Re: tcbhead_t gdb access for nonthreaded, gdb for longjmp()

[Jan Kratochvil <jan.kratochvil@redhat.com>]

Hi Daniel,

On Sun, 10 Sep 2006 16:27:24 +0200, Daniel Jacobowitz wrote:
> On Fri, Sep 08, 2006 at 12:22:35PM +0200, Jan Kratochvil wrote:
> > glibc part:
> > 
> >  * Provide some access to the `tcbhead_t.pointer_guard' field for gdb.
> >    Currently implemented by `td_thr_getxregs' providing only `pointer_guard'.
> >    New non-Solaris `td_thr_*' function could be provided instead.
> > 
> >  * All the `libthread_db' functions accessing inferior's `_thread_db*' symbols
> >    of `libpthread' fallback to the new `_local_db*' symbols in `libthread_db'
> >    itself. `libthread_db'<=>`libpthread' versions must match anyway.
> >    I admit I do not know how may `libthread_db' and `libpthread' as there is
> >    already required in `td_ta_new' their versions match.  Anyway it should be
> >    enough for 99% of cases - as the fallback option.
> 
> Your new libthread_db will accept any version of glibc, even one which
> does not match - that seems like a good way to get in a lot of trouble.

Is it fine to make `__libc_version' public?
`libthread_db' would check for it if running in `TD_MINIMALONLY' mode.


> I wonder if we really need to use libthread_db here anyway.

Besides longjmp() target PTR_DEMANGLE()ing there is also need to access glibc
TLS symbol `errno' but - in fact - AFAIK no other TLS symbol.
One may say there is no TLS `errno' for nonthreaded programs as they use only
	(*__errno_location ())

But from the user perspective I consider not acceptable to say something like
	No symbol "errno" in current context.

Also one cannot expect everyone will compile everything with bloated -ggdb3 to
automatically expand `errno' to `(*__errno_location ())'.

I see multiple solutions (maybe first one enough?).

 * Hardcoding `#define errno (*__errno_location ())' into gdb.

 * Providing full custom TLS resolving for gdb - no glibc change needed.

 * Extending current glibc libthread_db for non-threaded inferiors.
   (the patch sent before <20060908102235.GA31335@host0.dyn.jankratochvil.net>)

 * Merging the basic libpthread part for TLS resolving into glibc core
   as nonthreaded glibc is using the TLS support for threading anyway.

 * Merging the whole libpthread to glibc, making libpthread just a stub,
   forgetting there ever existed nonthreaded programs before,
   the same way UP (vs. SMP) was forgotten.

...
> a function in glibc which we could call that would return the target
> of a jmp_buf.  Then GDB wouldn't have to know how PTR_MANGLE worked.

Nice idea.


Regards,
Jan