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Re: [PATCH v2 1/2] Debugging without a binary (regression)
- From: Pedro Alves <palves at redhat dot com>
- To: Luis Machado <lgustavo at codesourcery dot com>, gdb-patches at sourceware dot org
- Date: Wed, 13 Apr 2016 16:00:21 +0100
- Subject: Re: [PATCH v2 1/2] Debugging without a binary (regression)
- Authentication-results: sourceware.org; auth=none
- References: <1460489060-17307-1-git-send-email-lgustavo at codesourcery dot com> <1460489060-17307-2-git-send-email-lgustavo at codesourcery dot com>
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?
> (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.
> + 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;
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,
Pedro Alves