weird errors when notifying gdb of linkmap changes

weird errors when notifying gdb of linkmap changes

[Mathieu Lacage]

hi,

I get the following errors:
Starting program: /home/mathieu/code/elf-loader/ls
[Thread debugging using libthread_db enabled]
Error while reading shared library symbols:
Cannot find new threads: generic error
Cannot find new threads: generic error
(gdb) bt
#0  0xb7f228d1 in _r_debug_state () at gdb.c:10
#1  0xb7f22964 in gdb_notify () at gdb.c:36
#2  0xb7f1d7fd in stage2 (args=
      {interpreter_load_base = 3086077952, program_phdr = 0x8048034,
program_phnum = 8, sysinfo = 3086074880, program_argc = 1,
program_argv = 0xbfa13ff4, program_envp = 0xbfa13ffc}) at ldso.c:253
#3  0xb7f1d9b0 in stage1 () at ldso.c:312
#4  0x00000001 in ?? ()

I wonder if there is an obvious well-known reason for this.

Mathieu
-- 
Mathieu Lacage <mathieu.lacage@gmail.com>

Re: weird errors when notifying gdb of linkmap changes

[Daniel Jacobowitz]

On Mon, Jan 12, 2009 at 08:20:01PM +0100, Mathieu Lacage wrote:
> hi,
> 
> I get the following errors:
> Starting program: /home/mathieu/code/elf-loader/ls
> [Thread debugging using libthread_db enabled]
> Error while reading shared library symbols:
> Cannot find new threads: generic error
> Cannot find new threads: generic error
> (gdb) bt
> #0  0xb7f228d1 in _r_debug_state () at gdb.c:10
> #1  0xb7f22964 in gdb_notify () at gdb.c:36
> #2  0xb7f1d7fd in stage2 (args=
>       {interpreter_load_base = 3086077952, program_phdr = 0x8048034,
> program_phnum = 8, sysinfo = 3086074880, program_argc = 1,
> program_argv = 0xbfa13ff4, program_envp = 0xbfa13ffc}) at ldso.c:253
> #3  0xb7f1d9b0 in stage1 () at ldso.c:312
> #4  0x00000001 in ?? ()
> 
> I wonder if there is an obvious well-known reason for this.

You'll have to build a debuggable libthread_db to figure out what it's
complaining about.  Most likely you have told it that the thread
library is loaded, but provided some incorrect information that led to
GDB not finding the thread library's data structures.

-- 
Daniel Jacobowitz
CodeSourcery

Re: weird errors when notifying gdb of linkmap changes

[Mathieu Lacage]

On Mon, Jan 12, 2009 at 8:28 PM, Daniel Jacobowitz <drow@false.org> wrote:

>> Starting program: /home/mathieu/code/elf-loader/ls
>> [Thread debugging using libthread_db enabled]
>> Error while reading shared library symbols:
>> Cannot find new threads: generic error
>> Cannot find new threads: generic error
>> (gdb) bt
>> #0  0xb7f228d1 in _r_debug_state () at gdb.c:10
>> #1  0xb7f22964 in gdb_notify () at gdb.c:36
>> #2  0xb7f1d7fd in stage2 (args=
>>       {interpreter_load_base = 3086077952, program_phdr = 0x8048034,
>> program_phnum = 8, sysinfo = 3086074880, program_argc = 1,
>> program_argv = 0xbfa13ff4, program_envp = 0xbfa13ffc}) at ldso.c:253
>> #3  0xb7f1d9b0 in stage1 () at ldso.c:312
>> #4  0x00000001 in ?? ()
>>
>> I wonder if there is an obvious well-known reason for this.
>
> You'll have to build a debuggable libthread_db to figure out what it's
> complaining about.  Most likely you have told it that the thread
> library is loaded, but provided some incorrect information that led to
> GDB not finding the thread library's data structures.

Ok. I looked a bit a this. What is happening is that I invoke
_r_debug_state _before_ calling the ELF initializers so,
__pthread_initialize_minimal has not been invoked yet so the data
structure for the main thread is not yet initialized so, nptl_db is
confused.

The bug https://bugzilla.redhat.com/show_bug.cgi?id=179072#c14 pointed
out by Jan sheds some light on a similar problem and points out that,
theoretically, the content of the libthread data structures is
parseable by libpthread_db _before_ _pthread_initialize_minimal is
called which would lead me to believe that I have screwed up something
in the memory layout of libpthread or something similar.

To summarize, I should be able to still call _r_debug_state _before_
calling __pthread_initialize_minimal but I have probably screwed up
something else.

bah, more debugging.
Mathieu
-- 
Mathieu Lacage <mathieu.lacage@gmail.com>