[gdb] Fix segfault in for_each_block, part 2

The previous commit describes PR gdb/30547, a segfault when running test-case
gdb.base/vfork-follow-parent.exp on powerpc64 (likewise on s390x).

The root cause for the segmentation fault is that linux_is_uclinux gives an
incorrect result: it returns true instead of false.

So, why does linux_is_uclinux:
...
int
linux_is_uclinux (void)
{
  CORE_ADDR dummy;

  return (target_auxv_search (AT_NULL, &dummy) > 0
	  && target_auxv_search (AT_PAGESZ, &dummy) == 0);
...
return true?

This is because ppc_linux_target_wordsize returns 4 instead of 8, causing
ppc_linux_nat_target::auxv_parse to misinterpret the auxv vector.

So, why does ppc_linux_target_wordsize:
...
int
ppc_linux_target_wordsize (int tid)
{
  int wordsize = 4;

  /* Check for 64-bit inferior process.	 This is the case when the host is
     64-bit, and in addition the top bit of the MSR register is set.  */
  long msr;

  errno = 0;
  msr = (long) ptrace (PTRACE_PEEKUSER, tid, PT_MSR * 8, 0);
  if (errno == 0 && ppc64_64bit_inferior_p (msr))
    wordsize = 8;

  return wordsize;
}
...
return 4?

Specifically, we get this result because because tid == 0, so we get
errno == ESRCH.

The tid == 0 is caused by the switch_to_no_thread in
handle_vfork_child_exec_or_exit:
...
	  /* Switch to no-thread while running clone_program_space, so
	     that clone_program_space doesn't want to read the
	     selected frame of a dead process.  */
	  scoped_restore_current_thread restore_thread;
	  switch_to_no_thread ();

	  inf->pspace = new program_space (maybe_new_address_space ());
...
but moving the maybe_new_address_space call to before that gives us the
same result.  The tid is no longer 0, but we still get ESRCH because the
thread has exited.

Fix this in handle_vfork_child_exec_or_exit by doing the
maybe_new_address_space call in the context of the vfork parent.

Tested on top of trunk on x86_64-linux and ppc64le-linux.
Tested on top of gdb-14-branch on ppc64-linux.

Co-Authored-By: Simon Marchi <simon.marchi@polymtl.ca>

PR gdb/30547
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30547

gdb/infrun.c

@@ -1105,13 +1105,19 @@ handle_vfork_child_exec_or_exit (int exec)
 	     go ahead and create a new one for this exiting
 	     inferior.  */
 
-	  /* Switch to no-thread while running clone_program_space, so
-	     that clone_program_space doesn't want to read the
-	     selected frame of a dead process.  */
 	  scoped_restore_current_thread restore_thread;
-	  switch_to_no_thread ();
 
-	  inf->pspace = new program_space (maybe_new_address_space ());
+	  /* Temporarily switch to the vfork parent, to facilitate ptrace
+	     calls done during maybe_new_address_space.  */
+	  switch_to_thread (any_live_thread_of_inferior (vfork_parent));
+	  address_space_ref_ptr aspace = maybe_new_address_space ();
+
+	  /* Switch back to the vfork child inferior.  Switch to no-thread
+	     while running clone_program_space, so that clone_program_space
+	     doesn't want to read the selected frame of a dead process.  */
+	  switch_to_inferior_no_thread (inf);
+
+	  inf->pspace = new program_space (std::move (aspace));
 	  inf->aspace = inf->pspace->aspace;
 	  set_current_program_space (inf->pspace);
 	  inf->removable = true;

gdb/nat/ppc-linux.c

@@ -78,6 +78,8 @@ ppc64_64bit_inferior_p (long msr)
 int
 ppc_linux_target_wordsize (int tid)
 {
+  gdb_assert (tid != 0);
+
   int wordsize = 4;
 
   /* Check for 64-bit inferior process.  This is the case when the host is

gdb/ppc-linux-nat.c

@@ -1914,6 +1914,8 @@ ppc_linux_nat_target::auxv_parse (const gdb_byte **readptr,
 				  const gdb_byte *endptr, CORE_ADDR *typep,
 				  CORE_ADDR *valp)
 {
+  gdb_assert (inferior_ptid != null_ptid);
+
   int tid = inferior_ptid.lwp ();
   if (tid == 0)
     tid = inferior_ptid.pid ();

gdb/s390-linux-nat.c

@@ -949,10 +949,12 @@ s390_target_wordsize (void)
   /* Check for 64-bit inferior process.  This is the case when the host is
      64-bit, and in addition bit 32 of the PSW mask is set.  */
 #ifdef __s390x__
+  int tid = s390_inferior_tid ();
+  gdb_assert (tid != 0);
   long pswm;
 
   errno = 0;
-  pswm = (long) ptrace (PTRACE_PEEKUSER, s390_inferior_tid (), PT_PSWMASK, 0);
+  pswm = (long) ptrace (PTRACE_PEEKUSER, tid, PT_PSWMASK, 0);
   if (errno == 0 && (pswm & 0x100000000ul) != 0)
     wordsize = 8;
 #endif
@@ -965,6 +967,7 @@ s390_linux_nat_target::auxv_parse (const gdb_byte **readptr,
 				   const gdb_byte *endptr, CORE_ADDR *typep,
 				   CORE_ADDR *valp)
 {
+  gdb_assert (inferior_ptid != null_ptid);
   int sizeof_auxv_field = s390_target_wordsize ();
   bfd_endian byte_order = gdbarch_byte_order (current_inferior ()->arch ());
   const gdb_byte *ptr = *readptr;