[PATCH] Guard against 'current_directory == NULL' on gdb_abspath (PR gdb/23613)

[Sergio Durigan Junior <sergiodj at redhat dot com>]

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1728147
Ref.: https://sourceware.org/bugzilla/show_bug.cgi?id=23613

Hi,

This bug has been reported a few days ago against Fedora GDB.  The
problem reported is that GDB segfaults when the working directory is
deleted.  It's pretty use to reproduce it:

  mkdir bla
  cd bla
  rmdir ../bla
  gdb echo

Debugging the problem is a bit tricky, because, since the current
directory doesn't exist anymore, a corefile cannot be saved there.
After a few attempts, I came up with the following:

  gdb -ex 'shell mkdir bla' -ex 'cd bla' -ex 'shell rmdir ../bla' -ex 'r echo' ./gdb/gdb

This assumes that you're inside a build directory which contains
./gdb/gdb, of course.

After investigating it, I found that the problem happens at
gdb_abspath, where we're dereferencing 'current_directory' without
checking if it's NULL:

    ...
    (concat (current_directory,
	     IS_DIR_SEPARATOR (current_directory[strlen (current_directory) - 1])
	     ? "" : SLASH_STRING,
    ...

So I fixed the problem with the patch below.  The idea is that, if
'current_directory' is NULL, then the final string returned should be
just the "path".

After fixing the bug, I found a similar one reported against our
bugzilla: PR gdb/23613.  The problem is the same, but the reproducer
is a bit different.

I really tried writing a testcase for this, but unfortunately it's
apparently not possible to start GDB inside a non-existent directory
with DejaGNU.

I regression tested this patch on the BuildBot, and no regressions
were found.

gdb/ChangeLog:
2019-07-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1728147
	PR gdb/23613
	* gdbsupport/pathstuff.c (gdb_abspath): Guard against
	'current_directory == NULL' case.
---
 gdb/gdbsupport/pathstuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/gdbsupport/pathstuff.c b/gdb/gdbsupport/pathstuff.c
index fafecd543d..aa51e8f36e 100644
--- a/gdb/gdbsupport/pathstuff.c
+++ b/gdb/gdbsupport/pathstuff.c
@@ -134,7 +134,7 @@ gdb_abspath (const char *path)
   if (path[0] == '~')
     return gdb_tilde_expand_up (path);
 
-  if (IS_ABSOLUTE_PATH (path))
+  if (IS_ABSOLUTE_PATH (path) || current_directory == NULL)
     return make_unique_xstrdup (path);
 
   /* Beware the // my son, the Emacs barfs, the botch that catch...  */
-- 
2.21.0