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 13:53:12 -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 1:40 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