[Linux][Backtracing] Don't try to use process_vm_readv().

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -655,7 +655,6 @@ wait_paused(uint32_t expected, const struct timespec *timeout)
 char memserver_stack[4096] __attribute__((aligned(SWIFT_PAGE_SIZE)));
 char memserver_buffer[4096];
 int memserver_fd;
-bool memserver_has_ptrace;
 sigjmp_buf memserver_fault_buf;
 pid_t memserver_pid;
 
@@ -712,21 +711,18 @@ memserver_fault(int sig) {
 
 ssize_t __attribute__((noinline))
 memserver_read(void *to, const void *from, size_t len) {
-  if (memserver_has_ptrace) {
-// This won't run for older Android APIs anyway, but it can't be compiled
-// either, as process_vm_readv() isn't available.
-#if !(defined(__ANDROID_API__) && __ANDROID_API__ < 23)
-    struct iovec local = { to, len };
-    struct iovec remote = { const_cast<void *>(from), len };
-    return process_vm_readv(memserver_pid, &local, 1, &remote, 1, 0);
-#endif
+  /* Earlier versions of this code tried to use process_vm_readv() if they
+     detected that CAP_SYS_PTRACE was enabled.  This is theoretically
+     slightly safer than using signals and memcpy(), but in practice it
+     turns out that some kernels don't support process_vm_readv() even
+     when CAP_SYS_PTRACE is enabled.
+
+     It seems simpler just to use the signal handlers instead. */
+  if (!sigsetjmp(memserver_fault_buf, 1)) {
+    memcpy(to, from, len);
+    return len;
   } else {
-    if (!sigsetjmp(memserver_fault_buf, 1)) {
-      memcpy(to, from, len);
-      return len;
-    } else {
-      return -1;
-    }
+    return -1;
   }
 }
 
@@ -739,21 +735,12 @@ memserver_entry(void *dummy __attribute__((unused))) {
   prctl(PR_SET_NAME, "[backtrace]");
 #endif
 
-// process_vm_readv() is not available for older Android APIs.
-#if defined(__ANDROID_API__) && __ANDROID_API__ < 23
-  memserver_has_ptrace = false;
-#else
-  memserver_has_ptrace = !!prctl(PR_CAPBSET_READ, CAP_SYS_PTRACE);
-#endif
-
-  if (!memserver_has_ptrace) {
-    struct sigaction sa;
-    sigfillset(&sa.sa_mask);
-    sa.sa_handler = memserver_fault;
-    sa.sa_flags = SA_NODEFER;
-    sigaction(SIGSEGV, &sa, NULL);
-    sigaction(SIGBUS, &sa, NULL);
-  }
+  struct sigaction sa;
+  sigfillset(&sa.sa_mask);
+  sa.sa_handler = memserver_fault;
+  sa.sa_flags = SA_NODEFER;
+  sigaction(SIGSEGV, &sa, NULL);
+  sigaction(SIGBUS, &sa, NULL);
 
   for (;;) {
     struct memserver_req req;