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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260529172331.356655-7-kas@kernel.org> X-Rspamd-Queue-Id: 6859240008 X-Stat-Signature: r1tt6hp55cgjtpiz1pzf58mpfnydt74k X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1780389183-856976 X-HE-Meta: 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 nIHKPF0t TD3vzq0ewJvgFDUZ3r2Jt1NJ+saELWkUKqZcL8mBaNiC16rOekvKoEsTnFS/1UWr1yhvSPzeHrf3pg8aJVzCi0bXeTccgDyds148SJey5is0afaZ49Ozo0sMS2Y7fjUo28m51jA5yZplZ6TPXGpljxxQwWO6PBKGdgJmwQEv2m176zTuyQzp79AXsL1HSXngRdTn/qekjKkaBlpf2TCN5tHZJkFtfZR7unXEepjqA44jZSxuncD15tzlxRH++Eun7veHYRBeovwJKeO6sUuvuU8viXdeP6wCHxzDoRRSqOf1+VK/8l5WYLnhQb6/E41Czqr9+CJZutXWIatQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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") > Cc: stable@vger.kernel.org > Reported-by: Sashiko AI review > Suggested-by: Lorenzo Stoakes > Signed-off-by: Kiryl Shutsemau > Assisted-by: Claude:claude-opus-4-8 Can you ask claude to produce more concise changelogs and better split it to paragraphs? Reviewed-by: Mike Rapoport (Microsoft) > --- > 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 > -- Sincerely yours, Mike.