gdb: push target earlier in procfs_target::attach (PR 27435)

Since this is a GDB 9 -> 10 regression, I would like to push it to
gdb-10-branch.

This is a follow-up to:

  https://sourceware.org/pipermail/gdb-patches/2021-February/176202.html

This patch fixes a segfault seen when attaching to a process on Solaris.
The steps leading to the segfault are:

 - procfs_target::attach calls do_attach, at this point the inferior's
   process slot in the target stack is empty.
 - do_attach adds a thread with `add_thread (&the_procfs_target, ptid)`
 - in add_thread_silent, the passed target (&the_procfs_target) is
   passed to find_inferior_ptid
 - find_inferior_ptid returns nullptr, as there is no inferior with this
   ptid that has &the_procfs_target as its process target
 - the nullptr `inf` is passed to find_thread_ptid, which dereferences
   it, causing a segfault
 - back in procfs_target::attach, after do_attach, we push the
   the_procfs_target on the inferior's target stack, although we never
   reach this because the segfault happens before.

To fix this, I think we need to do the same as is done in
inf_ptrace_target::attach: push the target early and unpush it in case
the attach fails (and keep it if the attach succeeds).

Implement it by moving target_unpush_up to target.h, so it can be
re-used here.  Make procfs_target::attach use it.  Note that just like
is mentioned in inf_ptrace_target::attach, we should push the target
before calling target_pid_to_str, so that calling target_pid_to_str ends
up in procfs_target::pid_to_str.

Tested by trying to attach on a process on gcc211 on the gcc compile
farm.

gdb/ChangeLog:

	PR gdb/27435
	* inf-ptrace.c (struct target_unpusher): Move to target.h.
	(target_unpush_up): Likewise.
	* procfs.c (procfs_target::attach): Push target early.  Use
	target_unpush_up to unpush target in case of error.
	* target.h (struct target_unpusher): Move here.
	(target_unpush_up): Likewise.

Change-Id: I88aff8b20204e1ca1d792e27ac6bc34fc1aa0d52

gdb/ChangeLog

@@ -1,3 +1,13 @@
+2021-02-22  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	PR gdb/27435
+	* inf-ptrace.c (struct target_unpusher): Move to target.h.
+	(target_unpush_up): Likewise.
+	* procfs.c (procfs_target::attach): Push target early.  Use
+	target_unpush_up to unpush target in case of error.
+	* target.h (struct target_unpusher): Move here.
+	(target_unpush_up): Likewise.
+
 2021-02-09  Shahab Vahedi  <shahab@synopsys.com>
 
 	PR build/27385

gdb/inf-ptrace.c

@@ -49,22 +49,6 @@ gdb_ptrace (PTRACE_TYPE_ARG1 request, ptid_t ptid, PTRACE_TYPE_ARG3 addr,
 #endif
 }
 
-/* A unique_ptr helper to unpush a target.  */
-
-struct target_unpusher
-{
-  void operator() (struct target_ops *ops) const
-  {
-    unpush_target (ops);
-  }
-};
-
-/* A unique_ptr that unpushes a target on destruction.  */
-
-typedef std::unique_ptr<struct target_ops, target_unpusher> target_unpush_up;
-
-
-
 inf_ptrace_target::~inf_ptrace_target ()
 {}
 

gdb/procfs.c

@@ -1767,6 +1767,14 @@ procfs_target::attach (const char *args, int from_tty)
   if (pid == getpid ())
     error (_("Attaching GDB to itself is not a good idea..."));
 
+  /* Push the target if needed, ensure it gets un-pushed it if attach fails.  */
+  target_unpush_up unpusher;
+  if (!target_is_pushed (this))
+    {
+      push_target (this);
+      unpusher.reset (this);
+    }
+
   if (from_tty)
     {
       const char *exec_file = get_exec_file (0);
@@ -1780,9 +1788,11 @@ procfs_target::attach (const char *args, int from_tty)
 
       fflush (stdout);
     }
+
   do_attach (ptid_t (pid));
-  if (!target_is_pushed (this))
-    push_target (this);
+
+  /* Everything went fine, keep the target pushed.  */
+  unpusher.release ();
 }
 
 void

gdb/target.h

@@ -2399,6 +2399,20 @@ extern void push_target (target_ops_up &&);
 
 extern int unpush_target (struct target_ops *);
 
+/* A unique_ptr helper to unpush a target.  */
+
+struct target_unpusher
+{
+  void operator() (struct target_ops *ops) const
+  {
+    unpush_target (ops);
+  }
+};
+
+/* A unique_ptr that unpushes a target on destruction.  */
+
+typedef std::unique_ptr<struct target_ops, target_unpusher> target_unpush_up;
+
 extern void target_pre_inferior (int);
 
 extern void target_preopen (int);