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

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 against Fedora GDB, but there's also an
upstream bug.  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:
2019-12-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1728147
	PR gdb/23613
	* bsd-kvm.c (bsd_kvm_target_open): Use 'gdb_abspath'.
	* corelow.c: Include 'gdbsupport/pathstuff.h'.
	(core_target_open): Use 'gdb_abspath'.
	* gdbsupport/pathstuff.c (gdb_abspath): Guard against
	'current_directory == NULL' case.
	* gdbsupport/pathstuff.h (gdb_abspath): Expand comment and
	explain what happens when 'current_directory' is NULL.
	* go32-nat.c (go32_nat_target::wait): Check if
	'current_directory' is NULL before call to 'chdir'.
	* source.c (add_path): Use 'gdb_abspath'.
	* top.c: Include 'gdbsupport/pathstuff.h'.
	(init_history): Use 'gdb_abspath'.
	(set_history_filename): Likewise.
	* tracefile-tfile.c: Include 'gdbsupport/pathstuff.h'.
	(tfile_target_open): Use 'gdb_abspath'.

Change-Id: Ibb0932fa25bc5c2d3ae4a7f64bd7f32885ca403b

gdb/ChangeLog

@@ -1,3 +1,23 @@
+2019-12-14  Sergio Durigan Junior  <sergiodj@redhat.com>
+
+	https://bugzilla.redhat.com/show_bug.cgi?id=1728147
+	PR gdb/23613
+	* bsd-kvm.c (bsd_kvm_target_open): Use 'gdb_abspath'.
+	* corelow.c: Include 'gdbsupport/pathstuff.h'.
+	(core_target_open): Use 'gdb_abspath'.
+	* gdbsupport/pathstuff.c (gdb_abspath): Guard against
+	'current_directory == NULL' case.
+	* gdbsupport/pathstuff.h (gdb_abspath): Expand comment and
+	explain what happens when 'current_directory' is NULL.
+	* go32-nat.c (go32_nat_target::wait): Check if
+	'current_directory' is NULL before call to 'chdir'.
+	* source.c (add_path): Use 'gdb_abspath'.
+	* top.c: Include 'gdbsupport/pathstuff.h'.
+	(init_history): Use 'gdb_abspath'.
+	(set_history_filename): Likewise.
+	* tracefile-tfile.c: Include 'gdbsupport/pathstuff.h'.
+	(tfile_target_open): Use 'gdb_abspath'.
+
 2019-12-13  Tom Tromey  <tromey@adacore.com>
 
 	* contrib/ari/gdb_ari.sh: Remove check for multiple calls to

gdb/bsd-kvm.c

@@ -114,14 +114,13 @@ bsd_kvm_target_open (const char *arg, int from_tty)
 
   if (arg)
     {
-      char *temp;
-
       filename = tilde_expand (arg);
       if (filename[0] != '/')
 	{
-	  temp = concat (current_directory, "/", filename, (char *)NULL);
+	  gdb::unique_xmalloc_ptr<char> temp (gdb_abspath (filename));
+
 	  xfree (filename);
-	  filename = temp;
+	  filename = temp.release ();
 	}
     }
 

gdb/corelow.c

@@ -44,6 +44,7 @@
 #include "completer.h"
 #include "gdbsupport/filestuff.h"
 #include "build-id.h"
+#include "gdbsupport/pathstuff.h"
 
 #ifndef O_LARGEFILE
 #define O_LARGEFILE 0
@@ -395,8 +396,7 @@ core_target_open (const char *arg, int from_tty)
 
   gdb::unique_xmalloc_ptr<char> filename (tilde_expand (arg));
   if (!IS_ABSOLUTE_PATH (filename.get ()))
-    filename.reset (concat (current_directory, "/",
-			    filename.get (), (char *) NULL));
+    filename = gdb_abspath (filename.get ());
 
   flags = O_BINARY | O_LARGEFILE;
   if (write_files)

