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

1 files changed, 12 insertions, 2 deletions

diff --git a/gdb/procfs.c b/gdb/procfs.c
index e73faa8..cab29c3 100644
--- a/gdb/procfs.c
+++ b/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