From nobody Thu Apr 9 12:00:54 2026 Received: from stravinsky.debian.org (stravinsky.debian.org [82.195.75.108]) (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 2EFD13C6A5C for ; Mon, 9 Mar 2026 13:42:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=82.195.75.108 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773063762; cv=none; b=QCYDSfTVkjKTOongAKXqtHDxktxbUZ3kf63k/7YgD0yCoTHLZIxgOclspVBIScHSpuP22N1TDr1VGPy5KKFI9Cdmc69N42wHvYBzwrk523sZguPEvC78+AgyxXj/UwroYAhJAUC2IqUr+VWmZFr3n8dz/eCPMjvIxhjotWiUs5g= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773063762; c=relaxed/simple; bh=wFUQv960bxV6n7TCJD4FMZFCiB/JTLz1Skv4YLrEGDo=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=ZFF2wNuVVUy2Gl9ijAXM1VE9DPwQKIrklkHQSKORDbDiFl4mOsYox2vhnaaUl0Dz4iQ9Krt0zqwyKLJEMfZT3c+1cK+ifd/0cCQtn2j06q+VBCf7ltqs8qn+DHMDAZcOWHZy2qyt7AEcHhrCCxQzmVwJYW7Ta8YnAEEbe0KPekQ= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=debian.org; spf=none smtp.mailfrom=debian.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b=FHCl0iht; arc=none smtp.client-ip=82.195.75.108 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=debian.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=debian.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b="FHCl0iht" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=debian.org; s=smtpauto.stravinsky; h=X-Debian-User:Cc:To:In-Reply-To:References: Message-Id:Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date: From:Reply-To:Content-ID:Content-Description; bh=blrIoTJexK9V0SlE9m9/WXlOitlTxjozBKun7UC+eh8=; b=FHCl0ihtv2OIcDfxgAcjQsZ13S bVrK4lbShDHCpY4HOHDe31fGqD+MOaqYf3HJ7OmAqhsUpH4+BAQMhGTBQu4+eI6rhKzskHaaVVdPS x9mV9YDX+NbL4UuTcA51fXuaTqIPHTospvcWEvNxzNRNBM1eWSGg21jrA0TWAvKdz4dy+fczghxT0 Z/rC61fvjGn8FclbAMdRIqihWnChq3jI+6NerOwUI6G6D/dOtSEaZ9jDkO/iQNt54EJ/Cu3/+UeRA 1VK++OJP8t60D5Lj4+uQYvhrrwjH5kuersshZWVI5wEib2AtpWJUL97dHbu+Xh6iqp9Q62PR9bAWz Mi3YgEBA==; Received: from authenticated user by stravinsky.debian.org with esmtpsa (TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.94.2) (envelope-from ) id 1vzasW-002G47-HZ; Mon, 09 Mar 2026 13:42:36 +0000 From: Breno Leitao Date: Mon, 09 Mar 2026 06:41:48 -0700 Subject: [PATCH v8 5/6] kho: kexec-metadata: track previous kernel chain 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="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260309-kho-v8-5-c3abcf4ac750@debian.org> References: <20260309-kho-v8-0-c3abcf4ac750@debian.org> In-Reply-To: <20260309-kho-v8-0-c3abcf4ac750@debian.org> To: Alexander Graf , Mike Rapoport , Pasha Tatashin , Pratyush Yadav Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, usamaarif642@gmail.com, Breno Leitao , SeongJae Park , kernel-team@meta.com X-Mailer: b4 0.15-dev-363b9 X-Developer-Signature: v=1; a=openpgp-sha256; l=6984; i=leitao@debian.org; h=from:subject:message-id; bh=wFUQv960bxV6n7TCJD4FMZFCiB/JTLz1Skv4YLrEGDo=; b=owEBbQKS/ZANAwAIATWjk5/8eHdtAcsmYgBprs4zTCZed02jaYw/bWAOKaoOVGmOZXpy/X8YT cQPE+vNHfOJAjMEAAEIAB0WIQSshTmm6PRnAspKQ5s1o5Of/Hh3bQUCaa7OMwAKCRA1o5Of/Hh3 bQbPD/9aJTG4urF72fJIr1nRxjL5YeMpUnN/2wI3qvPGXyebkyYTLLxD4GTqj7rLxkiqYrTtDRl t276tDdGD2Vl0S9X62sPgZagsEa2nMAyTmq03uZE/TXjM5dyVvKGIyZW1Y5GFgmLskD+t+uzrqE nuZYh78BaRoT2Ni0pnyUyL3vx0mSGW9wC2pdF3rFf7HrTM9AuVXFgHs0toqaQaPvZlda2YLLnfp j1ZcC9pBMeQhAl8vZUghV9QdRgfmEjsMJx/ybghubXZOS+I4nB5pqAo22+vu/3YhF1vK8M0d43e g1Spk+6JWdxlsPG3fu37hWB6U20KtmJPGikbDDjv5pgxs6kgg2JI6zFGp+QXBVXT6FNSc4YCn1T