[gdb] Fix segfault in for_each_block, part 1

When running test-case gdb.base/vfork-follow-parent.exp on powerpc64 (likewise
on s390x), I run into:
...
(gdb) PASS: gdb.base/vfork-follow-parent.exp: \
  exec_file=vfork-follow-parent-exit: target-non-stop=on: non-stop=off: \
  resolution_method=schedule-multiple: print unblock_parent = 1
continue^M
Continuing.^M
Reading symbols from vfork-follow-parent-exit...^M
^M
^M
Fatal signal: Segmentation fault^M
----- Backtrace -----^M
0x1027d3e7 gdb_internal_backtrace_1^M
        src/gdb/bt-utils.c:122^M
0x1027d54f _Z22gdb_internal_backtracev^M
        src/gdb/bt-utils.c:168^M
0x1057643f handle_fatal_signal^M
        src/gdb/event-top.c:889^M
0x10576677 handle_sigsegv^M
        src/gdb/event-top.c:962^M
0x3fffa7610477 ???^M
0x103f2144 for_each_block^M
        src/gdb/dcache.c:199^M
0x103f235b _Z17dcache_invalidateP13dcache_struct^M
        src/gdb/dcache.c:251^M
0x10bde8c7 _Z24target_dcache_invalidatev^M
        src/gdb/target-dcache.c:50^M
...
or similar.

The root cause for the segmentation fault is that linux_is_uclinux gives an
incorrect result: it should always return false, given that we're running on a
regular linux system, but instead it returns first true, then false.

In more detail, the segmentation fault happens as follows:
- a program space with an address space is created
- a second program space is about to be created. maybe_new_address_space
  is called, and because linux_is_uclinux returns true, maybe_new_address_space
  returns false, and no new address space is created
- a second program space with the same address space is created
- a program space is deleted. Because linux_is_uclinux now returns false,
  gdbarch_has_shared_address_space (current_inferior ()->arch ()) returns
  false, and the address space is deleted
- when gdb uses the address space of the remaining program space, we run into
  the segfault, because the address space is deleted.

Hardcoding linux_is_uclinux to false makes the test-case pass.

We leave addressing the root cause for the following commit in this series.

For now, prevent the segmentation fault by making the address space a refcounted
object.

This was already suggested here [1]:
...
A better solution might be to have the address spaces be reference counted
...

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

[1] https://sourceware.org/pipermail/gdb-patches/2023-October/202928.html

1 files changed, 8 insertions, 14 deletions

diff --git a/gdb/progspace.c b/gdb/progspace.c
index 839707e..0fc1fdd 100644
--- a/gdb/progspace.c
+++ b/gdb/progspace.c
@@ -55,8 +55,8 @@ address_space::address_space ()
    return a pointer to an existing address space, in case inferiors
    share an address space on this target system.  */
 
-struct address_space *
-maybe_new_address_space (void)
+address_space_ref_ptr
+maybe_new_address_space ()
 {
   int shared_aspace
     = gdbarch_has_shared_address_space (current_inferior ()->arch ());
@@ -67,7 +67,7 @@ maybe_new_address_space (void)
       return program_spaces[0]->aspace;
     }
 
-  return new address_space ();
+  return new_address_space ();
 }
 
 /* Start counting over from scratch.  */
@@ -95,9 +95,9 @@ remove_program_space (program_space *pspace)
 
 /* See progspace.h.  */
 
-program_space::program_space (address_space *aspace_)
+program_space::program_space (address_space_ref_ptr aspace_)
   : num (++last_program_space_num),
-    aspace (aspace_)
+    aspace (std::move (aspace_))
 {
   program_spaces.push_back (this);
   gdb::observers::new_program_space.notify (this);
@@ -122,8 +122,6 @@ program_space::~program_space ()
   /* Defer breakpoint re-set because we don't want to create new
      locations for this pspace which we're tearing down.  */
   clear_symtab_users (SYMFILE_DEFER_BP_RESET);
-  if (!gdbarch_has_shared_address_space (current_inferior ()->arch ()))
-    delete this->aspace;
 }
 
 /* See progspace.h.  */
@@ -411,18 +409,14 @@ update_address_spaces (void)
 
   if (shared_aspace)
     {
-      struct address_space *aspace = new address_space ();
+      address_space_ref_ptr aspace = new_address_space ();
 
-      delete current_program_space->aspace;
       for (struct program_space *pspace : program_spaces)
 	pspace->aspace = aspace;
     }
   else
     for (struct program_space *pspace : program_spaces)
-      {
-	delete pspace->aspace;
-	pspace->aspace = new address_space ();
-      }
+      pspace->aspace = new_address_space ();
 
   for (inferior *inf : all_inferiors ())
     if (gdbarch_has_global_solist (current_inferior ()->arch ()))
@@ -459,5 +453,5 @@ initialize_progspace (void)
      modules have done that.  Do this before
      initialize_current_architecture, because that accesses the ebfd
      of current_program_space.  */
-  current_program_space = new program_space (new address_space ());
+  current_program_space = new program_space (new_address_space ());
 }