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Howlett" , Mike Rapoport , Michal Hocko , Jason Gunthorpe , Peter Xu , Matthew Wilcox , Usama Arif References: <20260724222934.1463812-1-riel@surriel.com> <20260724222934.1463812-5-riel@surriel.com> Content-Language: en-US From: John Hubbard In-Reply-To: <20260724222934.1463812-5-riel@surriel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY1P220CA0026.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:5c3::17) To DM3PR12MB9416.namprd12.prod.outlook.com (2603:10b6:0:4b::8) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM3PR12MB9416:EE_|SA1PR12MB999083:EE_ X-MS-Office365-Filtering-Correlation-Id: 9e083249-f01b-4ffe-0f54-08deec51a41b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|23010399003|366016|18002099003|22082099003|10067099003|56012099006|11063799006|4143699003; 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A caller > that wants to reach a COWed page in a VM_IO or VM_PFNMAP VMA, where that > page does have a struct page, cannot express the exception and has to > repeat the flag test itself. > > Add an ignore_flags argument that check_vma_flags() clears from its local > copy of vma->vm_flags before the flag checks. A caller can drop a single > rejection this way while keeping every other check. All current callers > pass 0. > > This prepares for get_user_page_vma(), which passes VM_IO | VM_PFNMAP to > reach those COWed pages. > > No functional change intended. > > Assisted-by: Claude:claude-opus-4.8 > Signed-off-by: Rik van Riel > --- > mm/gup.c | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 0692119b7904..dcece7b5253c 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -1197,13 +1197,21 @@ static bool writable_file_mapping_allowed(struct vm_area_struct *vma, > return !vma_needs_dirty_tracking(vma); > } > > -static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) > +static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags, > + vm_flags_t ignore_flags) All callers pass zero in this patch, so there is no behavior change yet. The problem is the interface being added. The same local vm_flags value is used for the VM_IO/VM_PFNMAP gate, VM_WRITE, VM_SHADOW_STACK, VM_READ, VM_MAYREAD, and is_cow_mapping(). An arbitrary mask can therefore alter permission, shadow-stack, and COW validation. For example, masking VM_SHARED changes the result of is_cow_mapping(), while masking VM_SHADOW_STACK removes its write rejection. Checks implemented through helpers still read the original vma->vm_flags, so the meaning of ignore_flags also depends on how each check happens to be implemented. Perhaps it is best to replace the mask with a single-purpose internal GUP flag, such as FOLL_PFNMAP_COW, and apply it only to the special-mapping gate: if (vm_flags & (VM_IO | VM_PFNMAP) && (!(gup_flags & FOLL_PFNMAP_COW) || !(vm_flags & VM_PFNMAP))) return -EFAULT; This permits the later caller to examine a VM_PFNMAP mapping while keeping VM_IO-only mappings and every other VMA check unchanged. thanks, -- John Hubbard