From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 03A1B3E7BA1; Mon, 1 Jun 2026 18:34:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780338877; cv=none; b=V0UmWFWPiwNpFs2zAykRq9rnTg/J0EbMUKtLR4cOYZo/6AhhKUQ++FU7efnfULnM6dutpvYQJNsBuq4zkp7DQMAzveGo26rpblyXrA1E3WCAHfn25cgH3IIvG87qzLmVYC0ZppjkZinvtUd/4OjU0xUn1LXV7avUEoBWRMBrhRY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780338877; c=relaxed/simple; bh=coGJdyMqMK1JH8cdlQFfetob2/us+76M7fRKQAoG9W4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KtJIXf+pBLTJVBBGWggzPvggi04UsMAsGYLqNtlNEdcQ8CougeYG9L06ntbzDO5MdUKGmYv+bRVl/EiwrAV2C38YaMnYDUil6V8lFO2tWdsy9NCQQKq541qkDLOYNThiY35wxBf4bs1WTiLZKYLTCM/ToNG9KAPcokWzJj3lawY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HaIOJ6Hs; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HaIOJ6Hs" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 128DA1F00898; Mon, 1 Jun 2026 18:34:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780338875; bh=tm2Fbx23JzRqTYmnF90OiEMdXn92kYWSLQqsjNTPRE0=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=HaIOJ6Hs9RykH8Rht/3CdurT8Hb5OgUutwe1KKSe6r6VUU+yDVkZvyPwnUax85YV3 9oTZzIhhID/N+yast2IooIV/nDd2zBhCm6bUJG80iKPSH5CwTU59DwQJ028PIOUqCy mCO+YkucKnyXclGWbbiocJTmEUcUXcLtj0/IjXKeF4JIjFkWUfbrsR/tvOtWj0wFtE UhBzaB3Cd91+4MMTVjZD/xVFuRACpQCZQbBJ/08zlU+roKq5leHJhXqIAez8oE6U7T v6xDlys+E45ZZPm3GA59H0861Q8/smyp1Gjv1xg66TOf7cLRNdaKH3oSmY5v+99ARg NJdOb71T+vYQw== Date: Mon, 1 Jun 2026 19:34:29 +0100 From: Lorenzo Stoakes To: "Kiryl Shutsemau (Meta)" Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mike Rapoport , David Hildenbrand , stable@vger.kernel.org, Sashiko AI review , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Peter Xu , Pedro Falcato , Alice Ryhl Subject: Re: [PATCH 6/6] userfaultfd: build __VMA_UFFD_FLAGS from config-gated masks Message-ID: References: <20260529172331.356655-1-kas@kernel.org> <20260529172331.356655-7-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260529172331.356655-7-kas@kernel.org> On Fri, May 29, 2026 at 06:23:30PM +0100, Kiryl Shutsemau (Meta) wrote: > The VMA flags bitmap is a single word today: NUM_VMA_FLAG_BITS is > BITS_PER_LONG, so on 32-bit vma_flags_t holds only 32 bits. (The bitmap > type exists so this can grow past BITS_PER_LONG later; until it does, > anything declared above the first word is out of range on 32-bit.) The bit > enum nevertheless declares some bits unconditionally above BITS_PER_LONG -- > VMA_UFFD_MINOR_BIT is 41, with VM_UFFD_MINOR == VM_NONE on 32-bit so no VMA > actually carries the bit. > > __VMA_UFFD_FLAGS feeds VMA_UFFD_MINOR_BIT to mk_vma_flags() unconditionally. > On 32-bit that becomes __set_bit(41, &one_long), a write one word past the > end of the single-word bitmap. The compiler folds the out-of-bounds store > with wraparound (1UL << (41 % 32) == bit 9) into the first word; bit 9 is > already in __VMA_UFFD_FLAGS so the mask happens to come out right today, but > it is an out-of-bounds write all the same, and any high-numbered bit whose > mod-BITS_PER_LONG position is otherwise unused would silently OR an extra > bit into the mask. > > Rather than feed bit numbers that may not exist on the current build to > mk_vma_flags(), build the mask from whole per-mode masks that collapse to > EMPTY_VMA_FLAGS when their feature is unavailable. Add > mk_vma_flags_from_masks() for that, and define VMA_UFFD_MISSING / _WP / > _MINOR alongside the VM_UFFD_* flags, gating VMA_UFFD_MINOR on the same > config as