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

[Luis Machado <lgustavo@codesourcery.com>]

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 ?? ()
(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)

This is caused by a couple of function calls within exec_file_locate_attach
that can potentially throw errors.

The following patch guards both exec_file_attach and symbol_file_add_main to
prevent the errors from disrupting the connection process.

There was also a case where native GDB tripped on this problem, but it was
mostly fixed by bf74e428bca61022bd5cdf6bf28789a184748b4d.

Regression-tested on x86-64/Ubuntu.

Changes in v2:

- We are using two TRY/CATCH blocks to guard both exec_file_attach and
symbol_file_add_main.
- Adjusted code to display only a single warning message if both TRY/CATCH
blocks throw with the same error message.
- I had to duplicate the exception message string because the second CATCH
block will overwrite the contents of the pointer we saved.

gdb/ChangeLog:

2016-04-12  Luis Machado  <lgustavo@codesourcery.com>

	* exec.c (exec_file_locate_attach): Guard a couple functions
	that can throw errors.
	(exception_print_same): New helper function.
---
 gdb/exec.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 59 insertions(+), 2 deletions(-)

diff --git a/gdb/exec.c b/gdb/exec.c
index a10ab9b..051a585 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -135,6 +135,25 @@ exec_file_clear (int from_tty)
     printf_unfiltered (_("No executable file now.\n"));
 }
 
+/* Returns non-zero if exceptions E1 and E2 are equal.  Returns zero
+   otherwise.  */
+
+static int
+exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
+{
+  const char *msg1 = e1.message;
+  const char *msg2 = e2.message;
+
+  if (msg1 == NULL)
+    msg1 = "";
+  if (msg2 == NULL)
+    msg2 = "";
+
+  return (e1.reason == e2.reason
+	  && e1.error == e2.error
+	  && strcmp (e1.message, e2.message) == 0);
+}
+
 /* See gdbcore.h.  */
 
 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
+	 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);
   do_cleanups (old_chain);
 }
 
-- 
1.9.1