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Message-ID: <20191018133735.77e90e36@gandalf.local.home>
Date: Fri, 18 Oct 2019 13:37:35 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Borislav Petkov <bp@...e.de>
Cc: tip-bot2 for Jiri Slaby <tip-bot2@...utronix.de>,
linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
Jiri Slaby <jslaby@...e.cz>, "H. Peter Anvin" <hpa@...or.com>,
Ingo Molnar <mingo@...hat.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
linux-arch@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>,
Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [tip: x86/asm] x86/asm/ftrace: Mark function_hook as function
On Fri, 18 Oct 2019 19:13:54 +0200
Borislav Petkov <bp@...e.de> wrote:
> On Fri, Oct 18, 2019 at 12:49:56PM -0400, Steven Rostedt wrote:
> > On Fri, 18 Oct 2019 12:48:00 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > > > Relabel function_hook to be marked really as a function. It is called
> > > > from C and has the same expectations towards the stack etc.
> > >
> >
> > And to go even further, it *does not* have the same expectations
> > towards the stack.
> >
> > I think this patch should not be applied.
>
> There are a couple more markings like that now:
>
> $ git grep function_hook
> Documentation/asm-annotations.rst:120: SYM_FUNC_START(function_hook)
> Documentation/asm-annotations.rst:122: SYM_FUNC_END(function_hook)
> arch/x86/kernel/ftrace_32.S:15:# define function_hook __fentry__
> arch/x86/kernel/ftrace_32.S:24:SYM_FUNC_START(function_hook)
> arch/x86/kernel/ftrace_32.S:26:SYM_FUNC_END(function_hook)
> arch/x86/kernel/ftrace_64.S:17:# define function_hook __fentry__
> arch/x86/kernel/ftrace_64.S:135:SYM_FUNC_START(function_hook)
> arch/x86/kernel/ftrace_64.S:137:SYM_FUNC_END(function_hook)
> arch/x86/kernel/ftrace_64.S:251:SYM_FUNC_START(function_hook)
> arch/x86/kernel/ftrace_64.S:282:SYM_FUNC_END(function_hook)
>
> Frankly, I wouldn't mark this function at all as it is special and I see
> a little sense to have it in stack traces but maybe Jiri has another
> angle here. I'll let him comment.
It just needs to be visible by modules and what not, otherwise linking
will fail.
>
> I guess with the new nomenclature that can be SYM_CODE_* now...
>
> Then, this magic "function" or a global symbol with an address or
> whatever that is (oh, there's #define trickery too) definitely deserves
> a comment above it to explain what it is. I even have to build the .s
> file to see what it turns into:
The #define was because we use to support mcount or __fentry__, now we
just support __fentry__, and function_hook describes it better ;-)
>
> .globl __fentry__ ; .p2align 4, 0x90 ; __fentry__:
> retq
> .type __fentry__, @function ; .size __fentry__, .-__fentry__
>
> Yeah, it is called on every function entry:
>
> callq ffffffff81a01760 <__fentry__>
>
> but can we please explain with a comment above it what it is?
Heh, I guess we could, which would probably be quite a long comment as
it's the key behind ftrace itself.
-- Steve
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