VM_UFFD_MINOR (which implies 64BIT, where bit 41 fits). An > out-of-range bit is then never materialised, on any arch, and the in-range > fast path stays a compile-time constant. > > Fixes: 9ea35a25d51b ("mm: introduce VMA flags bitmap type") Hmm, I think commit a06eb2f8279e ("mm/vma: convert vma_modify_flags[_uffd]() to use vma_flags_t") is more appropriate isn't it? As that's the commit that added __VMA_UFFD_FLAGS :) > Cc: stable@vger.kernel.org > Reported-by: Sashiko AI review > Suggested-by: Lorenzo Stoakes > Signed-off-by: Kiryl Shutsemau The change LGTM, so: Reviewed-by: Lorenzo Stoakes > Assisted-by: Claude:claude-opus-4-8 > --- > include/linux/mm.h | 39 +++++++++++++++++++++++++++++++++++ > include/linux/userfaultfd_k.h | 4 ++-- > 2 files changed, 41 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 0f2612a70fb1..485df9c2dbdd 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -496,6 +496,21 @@ enum { > #else > #define VM_UFFD_MINOR VM_NONE > #endif > + > +/* > + * vma_flags_t masks for the userfaultfd VMA flags. VMA_UFFD_MINOR is gated on > + * the same config as VM_UFFD_MINOR -- which implies 64BIT, where the bit fits > + * -- so an out-of-range bit is never fed to mk_vma_flags() on a build whose > + * bitmap cannot hold it. > + */ > +#define VMA_UFFD_MISSING mk_vma_flags(VMA_UFFD_MISSING_BIT) > +#define VMA_UFFD_WP mk_vma_flags(VMA_UFFD_WP_BIT) > +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR > +#define VMA_UFFD_MINOR mk_vma_flags(VMA_UFFD_MINOR_BIT) > +#else > +#define VMA_UFFD_MINOR EMPTY_VMA_FLAGS > +#endif > + > #ifdef CONFIG_64BIT > #define VM_ALLOW_ANY_UNCACHED INIT_VM_FLAG(ALLOW_ANY_UNCACHED) > #define VM_SEALED INIT_VM_FLAG(SEALED) > @@ -1238,6 +1253,30 @@ static __always_inline void vma_flags_set_mask(vma_flags_t *flags, > #define vma_flags_set(flags, ...) \ > vma_flags_set_mask(flags, mk_vma_flags(__VA_ARGS__)) > > +static __always_inline vma_flags_t __mk_vma_flags_from_masks(size_t count, > + const vma_flags_t *masks) > +{ > + vma_flags_t flags = EMPTY_VMA_FLAGS; > + size_t i; > + > + for (i = 0; i < count; i++) > + vma_flags_set_mask(&flags, masks[i]); > + return flags; > +} > + > +/* > + * Combine pre-computed vma_flags_t masks into one value, e.g.: > + * > + * vma_flags_t flags = mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_MINOR); > + * > + * Unlike mk_vma_flags(), which takes bit numbers, this takes whole masks -- > + * each of which may be EMPTY_VMA_FLAGS when its feature is unavailable -- so a > + * bit that does not exist on the current build is never materialised. > + */ > +#define mk_vma_flags_from_masks(...) \ > + __mk_vma_flags_from_masks(COUNT_ARGS(__VA_ARGS__), \ > + (const vma_flags_t []){__VA_ARGS__}) > + > /* Clear all of the to-clear flags in flags, non-atomically. */ > static __always_inline void vma_flags_clear_mask(vma_flags_t *flags, > vma_flags_t to_clear) > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h > index 3ec8e1071673..68edac4dcd78 100644 > --- a/include/linux/userfaultfd_k.h > +++ b/include/linux/userfaultfd_k.h > @@ -23,8 +23,8 @@ > /* The set of all possible UFFD-related VM flags. */ > #define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR) > > -#define __VMA_UFFD_FLAGS mk_vma_flags(VMA_UFFD_MISSING_BIT, VMA_UFFD_WP_BIT, \ > - VMA_UFFD_MINOR_BIT) > +#define __VMA_UFFD_FLAGS mk_vma_flags_from_masks(VMA_UFFD_MISSING, VMA_UFFD_WP, \ > + VMA_UFFD_MINOR) > > /* > * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining > -- > 2.54.0 >