On Tue, 2 Aug 2016 16:00:11 -0500
Josh Poimboeuf <jpoim...@redhat.com> wrote:
[<ffffffff81061d8b>] nmi_raise_cpu_backtrace+0x1b/0x20
>
> The ret_stack is out of sync with the stack dump because the stack dump
> was started with the regs from the NMI, instead of being started from
> the current frame.
>
> So I guess there are a couple of ways to fix it:
>
> a) keep track of the return address pointer like we discussed above;
>
> or
>
> b) have the unwinder count the # of skipped frames which refer to
> 'return_to_handler', and pass that as the initial index value to
> ftrace_graph_ret_addr().
>
> Option a) would be much cleaner. But to fix it for both mcount and
> fentry, we couldn't override 'fp' so I guess we'd need to add a new
> field to ftrace_ret_stack.
Actually, what about calling ftrace_graph_ret_addr() to figure out the
next stack conversion only if reliable or CONFIG_FRAME_POINTER is not
enabled?
unsigned long real_addr = addr;
[...]
if (!IS_ENABLED(CONFIG_FRAME_POINTER) || reliable)
real_addr = ftrace_graph_ret_addr(task, graph, addr);
if (addr != real_addr)
ops->address(data, addr, 0);
ops->address(data, real_addr, reliable);
Then we only need the fp use case when FRAME_POINTER is not set. As
mcount forces FRAME_POINTER, we only need to worry about the fentry
case.
-- Steve
>
> Option b) is uglier, but I could probably make it work with the new
> unwinder.
>
