diff --git a/libc/bionic/pthread_attr.cpp b/libc/bionic/pthread_attr.cpp
index 7ad3431..cfa58fc 100644
--- a/libc/bionic/pthread_attr.cpp
+++ b/libc/bionic/pthread_attr.cpp
@@ -114,6 +114,36 @@
   return 0;
 }
 
+static uintptr_t __get_main_stack_startstack() {
+  FILE* fp = fopen("/proc/self/stat", "re");
+  if (fp == nullptr) {
+    __libc_fatal("couldn't open /proc/self/stat: %s", strerror(errno));
+  }
+
+  char line[BUFSIZ];
+  if (fgets(line, sizeof(line), fp) == nullptr) {
+    __libc_fatal("couldn't read /proc/self/stat: %s", strerror(errno));
+  }
+
+  fclose(fp);
+
+  // See man 5 proc. There's no reason comm can't contain ' ' or ')',
+  // so we search backwards for the end of it. We're looking for this field:
+  //
+  //  startstack %lu (28) The address of the start (i.e., bottom) of the stack.
+  uintptr_t startstack = 0;
+  const char* end_of_comm = strrchr(line, ')');
+  if (sscanf(end_of_comm + 1, " %*c "
+             "%*d %*d %*d %*d %*d "
+             "%*u %*u %*u %*u %*u %*u %*u "
+             "%*d %*d %*d %*d %*d %*d "
+             "%*u %*u %*d %*u %*u %*u %" SCNuPTR, &startstack) != 1) {
+    __libc_fatal("couldn't parse /proc/self/stat");
+  }
+
+  return startstack;
+}
+
 static int __pthread_attr_getstack_main_thread(void** stack_base, size_t* stack_size) {
   ErrnoRestorer errno_restorer;
 
@@ -127,20 +157,19 @@
     stack_limit.rlim_cur = 8 * 1024 * 1024;
   }
 
-  // It shouldn't matter which thread we are because we're just looking for "[stack]", but
-  // valgrind seems to mess with the stack enough that the kernel will report "[stack:pid]"
-  // instead if you look in /proc/self/maps, so we need to look in /proc/pid/task/pid/maps.
-  char path[64];
-  snprintf(path, sizeof(path), "/proc/self/task/%d/maps", getpid());
-  FILE* fp = fopen(path, "re");
-  if (fp == NULL) {
-    return errno;
+  // Ask the kernel where our main thread's stack started.
+  uintptr_t startstack = __get_main_stack_startstack();
+
+  // Hunt for the region that contains that address.
+  FILE* fp = fopen("/proc/self/maps", "re");
+  if (fp == nullptr) {
+    __libc_fatal("couldn't open /proc/self/maps");
   }
   char line[BUFSIZ];
   while (fgets(line, sizeof(line), fp) != NULL) {
-    if (ends_with(line, " [stack]\n")) {
-      uintptr_t lo, hi;
-      if (sscanf(line, "%" SCNxPTR "-%" SCNxPTR, &lo, &hi) == 2) {
+    uintptr_t lo, hi;
+    if (sscanf(line, "%" SCNxPTR "-%" SCNxPTR, &lo, &hi) == 2) {
+      if (lo <= startstack && startstack <= hi) {
         *stack_size = stack_limit.rlim_cur;
         *stack_base = reinterpret_cast<void*>(hi - *stack_size);
         fclose(fp);
@@ -148,7 +177,7 @@
       }
     }
   }
-  __libc_fatal("No [stack] line found in \"%s\"!", path);
+  __libc_fatal("Stack not found in /proc/self/maps");
 }
 
 int pthread_attr_getstack(const pthread_attr_t* attr, void** stack_base, size_t* stack_size) {
diff --git a/tests/pthread_test.cpp b/tests/pthread_test.cpp
index eeb1541..3c686ef 100644
--- a/tests/pthread_test.cpp
+++ b/tests/pthread_test.cpp
@@ -1220,6 +1220,67 @@
 #endif
 }
 
+struct GetStackSignalHandlerArg {
+  volatile bool done;
+  void* signal_handler_sp;
+  void* main_stack_base;
+  size_t main_stack_size;
+};
+
+static GetStackSignalHandlerArg getstack_signal_handler_arg;
+
+static void getstack_signal_handler(int sig) {
+  ASSERT_EQ(SIGUSR1, sig);
+  // Use sleep() to make current thread be switched out by the kernel to provoke the error.
+  sleep(1);
+  pthread_attr_t attr;
+  ASSERT_EQ(0, pthread_getattr_np(pthread_self(), &attr));
+  void* stack_base;
+  size_t stack_size;
+  ASSERT_EQ(0, pthread_attr_getstack(&attr, &stack_base, &stack_size));
+  getstack_signal_handler_arg.signal_handler_sp = &attr;
+  getstack_signal_handler_arg.main_stack_base = stack_base;
+  getstack_signal_handler_arg.main_stack_size = stack_size;
+  getstack_signal_handler_arg.done = true;
+}
+
+// The previous code obtained the main thread's stack by reading the entry in
+// /proc/self/task/<pid>/maps that was labeled [stack]. Unfortunately, on x86/x86_64, the kernel
+// relies on sp0 in task state segment(tss) to label the stack map with [stack]. If the kernel
+// switches a process while the main thread is in an alternate stack, then the kernel will label
+// the wrong map with [stack]. This test verifies that when the above situation happens, the main
+// thread's stack is found correctly.
+TEST(pthread, pthread_attr_getstack_in_signal_handler) {
+  const size_t sig_stack_size = 16 * 1024;
+  void* sig_stack = mmap(NULL, sig_stack_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
+                         -1, 0);
+  ASSERT_NE(MAP_FAILED, sig_stack);
+  stack_t ss;
+  ss.ss_sp = sig_stack;
+  ss.ss_size = sig_stack_size;
+  ss.ss_flags = 0;
+  stack_t oss;
+  ASSERT_EQ(0, sigaltstack(&ss, &oss));
+
+  ScopedSignalHandler handler(SIGUSR1, getstack_signal_handler, SA_ONSTACK);
+  getstack_signal_handler_arg.done = false;
+  kill(getpid(), SIGUSR1);
+  ASSERT_EQ(true, getstack_signal_handler_arg.done);
+
+  // Verify if the stack used by the signal handler is the alternate stack just registered.
+  ASSERT_LE(sig_stack, getstack_signal_handler_arg.signal_handler_sp);
+  ASSERT_GE(reinterpret_cast<char*>(sig_stack) + sig_stack_size,
+            getstack_signal_handler_arg.signal_handler_sp);
+
+  // Verify if the main thread's stack got in the signal handler is correct.
+  ASSERT_LE(getstack_signal_handler_arg.main_stack_base, &ss);
+  ASSERT_GE(reinterpret_cast<char*>(getstack_signal_handler_arg.main_stack_base) +
+            getstack_signal_handler_arg.main_stack_size, reinterpret_cast<void*>(&ss));
+
+  ASSERT_EQ(0, sigaltstack(&oss, nullptr));
+  ASSERT_EQ(0, munmap(sig_stack, sig_stack_size));
+}
+
 static void pthread_attr_getstack_18908062_helper(void*) {
   char local_variable;
   pthread_attr_t attributes;