Re: [PATCH v2 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

David Hildenbrand
2023-05-22T09:03:29Z

On 17.05.23 17:03, Florent Revest wrote:
> This extends the current PR_SET_MDWE prctl arg with a bit to indicate
> that the process doesn't want MDWE protection to propagate to children.
>
> To implement this no-inherit mode, the tag in current->mm->flags must be
> absent from MMF_INIT_MASK. This means that the encoding for "MDWE but
> without inherit" is different in the prctl than in the mm flags. This
> leads to a bit of bit-mangling in the prctl implementation.
>
> Signed-off-by: Florent Revest <revest@chromium.org>
> ---
> include/linux/sched/coredump.h | 10 ++++++++++
> include/uapi/linux/prctl.h | 1 +
> kernel/fork.c | 2 +-
> kernel/sys.c | 24 +++++++++++++++++++++---
> tools/include/uapi/linux/prctl.h | 1 +
> 5 files changed, 34 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 0ee96ea7a0e9..11f5e3dacb4e 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -91,4 +91,14 @@ static inline int get_dumpable(struct mm_struct *mm)
> MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)
>
> #define MMF_VM_MERGE_ANY 29
> +#define MMF_HAS_MDWE_NO_INHERIT 30
> +
> +#define MMF_INIT_FLAGS(flags) ({ \
> + unsigned long new_flags = flags; \
> + if (new_flags & (1UL << MMF_HAS_MDWE_NO_INHERIT)) \
> + new_flags &= ~((1UL << MMF_HAS_MDWE) | \
> + (1UL << MMF_HAS_MDWE_NO_INHERIT)); \
> + new_flags & MMF_INIT_MASK; \
> +})

Why the desire for macros here? :)

We have a single user of MMF_INIT_FLAGS, why not inline or use a proper
inline function?

--
Thanks,

David / dhildenb