SIGTRAP during thr_new syscall

[Kostik Belousov]

On Thu, Sep 25, 2008 at 11:45:20AM +0800, 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.
>  	 */

Should this be done for amd64 version of the cpu_set_upcall() as well ?
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