Re: [Valgrind-users] Re: [uml-devel] Re: UML and valgrind

[Tom Hughes <thh@cyberscience.com>]

In message <200408031931.i73JVkvv003367@ccure.user-mode-linux.org>
        Jeff Dike <jdike@addtoit.com> wrote:

> dbahi@enterasys.com said:
>> ugh, so close - it bails - stopped by clone() !?!!?? : 
>
> OK, there were a bunch of problems that were fixed when me, Jeremy,
> and Julian were working on this.  The clone one seems to have not
> made it.  I've lost the patches I had, but I dug this out of a piece
> of email.  It applies to coregrind/vg_syscalls.c:

Clone is not (at least in general) supportable in valgrind without
a reasonably large amount of work.

>> @@ -39,6 +40,10 @@
>>  # code which copies from baseBlock before the call, into
>>  # m_state_static, and back afterwards.
>>  
>> +.section .data
>> +save_ip:
>> +        .long   0
>> +
>>  VG_(do_syscall):
>>         # Save all the int registers of the real machines state on the
>>         # simulators stack.
>> @@ -80,10 +85,27 @@
>>         movl    VG_(m_state_static)+48, %esi
>>         movl    VG_(m_state_static)+52, %edi
>>  
>> +       cmpl    $__NR_clone, %eax
>> +       jne     not_clone
>> +
>> +       pushl   %eax
>> +       movl    VG_(m_state_static)+60, %eax
>> +       movl    %eax, save_ip
>> +       popl    %eax
>> +
>> +       int     $0x80
>> +
>> +       cmpl    $0, %eax
>> +       jne     parent_finish
>> +
>> +       jmp     *save_ip
>> +
>> +not_clone:
>>         # esp now refers to the simulatees stack
>>         # Do the actual system call
>>         int     $0x80

I don't see how a patch that small can have made clone work for you in
any version - at the very least you would need a change to vg_syscall.c
to stop it trapping the clone and complaining about it or you would
never actually reach VG_(do_syscall).

> It handles the clone by calling clone itself, creating a new
> valgrind thread which will go on grinding the new UML thread.

But how did you cope with the fact that valgrind doesn't protect it's
internal data structures in any way? You would have all sorts of
problems with two threads trying to access the same data.

Or are you not specifying CLONE_VM among the flags? it is it more like
a fork than a thread creation? That valgrind may be able to handle
quite easily.

In fact, what is the exact set of flags you're using to clone?

Tom

-- 
Tom Hughes (thh@cyberscience.com)
Software Engineer, Cyberscience Corporation
http://www.cyberscience.com/


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