Hi, [...] > > Yeah, and probably not as important in the mips world, as it is used > more with embedded devices than desktops. We must always take the > "paranoid" approach for tracing. At least in PPC and x86, we assume > everything is broken ;-) And we want to be as robust as possible. If > something goes wrong, we want to detect it ASAP and report it. And keep > the system from crashing. > > At least with MIPS we don't need to worry about crashing Linus's > desktop. With is the #1 priority we have on x86 ... "Don't crash Linus's > desktop!". > > If Linus sees a warning, he'll bitch at us. If we crash his box, and he > was to lose any information, he'll strip out our code! > Okay, a new patch for all of the exception handling will go into -v7. > > > > > So, we just need to replace this: > > > > if ((code & MOV_FP_SP) == MOV_FP_SP) > > return parent_addr; > > > > by > > > > #define S_INSN (0xafb0 << 16) > > > > if ((code & S_INSN) != S_INSN) > > return parent_addr; > > I would be even more paranoid, and make sure each of those stores, store > into sp. get it :-) (I need to be more paranoid too, otherwise, Steven will not accept my patches!) > > > > > > > > > > + > > > > + sp = fp + (code & STACK_OFFSET_MASK); > > > > + ra = *(unsigned long *)sp; > > > > > > Also might want to make the above into a asm with exception handling. > > > > > > > + > > > > + if (ra == parent) > > > > + return sp; > > > > + > > > > + ftrace_graph_stop(); > > > > + WARN_ON(1); > > > > + return parent_addr; > > > > > > Hmm, may need to do more than this. See below. > > > [...] > > > > + > > > > + old = *parent; > > > > + > > > > + parent = (unsigned long *)ftrace_get_parent_addr(self_addr, old, > > > > + (unsigned long)parent, > > > > + fp); > > > > + > > > > + *parent = return_hooker; > > > > > > Although you may have turned off fgraph tracer in > > > ftrace_get_parent_addr, nothing stops the below from messing with the > > > stack. The return stack may get off sync and break later. If you fail > > > the above, you should not be calling the push function below. > > > > > > > We need to really stop before ftrace_push_return_trace to avoid messing > > with the stack :-) but if we have stopped the tracer, is it important to > > mess with the stack or not? > > The ftrace_push_return_trace does not test if the trace stopped, that is > expected to be done by the caller. If you mess with the stack set up, > you will crash the box. Remember, before the failure, you could have > already replaced return jumps. Those will still be falling back to the > return_to_handler. If you mess with the stack, but don't update the > return, the other returns will be out of sync and call the wrong return > address. > As you can see, after stopping the function graph tracer(here the function is non-leaf) with ftrace_graph_stop() in ftrace_get_parent_addr(), I return the old parent_addr, this is only the stack address in the stack space of ftrace_graph_caller, which means that, I never touch the real stack address of the non-leaf function, and it will not trap into the return_to_handler hooker 'Cause the non-leaf function will load it's own normal return address from it's own stack, and then just return back normally. -- This is another trick :-) Regards, Wu Zhangjin
