Re: [Qemu-devel] [PATCH 1/4] scripts/qemugdb: get pthread_self from "info threads" command

From: Paolo Bonzini <pbonzini@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	qemu-devel@nongnu.org
Cc: stefanha@redhat.com, den@openvz.org
Subject: Re: [Qemu-devel] [PATCH 1/4] scripts/qemugdb: get pthread_self from "info threads" command
Date: Wed, 4 Apr 2018 11:52:03 +0200	[thread overview]
Message-ID: <749bccbe-614a-143b-6520-d6e3f82b9da6@redhat.com> (raw)
In-Reply-To: <20180328173238.507470-2-vsementsov@virtuozzo.com>

On 28/03/2018 19:32, Vladimir Sementsov-Ogievskiy wrote:
> When debugging a coredump, pthread_self can't be obtained from
> function arch_prctl. Moreover if qemu crashed in coroutine, we
> can't find 'start_thread' in current stack-trace. So, add a method,
> actually proposed in 1138f24645e9e, which should work for gdb
> version >= 7.3.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

New-enough gdb can also use gdb.selected_thread()
> ---
>  scripts/qemugdb/coroutine.py | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/qemugdb/coroutine.py b/scripts/qemugdb/coroutine.py
> index ab699794ab..ffaa45c464 100644
> --- a/scripts/qemugdb/coroutine.py
> +++ b/scripts/qemugdb/coroutine.py
> @@ -14,6 +14,7 @@
>  # GNU GPL, version 2 or (at your option) any later version.
>  
>  import gdb
> +import re
>  
>  VOID_PTR = gdb.lookup_type('void').pointer()
>  
> @@ -28,7 +29,17 @@ def get_fs_base():
>      return fs_base
>  
>  def pthread_self():
> -    '''Fetch pthread_self() from the glibc start_thread function.'''
> +    # Try read pthread_self from gdb command 'info threads'.
> +    # Will fail for old gdb.
> +    try:
> +        threads = gdb.execute('info threads', False, True)
> +        m = re.search('^\* 1    Thread (0x[0-9a-f]+)', threads, re.MULTILINE)

I don't think hard-coding "1" works here, and the spacing of the table
might differ as well.  However, looking for the asterisk seems safe from
a quick look at gdb source, and "Thread" looks like it isn't localized.

Paolo

> +        return int(m.group(1), 16)
> +    except TypeError:
> +        # gdb doesn't support third parameter for execute
> +        pass
> +
> +    # Try fetch pthread_self() from the glibc start_thread function.
>      f = gdb.newest_frame()
>      while f.name() != 'start_thread':
>          f = f.older()
> 