linux-kernel - Re: [RFC][PATCH] ftrace/x86: Emulate call function while updating in breakpoint handler

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Date:   Tue, 30 Apr 2019 15:00:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC][PATCH] ftrace/x86: Emulate call function while updating
 in breakpoint handler

On Tue, 30 Apr 2019 11:33:21 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Apr 30, 2019 at 10:49 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > +
> > +asm(
> > +       ".text\n"
> > +
> > +       /* Trampoline for function update with interrupts enabled */
> > +       ".global ftrace_emulate_call_irqoff\n"
> > +       ".type ftrace_emulate_call_irqoff, @function\n"
> > +       "ftrace_emulate_call_irqoff:\n\t"
> > +               "push %gs:ftrace_bp_call_return\n\t"  
> 
> Well, as mentioned in my original suggestion, this won't work on
> 32-bit, or on UP. They have different models for per-cpu data (32-bti
> uses %fs, and UP doesn't use a segment override at all).

Ah, yeah, I forgot about 32-bit. I could easily make this use fs as
well, and for UP, just use a static variable.

> 
> Maybe we just don't care about UP at all for this code, of course.
> 
> And maybe we can make the decision to also make 32-bit just not use
> this either - so maybe the code is ok per se, just needs to make sure
> it never triggers for the cases that it's not written for..
> 
> > +       "ftrace_emulate_call_update_irqoff:\n\t"
> > +               "push %gs:ftrace_bp_call_return\n\t"
> > +               "sti\n\t"
> > +               "jmp *ftrace_update_func_call\n"  
> 
> .. and this should then use the "push push sti ret" model instead.
> 
> Plus get updated for objtool complaints.

Yeah, I see that now. Somehow it disappeared when I looked for it after
making some other changes. I can update it.

> 
> Anyway, since Andy really likes the entry code change, can we have
> that patch in parallel and judge the difference that way? Iirc, that
> was x86-64 specific too.

Note, I don't think live kernel patching supports 32 bit anyway, so
that may not be an issue.

Josh,

When you come back to the office, can you look into that method?

-- Steve