Re: [PATCH] powerpc/mm: make NULL pointer deferences explicit on bad page faults.

Christophe Leroy [off-list ref] writes:

Hi Michael,

Le 14/12/2018 à 01:57, Michael Ellerman a écrit :

quoted

Hi Christophe,

You know it's the trivial patches that are going to get lots of review
comments :)

I'm so happy to get comments.

Haha :)

quoted

Christophe Leroy [off-list ref] writes:

quoted

As several other arches including x86, this patch makes it explicit
that a bad page fault is a NULL pointer dereference when the fault
address is lower than PAGE_SIZE

I'm being pedantic, but it's not necessarily a NULL pointer dereference.
It might just be a direct access to a low address, eg:

  char *p = 0x100;
  *p = 0;

That's not a NULL pointer dereference.

But other arches do print this so I guess it's OK to add, and in most
cases it will be an actual NULL pointer dereference.

I wonder though if we should use 4096 rather than PAGE_SIZE, given
that's the actual value other arches are using. We support 256K pages on
some systems, which is getting quite large.

Those invalid accesses are catched because the first page is marked non 
present or non accessible in the page table, so I thing using PAGE_SIZE 
here is valid regardless of the page size.

It's not a question of whether we catch the fault it's what we print
when we catch it. Most of the time on 64-bit the first few GB of the
page tables will be empty, so those will all fault, but we don't call
them NULL pointer deferences.

So I'm just saying that this is a heuristic, ie. an access close to zero
is probably an access at a small offset from a NULL pointer, but it may
not be. And so it's kind of arbitrary where we decide to make the cut
off point between printing that it's a NULL pointer vs a regularly bad
access.

Anyway I'm happy to use PAGE_SHIFT for now, if anyone complains we can
always change it.

quoted

What about:

   BUG: Unable to handle kernel instruction fetch at 0x00000000

I think we still need to make it explicit that we jumped there due to a 
NULL function pointer, allthought I don't have a good text idea yet for 
this.

Being pedantic again, we don't know that it was a NULL function pointer.
You might have done a bad setcontext and set your NIP to zero.

But it's probably fine to print it as a hint, and it's probably right
most of the time.

cheers