Re: [PATCH v1] mm/userfaultfd: UFFDIO_MOVE implementation should use ptep_get()

[David Hildenbrand <david@redhat.com>]

On 23.01.24 15:17, Ryan Roberts wrote:
> Commit c33c794828f2 ("mm: ptep_get() conversion") converted all
> (non-arch) call sites to use ptep_get() instead of doing a direct
> dereference of the pte. Full rationale can be found in that commit's
> log.
> 
> Since then, UFFDIO_MOVE has been implemented which does 7 direct pte
> dereferences. Let's fix those up to use ptep_get().
> 
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> Hi All,
> 
> This applies on top of v6.8-rc1. I'm hoping this can be merged into the
> next rc.
> 
> I've asserted in the past that there is no reliable automated mechanism
> to catch these; I'm relying on a combination of Coccinelle (which throws
> up a lot of false positives) and some compiler magic to force a compiler
> error on dereference. But given the frequency with which new issues are
> coming up, I'll add it to my todo list to try to find an automated
> solution.
If we'd use a distinct type for pte pointers that are only passed around 
(and not worked on), the compiler would bail out when doing such 
assignments.

Like

typedef struct { unsigned long pte; } pte_t;
typedef struct { unsigned long pte; } pte2_t;


pte_t pte = { 2 };
pte2_t pte2;

pte2 = pte;

-> "error: incompatible types when assigning to type 'pte2_t' from type 
'pte_t'"


pte_get() would do the conversion.


... but just thinking about it this would require a lot of work.

-- 
Cheers,

David / dhildenb