Re: [RFA] Don't unwind past entry point

Andrew Cagney
2004-11-10T17:03:25Z

Randolph Chung wrote:
> Here's a proposed patch to fix the problem i mentioned in
> http://sources.redhat.com/ml/gdb/2004-11/msg00020.html
>
> Briefly, if a compiler does a tail-call optimization to main, a function
> called from main will return directly to the caller of main, and we
> never terminate a backtrace.
>
> It doesn't address Joel's problem with threads though.

Check eli for the doco.

The code change is ok (while your though though, can you do me a favour
and move inside_entry_func to frame.c and make it static?).

This is missing a testcase but for that I'll need to take a raincheck -
we've almost but not quite got a setbacktrace testcase and it should be
extended to include this. Can you watch for that.

Andrew

> randolph
>
>
> 2004-11-09 Randolph Chung <tausq@debian.org>
>
> * frame.c (backtrace_past_entry): New flag.
> (get_prev_frame): Stop backtrace at the entry function if enabled
> by flag. Update comments.
> (_initialize_frame): Add command to set backtrace_past_entry flag.
>
> doc/
> * gdb.texinfo: Document set/show backtrace past-entry commands.
> Rearrange index entries for set/show backtrace past-main.
>
> Index: frame.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/frame.c,v
> retrieving revision 1.192
> diff -u -p -r1.192 frame.c
> --- frame.c 29 Oct 2004 20:23:06 -0000 1.192
> +++ frame.c 9 Nov 2004 19:21:38 -0000
> @@ -115,6 +115,7 @@ static int frame_debug;
> /* Flag to indicate whether backtraces should stop at main et.al. */
>
> static int backtrace_past_main;
> +static int backtrace_past_entry;
> static unsigned int backtrace_limit = UINT_MAX;
>
> static void
> @@ -1213,8 +1214,6 @@ get_prev_frame (struct frame_info *this_
> dummy frame PCs typically land in the entry func. Don't apply
> this test to the sentinel frame. Sentinel frames should always
> be allowed to unwind. */
> - /* NOTE: cagney/2003-02-25: Don't enable until someone has found
> - hard evidence that this is needed. */
> /* NOTE: cagney/2003-07-07: Fixed a bug in inside_main_func() -
> wasn't checking for "main" in the minimal symbols. With that
> fixed asm-source tests now stop in "main" instead of halting the
> @@ -1227,13 +1226,12 @@ get_prev_frame (struct frame_info *this_
> I guess) to determine the address range of the start function.
> That should provide a far better stopper than the current
> heuristics. */
> - /* NOTE: cagney/2003-07-15: Need to add a "set backtrace
> - beyond-entry-func" command so that this can be selectively
> - disabled. */
> - if (0
> -#if 0
> - && backtrace_beyond_entry_func
> -#endif
> + /* NOTE: tausq/2004-10-09: this is needed if, for example, the compiler
> + applied tail-call optimizations to main so that a function called
> + from main returns directly to the caller of main. Since we don't
> + stop at main, we should at least stop at the entry point of the
> + application. */
> + if (!backtrace_past_entry
> && this_frame->unwind->type != DUMMY_FRAME && this_frame->level >= 0
> && inside_entry_func (this_frame))
> {
> @@ -1531,6 +1529,17 @@ Whether backtraces should continue past
> NULL, NULL, &set_backtrace_cmdlist,
> &show_backtrace_cmdlist);
>
> + add_setshow_boolean_cmd ("past-entry", class_obscure,
> + &backtrace_past_entry, "\
> +Set whether backtraces should continue past the entry point of a program.", "\
> +Show whether backtraces should continue past the entry point of a program.", "\
> +Normally there are no callers beyond the entry point of a program, so GDB\n\
> +will terminate the backtrace there. Set this variable if you need to see \n\
> +the rest of the stack trace.", "\
> +Whether backtraces should continue past the entry point is %s.",
> + NULL, NULL, &set_backtrace_cmdlist,
> + &show_backtrace_cmdlist);
> +
> add_setshow_uinteger_cmd ("limit", class_obscure,
> &backtrace_limit, "\
> Set an upper bound on the number of backtrace levels.", "\
> Index: doc/gdb.texinfo
> ===================================================================
> RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
> retrieving revision 1.223
> diff -u -p -r1.223 gdb.texinfo
> --- doc/gdb.texinfo 8 Nov 2004 17:25:49 -0000 1.223
> +++ doc/gdb.texinfo 9 Nov 2004 19:21:47 -0000
> @@ -4109,7 +4109,7 @@ in a backtrace, you can change this beha
> @table @code
> @item set backtrace past-main
> @itemx set backtrace past-main on
> -@kindex set backtrace
> +@kindex set backtrace past-main
> Backtraces will continue past the user entry point.
>
> @item set backtrace past-main off
> @@ -4117,9 +4117,24 @@ Backtraces will stop when they encounter
> default.
>
> @item show backtrace past-main
> -@kindex show backtrace
> +@kindex show backtrace past-main
> Display the current user entry point backtrace policy.
>
> +@item set backtrace past-entry
> +@itemx set backtrace past-entry on
> +@kindex set backtrace past-entry
> +Backtraces will continue past the internal entry point of an application.
> +This entry point is encoded by the linker when the application is built,
> +and is likely before the user entry point ``main'' (or equivalent) is called.
> +
> +@item set backtrace past-entry off
> +Backtraces will stop when they encouter the internal entry point of an
> +application. This is the default.
> +
> +@item show backtrace past-entry
> +@kindex show backtrace past-entry
> +Display the current internal entry point backtrace policy.
> +
> @item set backtrace limit @var{n}
> @itemx set backtrace limit 0
> @cindex backtrace limit
>