SIGTRAP during thr_new syscall

[Amol Dixit]

David,
I tried your patch, but it doesn't seem to fix the problem because even 
though _pthread_create() calls thr_new(), this system call is never made 
for the first thread created. Breakpoint set in kernel code of thr_new 
is hit only for the initial_thread (_libpthread_init())...skips the 
first thread...and then is hit again for 2nd thread onwards.
This patch may be necessary, but it doesn't fix my issue of SIGTRAP on 
thread_start().
Any clue why the first thread created using pthread_create() doesn't 
enter thr_new() syscall inside the kernel at all?
I get the same SIGTRAP if I single-step through this thr_new() line in 
_pthread_create() using _local_ gdb.
I am running freebsd6.0 configured for SYSTEM_SCOPE threading (1:1).
Thanks,
Amol

David Xu wrote:
> Amol Dixit wrote:
>> Hi,
>> I am seeing an unexpected SIGTRAP being reported to gdbserver when 
>> the debugged process creates a new thread via the _pthread_create() 
>> call of libthr library.  [libthr/thread/thr_create.c,v 1.22.4.1, 
>> Freebsd 6.0]
>> Gdbserver has internally set a breakpoint on address of 
>> _thread_bp_create() so that it gets notified on thread creation and 
>> is expecting a SIGTRAP at address (stop pc) of _thread_bp_create(). 
>> But instead SIGTRAP happens as a side-effect of thr_new() system call 
>> and the stop pc at that point is that of routine thread_start() which 
>> is the starting function of new thread. So gdbserver cannot match 
>> expected breakpoint (ie. _thread_bp_create) and is confused.
>> For testing purpose, if I call _thread_bp_create() before thr_new() 
>> in _pthread_create(), I get the _expected_ SIGTRAP with address of 
>> _thread_bp_create. But that is not the fix.
>> Does anyone have any idea about this SIGTRAP being reported to 
>> tracing process gdbserver as part of thr_new? Where is it originating 
>> from and why?
>> Thanks,
>> Amol
>>
>
> I found kernel clears trap flag for new process but not for new thread
> in cpu_fork(), you may try following patch:
>
> Index: i386/i386/vm_machdep.c
> ===================================================================
> --- i386/i386/vm_machdep.c    (revision 183337)
> +++ i386/i386/vm_machdep.c    (working copy)
> @@ -413,6 +413,15 @@
>      bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
>
>      /*
> +     * If the current thread has the trap bit set (i.e. a debugger had
> +     * single stepped the process to the system call), we need to clear
> +     * the trap flag from the new frame. Otherwise, the new thread will
> +     * receive a (likely unexpected) SIGTRAP when it executes the first
> +     * instruction after returning to userland.
> +     */
> +    td->td_frame->tf_eflags &= ~PSL_T;
> +
> +    /*
>       * Set registers for trampoline to user mode.  Leave space for the
>       * return address on stack.  These are the kernel mode register 
> values.
>       */