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Re: [PATCH v2 1/2] Debugging without a binary (regression)


On 04/13/2016 10:00 AM, Pedro Alves wrote:
On 04/12/2016 08:24 PM, Luis Machado wrote:
When we attempt to debug a process using GDBserver in standard remote mode
without a symbol file on GDB's end, we may run into an issue where GDB cuts
the connection attempt short due to an error. The error is caused by not
being able to open a symbol file, like so:

--

(gdb) set sysroot
(gdb) tar rem :2345
Remote debugging using :2345
/proc/23769/exe: Permission denied.
(gdb) i r
The program has no registers now.
(gdb)

It should've been like this:

(gdb) set sysroot
(gdb) tar rem :2345
Remote debugging using :2345
0xf7ddb2d0 in ?? ()

The warning output is missing here, right?


It is, because i did not update it after the patch. :-)

Should be fixed in v3.

(gdb) i r
eax            0x0  0
ecx            0x0  0
edx            0x0  0
ebx            0x0  0
esp            0xffffdfa0 0xffffdfa0
ebp            0x0  0x0
esi            0x0  0
edi            0x0  0
eip            0xf7ddb2d0 0xf7ddb2d0
eflags         0x200  [ IF ]
cs             0x33 51
ss             0x2b 43
ds             0x0  0
es             0x0  0
fs             0x0  0
gs             0x0  0
(gdb)


  void
@@ -142,6 +161,7 @@ exec_file_locate_attach (int pid, int from_tty)
  {
    char *exec_file, *full_exec_path = NULL;
    struct cleanup *old_chain;
+  struct gdb_exception prev_err;

    /* Do nothing if we already have an executable filename.  */
    exec_file = (char *) get_exec_file (0);
@@ -182,9 +202,46 @@ exec_file_locate_attach (int pid, int from_tty)

    old_chain = make_cleanup (xfree, full_exec_path);

-  exec_file_attach (full_exec_path, from_tty);
-  symbol_file_add_main (full_exec_path, from_tty);
+  /* exec_file_attach and symbol_file_add_main may throw an error if the file
+     cannot be opened either locally or remotely.
+
+     This happens for example, when the file is first found in the local
+     sysroot (above), and then disappears (a TOCTOU race), or when it doesn't
+     exist in the target filesystem, or when the file does exist, but
+     is not readable.
+
+     Even without a symbol file, the remote-based debugging session should
+     continue normally instead of ending abruptly.  Hence we catch thrown
+     errors/exceptions in the following code.  */
+  TRY
+    {
+      exec_file_attach (full_exec_path, from_tty);
+    }
+  CATCH (err, RETURN_MASK_ERROR)
+    {
+      if (err.message != NULL)
+	warning ("%s", err.message);
+
+      prev_err = err;
+
+      /* Save message so it doesn't get trashed by the catch problem

"the catch problem below" should be either the "the caught
problem below", or "the catch below", I think.


Yeah, i got things mixed up when coming up with it. "the catch below" is what i wanted to say.

+	 below.  */
+      prev_err.message = xstrdup (err.message);
+    }
+  END_CATCH
+
+  TRY
+    {
+      symbol_file_add_main (full_exec_path, from_tty);
+    }
+  CATCH (err, RETURN_MASK_ERROR)
+    {
+      if (!exception_print_same (prev_err, err))
+	warning ("%s", err.message);
+    }
+  END_CATCH

+  xfree ((void *) prev_err.message);

This will reference an uninitialized prev_err.message if the
first TRY doesn't throw.  Initialize it with:

  struct gdb_exception prev_err = exception_none;



Done.

Also, it will leak prev_err.message on Ctrl-C/QUIT, which is
not caught by RETURN_MASK_ERROR.  prev_err.message needs to be
freed with a cleanup.  Say:

   struct gdb_exception prev_err = exception_none;

   old_chain = make_cleanup (xfree, full_exec_path);

   make_cleanup (free_current_contents, &prev_err.message);

   TRY
     {
        ...
     }
   CATCH (err, RETURN_MASK_ERROR)
     {
       if (err.message != NULL)
	warning ("%s", err.message);

       prev_err = err;
       prev_err.message = xstrdup (err.message);
     }
    ...

   do_cleanups (old_chain);


Thanks. I've fixed these. I'll get v3 out once the testcase gets reviewed.

Thanks,
Pedro Alves



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