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...  */

gdb/gdbsupport/pathstuff.h

@@ -44,7 +44,10 @@ extern gdb::unique_xmalloc_ptr<char>
 
    Contrary to "gdb_realpath", this function uses CURRENT_DIRECTORY
    for the path expansion.  This may lead to scenarios the current
-   working directory (CWD) is different than CURRENT_DIRECTORY.  */
+   working directory (CWD) is different than CURRENT_DIRECTORY.
+
+   If CURRENT_DIRECTORY is NULL, this function returns a copy of
+   PATH.  */
 
 extern gdb::unique_xmalloc_ptr<char> gdb_abspath (const char *path);
 

gdb/go32-nat.c

@@ -507,7 +507,8 @@ go32_nat_target::wait (ptid_t ptid, struct target_waitstatus *status,
     }
 
   getcwd (child_cwd, sizeof (child_cwd)); /* in case it has changed */
-  chdir (current_directory);
+  if (current_directory != NULL)
+    chdir (current_directory);
 
   if (a_tss.tss_irqn == 0x21)
     {

gdb/source.c

@@ -540,8 +540,7 @@ add_path (const char *dirname, char **which_path, int parse_separators)
 	new_name_holder.reset (concat (name, ".", (char *) NULL));
 #endif
       else if (!IS_ABSOLUTE_PATH (name) && name[0] != '$')
-	new_name_holder.reset (concat (current_directory, SLASH_STRING, name,
-				       (char *) NULL));
+	new_name_holder = gdb_abspath (name);
       else
 	new_name_holder.reset (savestring (name, p - name));
       name = new_name_holder.get ();

gdb/top.c

@@ -54,6 +54,7 @@
 #include "gdb_select.h"
 #include "gdbsupport/scope-exit.h"
 #include "gdbarch.h"
+#include "gdbsupport/pathstuff.h"
 
 /* readline include files.  */
 #include "readline/readline.h"
@@ -1994,12 +1995,13 @@ init_history (void)
          that was read.  */
 #ifdef __MSDOS__
       /* No leading dots in file names are allowed on MSDOS.  */
-      history_filename = concat (current_directory, "/_gdb_history",
-				 (char *)NULL);
+      const char *fname = "_gdb_history";
 #else
-      history_filename = concat (current_directory, "/.gdb_history",
-				 (char *)NULL);
+      const char *fname = ".gdb_history";
 #endif
+
+      gdb::unique_xmalloc_ptr<char> temp (gdb_abspath (fname));
+      history_filename = temp.release ();
     }
   read_history (history_filename);
 }
@@ -2077,8 +2079,12 @@ set_history_filename (const char *args,
      directories the file written will be the same as the one
      that was read.  */
   if (!IS_ABSOLUTE_PATH (history_filename))
-    history_filename = reconcat (history_filename, current_directory, "/", 
-				 history_filename, (char *) NULL);
+    {
+      gdb::unique_xmalloc_ptr<char> temp (gdb_abspath (history_filename));
+
+      xfree (history_filename);
+      history_filename = temp.release ();
+    }
 }
 
 static void

gdb/tracefile-tfile.c

@@ -32,6 +32,7 @@
 #include "xml-tdesc.h"
 #include "target-descriptions.h"
 #include "gdbsupport/buffer.h"
+#include "gdbsupport/pathstuff.h"
 #include <algorithm>
 
 #ifndef O_LARGEFILE
@@ -470,8 +471,7 @@ tfile_target_open (const char *arg, int from_tty)
 
   gdb::unique_xmalloc_ptr<char> filename (tilde_expand (arg));
   if (!IS_ABSOLUTE_PATH (filename.get ()))
-    filename.reset (concat (current_directory, "/", filename.get (),
-			    (char *) NULL));
+    filename = gdb_abspath (filename.get ());
 
   flags = O_BINARY | O_LARGEFILE;
   flags |= O_RDONLY;