Re: [PATCH v2] ARM: mm: Provide better fault message for permission fault

[Kefeng Wang]

On 2022/9/26 18:13, Russell King (Oracle) wrote:
> On Mon, Sep 19, 2022 at 06:38:45PM +0800, Kefeng Wang wrote:
> > If there is a permission fault in __do_kernel_fault(), we only
> > print the generic "paging request" message which don't show
> > read, write or excute information, let's provide better fault
> > message for them.
> I don't like this change. With CPUs that do not have the ability to
> relocate the vectors to 0xffff0000, the vectors live at address 0,
> so NULL pointer dereferences can produce permission faults.
The __do_user_fault(), do_DataAbort() and do_PrefetchAbort() shows
the FSR when printing, we could do it in die_kernel_fault(), and
which will be easy for us to check whether the page fault is permision
fault,


--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -111,8 +111,8 @@ static void die_kernel_fault(const char *msg, struct 
mm_struct *mm,
 {
        bust_spinlocks(1);
        pr_alert("8<--- cut here ---\n");
-       pr_alert("Unable to handle kernel %s at virtual address %08lx\n",
-                msg, addr);
+       pr_alert("Unable to handle kernel %s (0x%08x) at virtual address 
%08lx\n",
+                msg, fsr, addr);

        show_pte(KERN_ALERT, mm, addr);
        die("Oops", regs, fsr);


or,

> I would much rather we did something similar to what x86 does:
>
>          pr_alert("#PF: %s %s in %s mode\n",
>                   (error_code & X86_PF_USER)  ? "user" : "supervisor",
>                   (error_code & X86_PF_INSTR) ? "instruction fetch" :
>                   (error_code & X86_PF_WRITE) ? "write access" :
>                                                 "read access",
>                               user_mode(regs) ? "user" : "kernel");
>
> As we already print whether we're in user or kernel mode in the
> register dump, there's no need to repeat that. I think we just
> need an extra line to decode the FSR PF and write bits.

We could decode the FSR register,

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c

index 46cccd6bf705..406e0210c3c5 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -113,6 +113,10 @@ static void die_kernel_fault(const char *msg, 
struct mm_struct *mm,
        pr_alert("8<--- cut here ---\n");
        pr_alert("Unable to handle kernel %s at virtual address %08lx\n",
                 msg, addr);
+       pr_alert("FSR: 0x%08x, LNX_PF = %u, CM = %u, WnR = %u\n", fsr,
+                (fsr & FSR_LNX_PF) >> FSR_LNX_PF_SHIFT,
+                (fsr & FSR_CM) >> FSR_CM_SHIFT,
+                (fsr & FSR_WRITE) >> FSR_WRITE_SHIFT);

        show_pte(KERN_ALERT, mm, addr);
        die("Oops", regs, fsr);
diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h
index 83b5ab32d7a4..18f882aa2b32 100644
--- a/arch/arm/mm/fault.h
+++ b/arch/arm/mm/fault.h
@@ -5,9 +5,12 @@
 /*
  * Fault status register encodings.  We steal bit 31 for our own purposes.
  */
-#define FSR_LNX_PF             (1 << 31)
-#define FSR_CM                 (1 << 13)
-#define FSR_WRITE              (1 << 11)
+#define FSR_LNX_PF_SHIFT       (31)
+#define FSR_LNX_PF             (1 << FSR_LNX_PF_SHIFT)
+#define FSR_CM_SHIFT           (13)
+#define FSR_CM                 (1 << FSR_CM_SHIFT)
+#define FSR_WRITE_SHIFT                (11)
+#define FSR_WRITE              (1 << FSR_WRITE_SHIFT)
 #define FSR_FS4                        (1 << 10)
 #define FSR_FS3_0              (15)
 #define FSR_FS5_0              (0x3f)


What's your option ?

>