linux-kernel - Re: [PATCH v2 4/5] mm: Add a NO_INHERIT flag to the PR_SET

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Message-ID: <7883b08b-6bdc-a214-96a3-3f5bc1d36da4@redhat.com>
Date:   Mon, 22 May 2023 11:01:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Florent Revest <revest@...omium.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, catalin.marinas@....com,
        anshuman.khandual@....com, joey.gouly@....com, mhocko@...e.com,
        keescook@...omium.org, peterx@...hat.com, izbyshev@...ras.ru,
        broonie@...nel.org, szabolcs.nagy@....com, kpsingh@...nel.org,
        gthelen@...gle.com, toiwoton@...il.com
Subject: Re: [PATCH v2 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

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@...omium.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