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[34.127.79.33]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2edb6d7d964sm11306915ad.32.2026.11.10.08.23.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Nov 2026 08:23:54 -0800 (PST) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 10 Nov 2026 16:23:53 +0000 Message-Id: Cc: , , "Pratyush Yadav" , , , , , , , , , Subject: Re: [PATCH v4 16/19] PCI: liveupdate: Add feature header to struct pci_dev_ser From: "David Matlack" To: "Tarun Sahu" , "Pasha Tatashin" , "Mike Rapoport" , , , , "David Matlack" , "Alexander Graf" , , , , , , , "Bjorn Helgaas" , , , , , "Andrew Morton" , , X-Mailer: aerc 0.21.0 References: <20261110050202.529473-1-tarunsahu@google.com> <20261110050202.529473-17-tarunsahu@google.com> In-Reply-To: <20261110050202.529473-17-tarunsahu@google.com> On Tue Nov 10, 2026 at 5:01 AM UTC, Tarun Sahu wrote: > Precede each serialized PCI device entry in the PCI Live Update KHO block > set with struct kho_ser_hdr via DECLARE_KHO_SER so future per-device PCI > Live Update features can append fields to struct pci_dev_ser without > breaking compatibility between adjacent kernels. Elements of a collection should not have their own feature headers. This applies to patches 10, 12, 14, 16 and 18. I raised this during the v2 discussion [1][2], but I don't think it was addressed. All entries in a KHO block set are written by the same kernel using the same struct, and the block set has a single entry_size, so every entry header is necessarily identical. They don't convey anything that the header of the struct that owns the collection can't. In this case, struct pci_ser's header can define features that describe fields in struct pci_dev_ser, and the PCI core checks them once before iterating. If a feature only applies to some devices, the field's value in each entry says whether that device uses it. Per-entry headers also cost a lot for small entries. struct pci_dev_ser goes from 8 to 16 bytes, and to 24 bytes once the first feature is added. Patch 18 does the same for every preserved memfd folio, which for a large guest backed by 4K pages adds gigabytes of preserved metadata. They also add new ways to read out of bounds. kho_ser_payload() trusts nr_required/nr_optional in each entry, and nothing checks that the entry header plus payload fit within entry_size. In patches 14 and 18 the walk uses each entry's fhdr->size to find the next entry, so one bad header throws off every entry after it. Instead I would keep struct pci_dev_ser as a plain struct, take the entry size from the KHO block header (patch 2), and in pci_flb_retrieve() check once that the entry size covers the base fields, plus the fields of any features set in struct pci_ser's header, e.g.: entry_size =3D kho_block_set_entry_size(&incoming->block_set); if (entry_size < offsetofend(struct pci_dev_ser, refcount)) panic(...); That also restores the minimum entry size check that the v10 PCI series had with PCI_DEV_SER_MIN_SIZE, which this version drops. [...] > - kho_block_set_init(&incoming->block_set, sizeof(struct pci_dev_ser)); > + kho_block_set_init(&incoming->block_set, kho_ser_size(pci_dev_ser)); This size is never used. kho_block_set_restore() overwrites it with the entry size recorded by the previous kernel. Passing this kernel's size here is misleading: pci_flb_retrieve() (and the LUO session/file restore paths) read as if they iterate incoming entries with the outgoing entry size, which would be a bug. Incoming block sets should not take an entry size at all. kho_block_set_restore() should initialize the block set itself from the incoming block headers, and the incoming paths should drop the kho_block_set_init() call. [1] https://lore.kernel.org/kexec/atl6K2Juy6Bh29lh@google.com/ [2] https://lore.kernel.org/kexec/at-8vkVpN2SF3XgB@google.com/