[PATCH] powerpc/64: Fix stacktrace on BE when function_graph is enabled

[Naveen N. Rao]

Michael Ellerman wrote:
> Currently if we oops or warn while function_graph is active the stack
> trace looks like:
>   .trace_graph_return+0xac/0x100
>   .ftrace_return_to_handler+0x98/0x140
>   .return_to_handler+0x20/0x40
>   .return_to_handler+0x0/0x40
>   .return_to_handler+0x0/0x40
>   .return_to_handler+0x0/0x40
>   .return_to_handler+0x0/0x40
>   .return_to_handler+0x0/0x40
>   .return_to_handler+0x0/0x40
>   .cpu_startup_entry+0x34/0x40
>   .start_secondary+0x680/0x6f0
>   start_secondary_prolog+0x10/0x14
> 
> Notice the multiple entries that just show .return_to_handler.
> 
> There is logic in show_stack() to detect this case and print the
> traced function, but we inadvertently broke it in commit
> 7d56c65a6ff9 ("powerpc/ftrace: Remove mod_return_to_handler") (2014),
> because that commit accidentally removed the dereference of rth which
> gets the text address from the function descriptor. Hence this is only
> broken on big endian (or technically ELFv1).
> 
> Fix it by using the proper accessor, which is ppc_function_entry().
> Result is we get a stack trace such as:
> 
>   .trace_graph_return+0x134/0x160
>   .ftrace_return_to_handler+0x94/0x140
>   .return_to_handler+0x20/0x40
>   .return_to_handler+0x0/0x40 (.shared_cede_loop+0x48/0x130)
>   .return_to_handler+0x0/0x40 (.cpuidle_enter_state+0xa0/0x690)
>   .return_to_handler+0x0/0x40 (.cpuidle_enter+0x44/0x70)
>   .return_to_handler+0x0/0x40 (.call_cpuidle+0x68/0xc0)
>   .return_to_handler+0x0/0x40 (.do_idle+0x37c/0x400)
>   .return_to_handler+0x0/0x40 (.cpu_startup_entry+0x30/0x50)
>   .rest_init+0x224/0x348
> 
> Fixes: 7d56c65a6ff9 ("powerpc/ftrace: Remove mod_return_to_handler")
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
>  arch/powerpc/kernel/process.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 8fc4de0d22b4..1601d7cfe45e 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -2048,7 +2048,7 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	struct ftrace_ret_stack *ret_stack;
>  	extern void return_to_handler(void);
> -	unsigned long rth = (unsigned long)return_to_handler;
> +	unsigned long rth = ppc_function_entry(return_to_handler);

Thanks! This looks good to me. A small suggestion though -- can we use 
dereference_kernel_function_descriptor() instead? It will be a nop for 
ABIv2, which would be nice, but not really a major deal.

In either case:
Reviewed-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>


- Naveen