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Wed, 21 Oct 2026 08:25:33 -0700 (PDT) Date: Wed, 21 Oct 2026 15:25:27 +0000 In-Reply-To: <20261021152527.375070-1-tarunsahu@google.com> X-Archived-By: Mime-Version: 1.0 References: <20261021152527.375070-1-tarunsahu@google.com> X-Mailer: git-send-email 2.56.0.212.g129ede20d9-goog Message-ID: <20261021152527.375070-5-tarunsahu@google.com> Subject: [RFC PATCH v2 4/4] mm: memfd_luo: Add the 2-bit pair feature header From: Tarun Sahu To: Andrew Morton , joro@8bytes.org, Mike Rapoport , Pratyush Yadav , maz@kernel.org, arnd@arndb.de, David Matlack , jgg@nvidia.com, robin.murphy@arm.com, loganodell@google.com, oupton@kernel.org, kevin.tian@intel.com, alex@shazbot.org, dwmw2@infradead.org, pbonzini@redhat.com, seanjc@google.com, will@kernel.org, Pasha Tatashin , Alexander Graf , Bjorn Helgaas , baolu.lu@linux.intel.com Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, sagis@google.com, linux-mm@kvack.org, kvmarm@lists.linux.dev, iommu@lists.linux.dev, linux-pci@vger.kernel.org, kexec@lists.infradead.org, skhawaja@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Tarun Sahu Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261021_082536_511797_E4CDAFC7 X-CRM114-Status: GOOD ( 22.48 ) X-Spam-Score: -9.5 (---------) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-26.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIMWL_WL_MED,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Embed the fixed-size 128-byte struct luo_feature_hdr at the start of struct memfd_luo_ser and define enum memfd_luo_feature (MEMFD_LUO_FEAT_CORE and MEMFD_LUO_FEAT_MAX) for memfd preservation. All current struct memfd_luo_ser fields (pos, size, seals, flags, nr_folios, folios) and MEMFD_LUO_ALL_SEALS belong to MEMFD_LUO_FEAT_CORE (LUO_FEAT_REQUIRED in memfd_luo_features). Future changes add new entries to enum memfd_luo_feature before MEMFD_LUO_FEAT_MAX and append any new fields to the end of struct memfd_luo_ser so existing field offsets never shift. Signed-off-by: Logan Odell Signed-off-by: Tarun Sahu --- include/linux/kho/abi/memfd.h | 35 ++++++++++++++++++++++++++++++----- mm/memfd_luo.c | 14 ++++++++++++++ 2 files changed, 44 insertions(+), 5 deletions(-) diff --git a/include/linux/kho/abi/memfd.h b/include/linux/kho/abi/memfd.h index 371ffd108a79..f0b5235a408d 100644 --- a/include/linux/kho/abi/memfd.h +++ b/include/linux/kho/abi/memfd.h @@ -13,6 +13,7 @@ #include #include +#include /** * DOC: memfd Live Update ABI @@ -21,11 +22,20 @@ * reboot using the LUO. * * The state is serialized into a packed structure `struct memfd_luo_ser` - * which is handed over to the next kernel via the KHO mechanism. + * which is handed over to the target kernel via the KHO mechanism. * - * This interface is a contract. Any modification to the structure layout - * constitutes a breaking change. Such changes require incrementing the - * version number in the MEMFD_LUO_FH_COMPATIBLE string. + * This interface is a contract. `struct memfd_luo_ser` starts with a fixed + * 128-byte `struct luo_feature_hdr`. The relative order and offsets of + * fields must remain unchanged as features are added or retired: new fields + * are appended at the end of `struct memfd_luo_ser` under a new feature + * flag in `enum memfd_luo_feature` (before `MEMFD_LUO_FEAT_MAX`), and + * retired fields are kept in place so subsequent offsets never shift. In + * the target kernel, fields of a `LUO_FEAT_REQUIRED` feature are guaranteed + * present once `luo_feature_hdr_validate()` succeeds, whereas fields of a + * `LUO_FEAT_OPTIONAL` feature should be accessed only after checking + * `luo_feature_is_active()`. All current fields (`pos`, `size`, `seals`, + * `flags`, `nr_folios`, `folios`) and `MEMFD_LUO_ALL_SEALS` belong to + * `MEMFD_LUO_FEAT_CORE` (`LUO_FEAT_REQUIRED` in `memfd_luo_features`). */ /** @@ -58,7 +68,7 @@ struct memfd_luo_folio_ser { /* * The set of seals this version supports preserving. If support for any new - * seals is needed, add it here and bump version. + * seals is needed, gate them behind a new feature flag. */ #define MEMFD_LUO_ALL_SEALS (F_SEAL_SEAL | \ F_SEAL_SHRINK | \ @@ -67,8 +77,22 @@ struct memfd_luo_folio_ser { F_SEAL_FUTURE_WRITE | \ F_SEAL_EXEC) +/** + * enum memfd_luo_feature - Feature indices for &struct memfd_luo_ser. + * @MEMFD_LUO_FEAT_CORE: Core feature covering `pos`, `size`, `seals`, `flags`, + * `nr_folios`, `folios`, and %MEMFD_LUO_ALL_SEALS. + * @MEMFD_LUO_FEAT_MAX: Number of defined &enum memfd_luo_feature features. + */ +enum memfd_luo_feature { + MEMFD_LUO_FEAT_CORE = 0, + MEMFD_LUO_FEAT_MAX, +}; + +static_assert((int)MEMFD_LUO_FEAT_MAX <= LUO_FEAT_MAX); + /** * struct memfd_luo_ser - Main serialization structure for a memfd. + * @features: ABI feature and size header for memfd_luo_ser. * @pos: The file's current position (f_pos). * @size: The total size of the file in bytes (i_size). * @seals: The seals present on the memfd. The seals are uABI so it is safe @@ -79,6 +103,7 @@ struct memfd_luo_folio_ser { * struct memfd_luo_folio_ser. */ struct memfd_luo_ser { + struct luo_feature_hdr features; u64 pos; u64 size; u32 seals; diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c index 64f8b8cb75bd..ea1f4c8cd259 100644 --- a/mm/memfd_luo.c +++ b/mm/memfd_luo.c @@ -84,6 +84,10 @@ #include "internal.h" +static const u8 memfd_luo_features[MEMFD_LUO_FEAT_MAX] = { + [MEMFD_LUO_FEAT_CORE] = LUO_FEAT_REQUIRED, +}; + static int memfd_luo_preserve_folios(struct file *file, struct kho_vmalloc *kho_vmalloc, struct memfd_luo_folio_ser **out_folios_ser, @@ -284,6 +288,8 @@ static int memfd_luo_preserve(struct liveupdate_file_op_args *args) goto err_free_ser; } + luo_feature_hdr_init(&ser->features, sizeof(*ser), + memfd_luo_features); ser->pos = args->file->f_pos; inode_size = i_size_read(inode); @@ -525,6 +531,14 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args) if (!ser) return -EINVAL; + err = luo_feature_hdr_validate(&ser->features, memfd_luo_features); + if (err) { + pr_err("memfd_luo_ser feature header validation failed (size=%llu, flags[0]=%#llx): %pe\n", + ser->features.size, ser->features.flags[0], + ERR_PTR(err)); + goto free_ser; + } + /* Make sure the file only has seals supported by this version. */ if (ser->seals & ~MEMFD_LUO_ALL_SEALS) { err = -EOPNOTSUPP; -- 2.56.0.212.g129ede20d9-goog