Re: frame.c backtrace limit issue and fix

Daniel Jacobowitz
2005-05-28T22:51:44Z

On Mon, May 23, 2005 at 07:19:04PM +0100, Jonathan Larmour wrote:
> On my embedded m68k target, where debugging appears problematic, sometimes
> frames called in frame.c:get_prev_frame() have their frame level set to
> -1. If you have a backtrace limit set (with "set backtrace limit") then
> this would mean the following test is done:
>
>
> if (this_frame->level > backtrace_limit)
> {
> error ("Backtrace limit of %d exceeded", backtrace_limit);
> }
>
> This looks like it would be right even if the level is -1. However
> backtrace_limit is unsigned and this results in this_frame->level being
> promoted to an unsigned 0xffffffff value.
>
> Therefore I'm attaching a patch to correct this, simply by making
> backtrace_limit now be signed. Anyone who needs more than 2 billion stack
> levels has bigger problems :-).
>
> I do have write access so can check this in if someone approves.

You seem to be submitting patches against an old version; there already
is a function named add_setshow_integer_cmd. Andrew added it in
February. Also, someone wrote a testcase for this problem, but I think
it's still in my queue somewhere... ah, no, it's on Paul's plate at the
moment.

So only about half this patch is needed. I took that, stuck a properly
formatted ChangeLog and PR number on it, and tested it.
Unfortunately... it didn't work. The errors went away, but backtraces
didn't stop in the right place.

(gdb) bt
#0 factorial (value=4) at /big/fsf/commit/src/gdb/testsuite/gdb.base/break.c:111
#1 0x080484e3 in factorial (value=5) at /big/fsf/commit/src/gdb/testsuite/gdb.base/break.c:112
#2 0x080484e3 in factorial (value=6) at /big/fsf/commit/src/gdb/testsuite/gdb.base/break.c:112
Backtrace limit of 1 exceeded

Oops, that's not a backtrace limit of 1, that's a backtrace limit of
3... the documentation suggests that "bt 3" and "set backtrace limit 3"
should behave similarly in this regard.

I also don't think that the error() is appropriate. "set backtrace
past-main" doesn't give an error at main.

With all those fixed, I get the attached patch, which I've tested,
regression tested, and checked in. Thanks.

--
Daniel Jacobowitz
CodeSourcery, LLC

2005-05-28 Daniel Jacobowitz <dan@codesourcery.com>
Jonathan Larmour <jifl@eCosCentric.com>

PR backtrace/1760
* frame.c (backtrace_limit): Change type to int.
(get_prev_frame): Update backtrace limit support.
(_initialize_frame): Use add_setshow_integer_cmd for backtrace_limit.

Index: frame.c
===================================================================
RCS file: /cvs/src/src/gdb/frame.c,v
retrieving revision 1.209
diff -u -p -r1.209 frame.c
--- frame.c 22 May 2005 14:53:33 -0000 1.209
+++ frame.c 28 May 2005 22:41:17 -0000
@@ -141,7 +141,7 @@ Whether backtraces should continue past
value);
}

-static unsigned int backtrace_limit = UINT_MAX;
+static int backtrace_limit = INT_MAX;
static void
show_backtrace_limit (struct ui_file *file, int from_tty,
struct cmd_list_element *c, const char *value)
@@ -1258,9 +1258,16 @@ get_prev_frame (struct frame_info *this_
return NULL;
}

- if (this_frame->level > backtrace_limit)
+ /* If the user's backtrace limit has been exceeded, stop. We must
+ add two to the current level; one of those accounts for backtrace_limit
+ being 1-based and the level being 0-based, and the other accounts for
+ the level of the new frame instead of the level of the current
+ frame. */
+ if (this_frame->level + 2 > backtrace_limit)
{
- error (_("Backtrace limit of %d exceeded"), backtrace_limit);
+ frame_debug_got_null_frame (gdb_stdlog, this_frame,
+ "backtrace limit exceeded");
+ return NULL;
}

/* If we're already inside the entry function for the main objfile,
@@ -1610,16 +1617,16 @@ the rest of the stack trace."),
&set_backtrace_cmdlist,
&show_backtrace_cmdlist);

- add_setshow_uinteger_cmd ("limit", class_obscure,
- &backtrace_limit, _("\
+ add_setshow_integer_cmd ("limit", class_obscure,
+ &backtrace_limit, _("\
Set an upper bound on the number of backtrace levels."), _("\
Show the upper bound on the number of backtrace levels."), _("\
No more than the specified number of frames can be displayed or examined.\n\
Zero is unlimited."),
- NULL,
- show_backtrace_limit,
- &set_backtrace_cmdlist,
- &show_backtrace_cmdlist);
+ NULL,
+ show_backtrace_limit,
+ &set_backtrace_cmdlist,
+ &show_backtrace_cmdlist);

/* Debug this files internals. */
add_setshow_zinteger_cmd ("frame", class_maintenance, &frame_debug, _("\