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Date:   Wed, 1 May 2019 21:13:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Joerg Roedel <jroedel@suse.de>,
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        live-patching@vger.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>
Subject: Re: [RFC][PATCH] ftrace/x86: Emulate call function while updating in
 breakpoint handler
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On Wed, May 01, 2019 at 12:03:52PM -0700, Linus Torvalds wrote:
> On Wed, May 1, 2019 at 6:11 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Here goes, compile tested only...
> 
> Ugh, two different threads. This has the same bug (same source) as the
> one Steven posted:

This is what Steve started from; lets continue in the other thread.

> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -1479,6 +1479,13 @@ ENTRY(int3)
> >         ASM_CLAC
> >         pushl   $-1                             # mark this as an int
> >
> > +       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
> > +       jnz     .Lfrom_usermode_no_gap
> > +       .rept 6
> > +       pushl   5*4(%esp)
> > +       .endr
> > +.Lfrom_usermode_no_gap:
> 
> This will corrupt things horribly if you still use vm86 mode. Checking
> CS RPL is simply not correct.

I'll go fix; I never really understood that vm86 crud and I cobbled this
32bit thing together based on the 64bit version (that Josh did a while
ago).