Re: [tip:x86/mm] x86/mm: Break out user address space handling

From: Ingo Molnar <mingo@kernel.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: tip-bot for Dave Hansen <tipbot@zytor.com>,
	linux-tip-commits@vger.kernel.org, dave.hansen@linux.intel.com,
	tglx@linutronix.de, linux-kernel@vger.kernel.org, hpa@zytor.com,
	jannh@google.com, sean.j.christopherson@intel.com,
	peterz@infradead.org, luto@kernel.org
Subject: Re: [tip:x86/mm] x86/mm: Break out user address space handling
Date: Fri, 19 Oct 2018 07:58:44 +0200	[thread overview]
Message-ID: <20181019055844.GA50013@gmail.com> (raw)
In-Reply-To: <87k1mjpxwi.fsf@xmission.com>

* Eric W. Biederman <ebiederm@xmission.com> wrote:

> tip-bot for Dave Hansen <tipbot@zytor.com> writes:
> 
> > Commit-ID:  aa37c51b9421d66f7931c5fdcb9ce80c450974be
> > Gitweb:     https://git.kernel.org/tip/aa37c51b9421d66f7931c5fdcb9ce80c450974be
> > Author:     Dave Hansen <dave.hansen@linux.intel.com>
> > AuthorDate: Fri, 28 Sep 2018 09:02:23 -0700
> > Committer:  Peter Zijlstra <peterz@infradead.org>
> > CommitDate: Tue, 9 Oct 2018 16:51:15 +0200
> >
> > x86/mm: Break out user address space handling
> >
> > The last patch broke out kernel address space handing into its own
> > helper.  Now, do the same for user address space handling.
> >
> > Cc: x86@kernel.org
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Link: http://lkml.kernel.org/r/20180928160223.9C4F6440@viggo.jf.intel.com
> > ---
> >  arch/x86/mm/fault.c | 47 ++++++++++++++++++++++++++++-------------------
> >  1 file changed, 28 insertions(+), 19 deletions(-)
> >
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index c7e32f453852..0d1f5d39fc63 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -966,6 +966,7 @@ bad_area_access_error(struct pt_regs *regs, unsigned long error_code,
> >  		__bad_area(regs, error_code, address, vma, SEGV_ACCERR);
> >  }
> >  
> > +/* Handle faults in the kernel portion of the address space */
>                            ^^^^^^
> I believe you mean the __user__ portion of the address space.
> Given that the call chain is:
> 
> do_user_addr_fault
>    handle_mm_fault
>       do_sigbus      

It's both:

  /* Handle faults in the kernel portion of the address space */
  static void
  do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
            u32 *pkey, unsigned int fault)
  {
        struct task_struct *tsk = current;
        int code = BUS_ADRERR;

        /* Kernel mode? Handle exceptions or die: */
        if (!(error_code & X86_PF_USER)) {
                no_context(regs, error_code, address, SIGBUS, BUS_ADRERR);
                return;
        }

        /* User-space => ok to do another page fault: */
        if (is_prefetch(regs, error_code, address))
                return;

        tsk->thread.cr2         = address;
        tsk->thread.error_code  = error_code;
        tsk->thread.trap_nr     = X86_TRAP_PF;

Note the X86_PF_USER check: that's what determines whether the fault was 
for user or system mappings.

I agree that the comment is misleading and should be clarified.

Thanks,

	Ingo