SIGTRAP during thr_new syscall
davidxu at freebsd.org
Thu Sep 25 03:43:38 UTC 2008
Amol Dixit wrote:
> 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 188.8.131.52, 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?
I found kernel clears trap flag for new process but not for new thread
in cpu_fork(), you may try following patch:
--- 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.
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