SuCXUhcRCAVXVmWM9tl4gC2JAa1dqe9V/SOK8dvu7gGVlsBd0ltaB7poT3PFPp/NrqFSMCdDG7j AXSKMy98e7jxuyi2bkfN/oCMGeN65GP2hYQT3Efa305saMqySSvVHbt6m8bSSIiIYAICaUk8O3H 7tVoxzfWlaxbQNmfJsoU1GPWNonwpZBWWE7jkWpalUOiccl+Esti2716A5CbTxxwI1zO+GzUYDf gtiwrWEm23A8t1rLFapudKYYH/8qnAgCEtzfI5RBC5wd4pvIlte29UdlqY0RYjvVrxCPdY5DFwX W5/izPj42rd01kA== X-Developer-Key: i=leitao@debian.org; a=openpgp; fpr=AC8539A6E8F46702CA4A439B35A3939FFC78776D X-Debian-User: leitao Use Kexec Handover (KHO) to pass the previous kernel's version string and the number of kexec reboots since the last cold boot to the next kernel, and print it at boot time. Example output: [ 0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1) Motivation =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Bugs that only reproduce when kexecing from specific kernel versions are difficult to diagnose. These issues occur when a buggy kernel kexecs into a new kernel, with the bug manifesting only in the second kernel. Recent examples include the following commits: * eb2266312507 ("x86/boot: Fix page table access in 5-level to 4-level pag= ing transition") * 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory for event log to avo= id corruption") * 64b45dd46e15 ("x86/efi: skip memattr table on kexec boot") As kexec-based reboots become more common, these version-dependent bugs are appearing more frequently. At scale, correlating crashes to the previous kernel version is challenging, especially when issues only occur in specific transition scenarios. Implementation =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D The kexec metadata is stored as a plain C struct (struct kho_kexec_metadata) rather than FDT format, for simplicity and direct field access. It is registered via kho_add_subtree() as a separate subtree, keeping it independent from the core KHO ABI. This design choice: - Keeps the core KHO ABI minimal and stable - Allows the metadata format to evolve independently - Avoids requiring version bumps for all KHO consumers (LUO, etc.) when the metadata format changes The struct kho_kexec_metadata contains two fields: - previous_release: The kernel version that initiated the kexec - kexec_count: Number of kexec boots since last cold boot On cold boot, kexec_count starts at 0 and increments with each kexec. The count helps identify issues that only manifest after multiple consecutive kexec reboots. Acked-by: SeongJae Park Reviewed-by: Mike Rapoport (Microsoft) Signed-off-by: Breno Leitao --- include/linux/kho/abi/kexec_handover.h | 31 ++++++++++++++ kernel/liveupdate/kexec_handover.c | 75 ++++++++++++++++++++++++++++++= ++++ 2 files changed, 106 insertions(+) diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi= /kexec_handover.h index 7e847a2339b09..832390f96f49c 100644 --- a/include/linux/kho/abi/kexec_handover.h +++ b/include/linux/kho/abi/kexec_handover.h @@ -14,6 +14,7 @@ #include #include #include +#include =20 #include =20 @@ -101,6 +102,36 @@ /* The FDT property for the size of preserved data blobs. */ #define KHO_SUB_TREE_SIZE_PROP_NAME "blob-size" =20 +/** + * DOC: Kexec Metadata ABI + * + * The "kexec-metadata" subtree stores optional metadata about the kexec c= hain. + * It is registered via kho_add_subtree(), keeping it independent from the= core + * KHO ABI. This allows the metadata format to evolve without affecting ot= her + * KHO consumers. + * + * The metadata is stored as a plain C struct rather than FDT format for + * simplicity and