From mboxrd@z Thu Jan 1 00:00:00 1970 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 C70AF2BEC5E for ; Mon, 26 Jan 2026 13:47:16 +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=1769435238; cv=none; b=Aq4lCzo+XfA7IkfvuThhln7XsfKCmGrGtuHCPdOOV8IzVqrr4dqB0vqDyx6QnCo2Y+6ju+VMTn2zM8pcaJIdqvajEpmGOwBCCPzXUfTgYSFwCrUN4maUOg/YNH/lnd89OSnizqNJbvktJSznl/YfLVXgK+1h0zSiKcFRKZ9rhs0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769435238; c=relaxed/simple; bh=H44SNeGBzs0DGr6ApSWZsEITzlvwMWzHy/a1JJE8GTM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cfrfLbpUxMDi9R40J3vx8HOJWrvcRHqVxW4Pwv3E8MLcEylrdLYpseYBk1Xs/qOeZgEtscA83fC2ebzBUemrfy3CnEzRC1KJ6ew+P+OaJSSEu6H92vevKy8P/rXavsvlU35o5XGM2SM0NaJJMvpDvgqzrD3gAo4h/ZpJ1Vfz/5w= 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=PTW8Tqhg; 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="PTW8Tqhg" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=debian.org; s=smtpauto.stravinsky; h=X-Debian-User:In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=aa1DMzjW5ihMgg92YYEEvi+hVWZtO1QQ1kl6+AaoZvQ=; b=PTW8TqhgFvvUWcRC3HJM028fcP M7Vxisjv/gGMNmxFO/SLOgwXfepfwBg8TfKn2bhZf1AVDlJkcvSDMbJLwOpj10bTWKE3kluom+K+N /L0DHiSRwa7TItLibUQbsjIgw5OQ5QBdu/vN9mLeWAu9Bt2yyDrmdLewyCBzHo1Q9fkJfSQdJfOka MCN20eKZFSUeIVHI1U2iyeA/KMrVnyztbocEcxEPWBtKFAZcXb76eg90AIrS+9ZSs2PscROltDqCp YsKRS5aiepm+1WZlSQvL1/1oDp0bVAyFumjQytrDAxjxZyw7mUrB54GxuaxhmlLkyF1ke7ddLpjHE Ynk/aOxg==; 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 1vkMvr-00GDT2-Op; Mon, 26 Jan 2026 13:47:08 +0000 Date: Mon, 26 Jan 2026 05:47:02 -0800 From: Breno Leitao To: Pratyush Yadav Cc: Alexander Graf , Mike Rapoport , Pasha Tatashin , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, usamaarif642@gmail.com, rmikey@meta.com, clm@fb.com, riel@surriel.com, kernel-team@meta.com, SeongJae Park Subject: Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain Message-ID: References: <20260121-kho-v4-1-5c8fe77b6804@debian.org> <2vxzikcoa4g1.fsf@kernel.org> 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=us-ascii Content-Disposition: inline In-Reply-To: <2vxzikcoa4g1.fsf@kernel.org> X-Debian-User: leitao On Mon, Jan 26, 2026 at 02:28:30PM +0100, Pratyush Yadav wrote: > > On Wed, Jan 21, 2026 at 06:50:38AM -0800, Breno Leitao wrote: > >> +static __init int kho_populate_kexec_metadata(void) > >> +{ > >> + struct kho_kexec_metadata *metadata; > >> + int err; > >> + > >> + metadata = 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 = kho_in.kexec_count + 1; > >> + > >> + err = kho_add_subtree(KHO_METADATA_NODE_NAME, metadata); > > > > There is a hidden bug in here when CONFIG_KEXEC_HANDOVER_DEBUGFS is set. > > Good catch! > > > > > kho_add_subtree() expects a fdt as the second argument, and we are > > passing a pure C struct. That works fine, except for debugfs, which > > does: > > > > 1. kho_add_subtree() calls kho_debugfs_fdt_add() > > 2. kho_debugfs_fdt_add() calls __kho_debugfs_fdt_add() > > 3. __kho_debugfs_fdt_add() executes fdt_totalsize(fdt) > > > > The fdt_totalsize() macro reads bytes 4-7 of the input as a big-endian u32, and > > this will hit struct kho_kexec_metadata, given I am passing a C struct instead > > of a FDT. > > > > struct kho_kexec_metadata { > > char previous_release[__NEW_UTS_LEN + 1]; // 65 bytes > > u32 kexec_count; > > } __packed; > > > > Bytes 4-7 would be characters from previous_release (e.g., "0-rc" from > > "6.19.0-rc4..."). Interpreted as big-endian u32, this gives a garbage size > > value. > > > > The alternatives I see here are: > > > > 1) Come back to FDT instead of plain C struct, similarly to the previous > > version [1] > > 2) Created some helpers to treat C struct fields specially just for this > > feature, and we can do it later if we have more users. > > 3) Move this kexec_metadata to work on top of LUO (similarly to memfd), but > > that would be an unnecessary dependency just to have this kexec_metadata. > > > > That said, for the next version, I am coming back to to FDT. > > Please, no. Don't go back to it just for the sake of this bug. > > I think KHO's assumption that the subtree will always point to an FDT is > broken, and we should fix that. I think KHO should expose the blob of > serialized data and let userspace figure out what the format is and how > to decode it. > > To do that, we would need to update kho_add_subtree() to take a size > parameter from callers, and pass that down to debugfs code. I count 3 > callers of kho_add_subtree() - memblock, LUO, and test_kho. I think all > 3 should be fairly easy to update, but I am happy to help out if you > need. Sure, let me hack and see what I can get here. Thanks for the direction, --breno