Re: [RFC PATCH 0/3] Pretty-printing for errno

Zack Weinberg
2017-09-06T13:05:27Z

On Tue, Sep 5, 2017 at 6:31 PM, Pedro Alves <palves@redhat.com> wrote:
> On 09/05/2017 10:15 PM, Zack Weinberg wrote:
>>
>> I don't understand why thread-local variables are inaccessible on my
>> perfectly ordinary x86_64-unknown-linux-gnu workstation (the base OS
>> is Debian 'stretch'). Do you have any idea what might be wrong?
>
> I assume your test program isn't actually linked with -pthread?

That is correct.

> When you do "print errno" in this situation, because there's no
> "#define errno ..." in sight, gdb ends up finding the real "errno" symbol,
> which, even though the program isn't threaded, is a TLS symbol, and as such has
> a DWARF location expression describing its location as an offset into the
> thread-local block for the current thread. GDB needs to resolve that address, and
> for threaded programs that is normally done with assistance from libthread_db.so.
> The problem is then that libthread_db.so only works with programs that
> link with libpthread.so, and if your test program is actually non-threaded,
> it doesn't link with libpthread.so.

I am not familiar with the glibc-side TLS implementation, nor with
libthread_db.so, nor the code in GDB that uses libthread_db.so.
However, reading the implementation of td_thr_tls_get_addr leads me to
believe that that function is *supposed* to work even if libpthread.so
has not been loaded into the 'inferior'. If it doesn't, perhaps that
is a bug on our side. Do you know if GDB even tries? It's not obvious
to me looking at linux-thread-db.c.

> A workaround specifically for errno, and only for live-process debugging [*]
> is the "macro define" trick I had suggested before:
>
> (gdb) macro define errno (*__errno_location ())
>
> After that, "p errno" ends up calling __errno_location just
> like when you compile the test program with -g3.

Again, is it possible to do (the equivalent of) this from the
initialization code of a pretty-printer module? Specifically, there
already exists a Python function that's doing this:

def register(objfile):
"""Register pretty printers for the current objfile."""

printer = gdb.printing.RegexpCollectionPrettyPrinter("glibc-errno")
printer.add_printer('error_t', r'^(?:__)?error_t', ErrnoPrinter)

if objfile == None:
objfile = gdb

gdb.printing.register_pretty_printer(objfile, printer)

called when the module is loaded; what would I need to add to that so
that the macro is defined (if it isn't already)?

zw