linux-kernel - Re: [PATCH 6/5] x86/fault: Clean up the page fault oops decoder a bit

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Date:   Wed, 5 Dec 2018 07:23:36 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 6/5] x86/fault: Clean up the page fault oops decoder a bit

On Tue, Dec 04, 2018 at 11:22:25AM -0800, Andy Lutomirski wrote:
> On Tue, Nov 27, 2018 at 7:32 AM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> >  arch/x86/mm/fault.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 2ff25ad33233..510e263c256b 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -660,8 +660,10 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
> >         err_str_append(error_code, err_txt, X86_PF_RSVD,  "[RSVD]" );
> >         err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
> >         err_str_append(error_code, err_txt, X86_PF_PK,    "[PK]"   );
> > -
> > -       pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]");
> > +       err_str_append(~error_code, err_txt, X86_PF_USER, "[KERNEL]");
> > +       err_str_append(~error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR,
> > +                                                         "[READ]");
> > +       pr_alert("#PF error code: %s\n", err_txt);
> >
> 
> Seems generally nice, but I would suggest making the bit-not-set name
> be another parameter to err_str_append().

I didn't recall why I chose to negate error_code until I revisited the
actual code.  The "READ" case is a combination of !WRITE && !USER, i.e.
doesn't fit into an existing err_str_append() call.  So we'd end up with
an extra err_str_append() call that would also have a null message for
the positive test, which seemed unnecessarily complex and more convoluted
than simply negating error_code.

E.g.:

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2ff25ad33233..48b420621825 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -607,12 +607,17 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
  * This helper function transforms the #PF error_code bits into
  * "[PROT] [USER]" type of descriptive, almost human-readable error strings:
  */
-static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)
+static void err_str_append(unsigned long error_code, char *buf, unsigned long mask,
+                          const char *pos, const char *neg)
 {
-       if (error_code & mask) {
+       if ((error_code & mask) == mask && pos) {
                if (buf[0])
                        strcat(buf, " ");
-               strcat(buf, txt);
+               strcat(buf, pos);
+       } else if (!(error_code & mask) && neg) {
+               if (buf[0])
+                       strcat(buf, " ");
+               strcat(buf, neg);
        }
 }

@@ -654,14 +659,15 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
         * Note: length of these appended strings including the separation space and the
         * zero delimiter must fit into err_txt[].
         */
-       err_str_append(error_code, err_txt, X86_PF_PROT,  "[PROT]" );
-       err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]");
-       err_str_append(error_code, err_txt, X86_PF_USER,  "[USER]" );
-       err_str_append(error_code, err_txt, X86_PF_RSVD,  "[RSVD]" );
-       err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
-       err_str_append(error_code, err_txt, X86_PF_PK,    "[PK]"   );
-
-       pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]");
+       err_str_append(error_code, err_txt, X86_PF_PROT,  "[PROT]" , NULL);
+       err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]", NULL);
+       err_str_append(error_code, err_txt, X86_PF_USER,  "[USER]" , "[SUPERVISOR]");
+       err_str_append(error_code, err_txt, X86_PF_RSVD,  "[RSVD]" , NULL);
+       err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]", NULL);
+       err_str_append(error_code, err_txt, X86_PF_PK,    "[PK]"   , NULL);
+       err_str_append(error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR, NULL,
+                                                         "[READ]");
+       pr_alert("#PF error code: %s\n", err_txt);

        if (!(error_code & X86_PF_USER) && user_mode(regs)) {
                struct desc_ptr idt, gdt;