GDB backtrace and signal trampolines

Hollis Blanchard hollisb at
Fri Aug 12 01:54:37 EST 2005

GDB 6.3 contains this code in ppc-linux-tdep.c:

static const struct frame_unwind *
ppc_linux_sigtramp_sniffer (struct frame_info *next_frame)
   struct gdbarch_tdep *tdep = gdbarch_tdep (get_frame_arch 

   if (frame_pc_unwind (next_frame)
       > frame_unwind_register_unsigned (next_frame, SP_REGNUM))
     /* Assume anything that is vaguely on the stack is a signal
        trampoline.  */
     return &ppc_linux_sigtramp_unwind;
     return NULL;

Essentially it says that any time the program counter is above the 
stack pointer, we must be in a signal trampoline, and so GDB proceeds 
to grope about for a struct rt_sigframe on the stack.

This is not a good assumption. I'm using a GDB stub to debug Xen, and 
as it so happens, the Xen stack is below the Xen text. That means that 
the above test always triggers, but of course there is no rt_sigframe 
on the stack, and my backtrace runs away.

Would it make sense to limit the test to within a few hundred bytes of 
the stack pointer? Or some better way to detect that the PC is in a 
signal trampoline?

(Also, how can I test backtraces within a signal trampoline? I've 
single-stepped my way into and out of a signal hander, and never saw 
the PC inside the stack.)

Hollis Blanchard
IBM Linux Technology Center

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