linux-kernel - Re: [RFC PATCH v1 3/4] arm64: Detect FTRACE cases that make the stack trace unreliable

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Date:   Thu, 1 Apr 2021 14:47:11 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     mark.rutland@....com, jpoimboe@...hat.com, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 3/4] arm64: Detect FTRACE cases that make the stack
 trace unreliable



On 4/1/21 1:53 PM, Madhavan T. Venkataraman wrote:
> 
> 
> On 4/1/21 1:40 PM, Madhavan T. Venkataraman wrote:
>>>> So, it is only defined if CONFIG_FUNCTION_GRAPH_TRACER is defined. I can address
>>>> this as well as your comment by defining another label whose name is more meaningful
>>>> to our use:
>>>> +SYM_INNER_LABEL(ftrace_trampoline, SYM_L_GLOBAL) // checked by the unwinder
>>>> #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
>>> I'm not sure we need to bother with that, you'd still need the & I think.
>> I think we need to bother with that. If CONFIG_FUNCTION_GRAPH_TRACER is not on but
>> CONFIG_DYNAMIC_FTRACE_WITH_REGS is, then ftrace_graph_call() will not occur in the stack
>> trace taken from a tracer function. The unwinder still needs to recognize an ftrace frame.
>> I don't want to assume ftrace_common_return which is the label that currently follows
>> the above code. So, we need a different label outside the above ifdef.
> 
> Alternatively, I could just move the SYM_INNER_LABEL(ftrace_graph_call..) to outside the ifdef.
> 
> Madhavan
> 

Or, even better, I could just use ftrace_call+4 because that would be the return
address for the tracer function at ftrace_call:

SYM_CODE_START(ftrace_common)
        sub     x0, x30, #AARCH64_INSN_SIZE     // ip (callsite's BL insn)
        mov     x1, x9                          // parent_ip (callsite's LR)
        ldr_l   x2, function_trace_op           // op
        mov     x3, sp                          // regs

SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
        bl      ftrace_stub

I think that would be cleaner. And, I don't need the complicated comments for ftrace_graph_call.

Is this acceptable?

Madhavan