On 3/23/21 5:51 AM, Mark Rutland wrote:
> On Mon, Mar 15, 2021 at 11:57:57AM -0500, madvenka@xxxxxxxxxxxxxxxxxxx wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx>
>>
>> When CONFIG_DYNAMIC_FTRACE_WITH_REGS is enabled and tracing is activated
>> for a function, the ftrace infrastructure is called for the function at
>> the very beginning. Ftrace creates two frames:
>>
>> - One for the traced function
>>
>> - One for the caller of the traced function
>>
>> That gives a reliable stack trace while executing in the ftrace
>> infrastructure code. When ftrace returns to the traced function, the frames
>> are popped and everything is back to normal.
>>
>> However, in cases like live patch, execution is redirected to a different
>> function when ftrace returns. A stack trace taken while still in the ftrace
>> infrastructure code will not show the target function. The target function
>> is the real function that we want to track.
>>
>> So, if an FTRACE frame is detected on the stack, just mark the stack trace
>> as unreliable.
>
> To identify this case, please identify the ftrace trampolines instead,
> e.g. ftrace_regs_caller, return_to_handler.
>
Yes. As part of the return address checking, I will check this. IIUC, I think that
I need to check for the inner labels that are defined at the point where the
instructions are patched for ftrace. E.g., ftrace_call and ftrace_graph_call.
SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
bl ftrace_stub <====================================
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
SYM_INNER_LABEL(ftrace_graph_call, SYM_L_GLOBAL) // ftrace_graph_caller();
nop <======= // If enabled, this will be replaced
// "b ftrace_graph_caller"
#endif
For instance, the stack trace I got while tracing do_mmap() with the stack trace
tracer looks like this:
...
[ 338.911793] trace_function+0xc4/0x160
[ 338.911801] function_stack_trace_call+0xac/0x130
[ 338.911807] ftrace_graph_call+0x0/0x4
[ 338.911813] do_mmap+0x8/0x598
[ 338.911820] vm_mmap_pgoff+0xf4/0x188
[ 338.911826] ksys_mmap_pgoff+0x1d8/0x220
[ 338.911832] __arm64_sys_mmap+0x38/0x50
[ 338.911839] el0_svc_common.constprop.0+0x70/0x1a8
[ 338.911846] do_el0_svc+0x2c/0x98
[ 338.911851] el0_svc+0x2c/0x70
[ 338.911859] el0_sync_handler+0xb0/0xb8
[ 338.911864] el0_sync+0x180/0x1c0
> It'd be good to check *exactly* when we need to reject, since IIUC when
> we have a graph stack entry the unwind will be correct from livepatch's
> PoV.
>
The current unwinder already handles this like this:
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
if (tsk->ret_stack &&
(ptrauth_strip_insn_pac(frame->pc) == (unsigned long)return_to_handler)) {
struct ftrace_ret_stack *ret_stack;
/*
* This is a case where function graph tracer has
* modified a return address (LR) in a stack frame
* to hook a function return.
* So replace it to an original value.
*/
ret_stack = ftrace_graph_get_ret_stack(tsk, frame->graph++);
if (WARN_ON_ONCE(!ret_stack))
return -EINVAL;
frame->pc = ret_stack->ret;
}
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
Is there anything else that needs handling here?
Thanks,
Madhavan