direct field access. + */ + +/** + * struct kho_kexec_metadata - Kexec metadata passed between kernels + * @previous_release: Kernel version string that initiated the kexec + * @kexec_count: Number of kexec boots since last cold boot + * + * This structure is preserved across kexec and allows the new kernel to + * identify which kernel it was booted from and how many kexec reboots + * have occurred. + * + * __NEW_UTS_LEN is part of uABI, so it safe to use it in here. + */ +struct kho_kexec_metadata { + char previous_release[__NEW_UTS_LEN + 1]; + u32 kexec_count; +} __packed; + +#define KHO_METADATA_NODE_NAME "kexec-metadata" + /** * DOC: Kexec Handover ABI for vmalloc Preservation * diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_h= andover.c index 1f22705d5d246..7bac80e9a29a4 100644 --- a/kernel/liveupdate/kexec_handover.c +++ b/kernel/liveupdate/kexec_handover.c @@ -18,6 +18,7 @@ #include #include #include +#include #include #include #include @@ -1285,6 +1286,8 @@ EXPORT_SYMBOL_GPL(kho_restore_free); struct kho_in { phys_addr_t fdt_phys; phys_addr_t scratch_phys; + char previous_release[__NEW_UTS_LEN + 1]; + u32 kexec_count; struct kho_debugfs dbg; }; =20 @@ -1408,6 +1411,74 @@ static __init int kho_out_fdt_setup(void) return err; } =20 +static void __init kho_in_kexec_metadata(void) +{ + struct kho_kexec_metadata *metadata; + phys_addr_t metadata_phys; + int err; + + err =3D kho_retrieve_subtree(KHO_METADATA_NODE_NAME, &metadata_phys, + NULL); + if (err) + /* This is fine, previous kernel didn't export metadata */ + return; + metadata =3D phys_to_virt(metadata_phys); + + /* + * Copy data to the kernel structure that will persist during + * kernel lifetime. + */ + kho_in.kexec_count =3D metadata->kexec_count; + strscpy(kho_in.previous_release, metadata->previous_release, + sizeof(kho_in.previous_release)); + + pr_info("exec from: %s (count %u)\n", kho_in.previous_release, + kho_in.kexec_count); +} + +/* + * Create kexec metadata to pass kernel version and boot count to the + * next kernel. This keeps the core KHO ABI minimal and allows the + * metadata format to evolve independently. + */ +static __init int kho_out_kexec_metadata(void) +{ + struct kho_kexec_metadata *metadata; + int err; + + metadata =3D kho_alloc_preserve(sizeof(*metadata)); + if (IS_ERR(metadata)) + return PTR_ERR(metadata); + + strscpy(metadata->previous_release, init_uts_ns.name.release, + sizeof(metadata->previous_release)); + /* kho_in.kexec_count is set to 0 on cold boot */ + metadata->kexec_count =3D kho_in.kexec_count + 1; + + err =3D kho_add_subtree(KHO_METADATA_NODE_NAME, metadata, + sizeof(*metadata)); + if (err) + kho_unpreserve_free(metadata); + + return err; +} + +static int __init kho_kexec_metadata_init(const void *fdt) +{ + int err; + + if (fdt) + kho_in_kexec_metadata(); + + /* Populate kexec metadata for the possible next kexec */ + err =3D kho_out_kexec_metadata(); + if (err) + pr_warn("failed to initialize kexec-metadata subtree: %d\n", + err); + + return err; +} + static __init int kho_init(void) { struct kho_radix_tree *tree =3D &kho_out.radix_tree; @@ -1441,6 +1512,10 @@ static __init int kho_init(void) if (err) goto err_free_fdt; =20 + err =3D kho_kexec_metadata_init(fdt); + if (err) + goto err_free_fdt; + for (int i =3D 0; i < kho_scratch_cnt; i++) { unsigned long base_pfn =3D PHYS_PFN(kho_scratch[i].addr); unsigned long count =3D kho_scratch[i].size >> PAGE_SHIFT; --=20 2.47.3