powerpc/64s: Don't use DSISR for SLB faults

Since commit 46ddcb39 ("powerpc/mm: Show if a bad page fault on data
is read or write.") we use page_fault_is_write(regs->dsisr) in
__bad_page_fault() to determine if the fault is for a read or write, and
change the message printed accordingly.

But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data
Storage Interrupt Status Register) to a useful value. All ISA versions
from v2.03 through v3.1 specify that the Data Segment Interrupt sets
DSISR "to an undefined value". As far as I can see there's no mention of
SLB faults setting DSISR in any BookIV content either.

This manifests as accesses that should be a read being incorrectly
reported as writes, for example, using the xmon "dump" command:

  0:mon> d 0x5deadbeef0000000
  5deadbeef0000000
  [359526.415354][    C6] BUG: Unable to handle kernel data access on write at 0x5deadbeef0000000
  [359526.415611][    C6] Faulting instruction address: 0xc00000000010a300
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf400]
      pc: c00000000010a300: mread+0x90/0x190

If we disassemble the PC, we see a load instruction:

  0:mon> di c00000000010a300
  c00000000010a300 89490000      lbz     r10,0(r9)

We can also see in exceptions-64s.S that the data_access_slb block
doesn't set IDSISR=1, which means it doesn't load DSISR into pt_regs. So
the value we're using to determine if the fault is a read/write is some
stale value in pt_regs from a previous page fault.

Rework the printing logic to separate the SLB fault case out, and only
print read/write in the cases where we can determine it.

The result looks like eg:

  0:mon> d 0x5deadbeef0000000
  5deadbeef0000000
  [  721.779525][    C6] BUG: Unable to handle kernel data access at 0x5deadbeef0000000
  [  721.779697][    C6] Faulting instruction address: 0xc00000000014cbe0
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]

  0:mon> d 0
  0000000000000000
  [  742.793242][    C6] BUG: Kernel NULL pointer dereference at 0x00000000
  [  742.793316][    C6] Faulting instruction address: 0xc00000000014cbe0
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]

Fixes: 46ddcb39 ("powerpc/mm: Show if a bad page fault on data is read or write.")
Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Link: https://lore.kernel.org/r/20220222113449.319193-1-mpe@ellerman.id.au

arch/powerpc/mm/fault.c

@@ -567,18 +567,24 @@ NOKPROBE_SYMBOL(hash__do_page_fault);
 static void __bad_page_fault(struct pt_regs *regs, int sig)
 {
 	int is_write = page_fault_is_write(regs->dsisr);
+	const char *msg;
 
 	/* kernel has accessed a bad area */
 
+	if (regs->dar < PAGE_SIZE)
+		msg = "Kernel NULL pointer dereference";
+	else
+		msg = "Unable to handle kernel data access";
+
 	switch (TRAP(regs)) {
 	case INTERRUPT_DATA_STORAGE:
-	case INTERRUPT_DATA_SEGMENT:
 	case INTERRUPT_H_DATA_STORAGE:
-		pr_alert("BUG: %s on %s at 0x%08lx\n",
-			 regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
-			 "Unable to handle kernel data access",
+		pr_alert("BUG: %s on %s at 0x%08lx\n", msg,
 			 is_write ? "write" : "read", regs->dar);
 		break;
+	case INTERRUPT_DATA_SEGMENT:
+		pr_alert("BUG: %s at 0x%08lx\n", msg, regs->dar);
+		break;
 	case INTERRUPT_INST_STORAGE:
 	case INTERRUPT_INST_SEGMENT:
 		pr_alert("BUG: Unable to handle kernel instruction fetch%s",