.
-  void* shadow_call_stack_guard_region;
-
   Lock startup_handshake_lock;
 
   size_t mmap_size;
@@ -154,7 +131,7 @@
 __LIBC_HIDDEN__ int __init_thread(pthread_internal_t* thread);
 __LIBC_HIDDEN__ bool __init_tls(pthread_internal_t* thread);
 __LIBC_HIDDEN__ void __init_thread_stack_guard(pthread_internal_t* thread);
-__LIBC_HIDDEN__ void __init_additional_stacks(pthread_internal_t*);
+__LIBC_HIDDEN__ void __init_alternate_signal_stack(pthread_internal_t*);
 
 __LIBC_HIDDEN__ pthread_t           __pthread_internal_add(pthread_internal_t* thread);
 __LIBC_HIDDEN__ pthread_internal_t* __pthread_internal_find(pthread_t pthread_id);
@@ -203,13 +180,6 @@
 // Leave room for a guard page in the internally created signal stacks.
 #define SIGNAL_STACK_SIZE (SIGNAL_STACK_SIZE_WITHOUT_GUARD + PTHREAD_GUARD_SIZE)
 
-// Size of the shadow call stack.
-#define SCS_SIZE (8 * 1024)
-
-// The shadow call stack is allocated at a random address within a guard region
-// of this size.
-#define SCS_GUARD_REGION_SIZE (16 * 1024 * 1024)
-
 // Needed by fork.
 __LIBC_HIDDEN__ extern void __bionic_atfork_run_prepare();
 __LIBC_HIDDEN__ extern void __bionic_atfork_run_child();
diff --git a/tests/Android.bp b/tests/Android.bp
index cb1f6c0..4ad51da 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -38,12 +38,6 @@
         // For glibc.
         "-D__STDC_LIMIT_MACROS",
     ],
-    // Make the bionic tests implicitly test bionic's shadow call stack support.
-    arch: {
-        arm64: {
-           cflags: ["-fsanitize=shadow-call-stack"],
-        },
-    },
     stl: "libc++",
     sanitize: {
         never: true,