linux-kernel - Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain

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Message-ID: <aXdwObCI5z6qbuwE@gmail.com>
Date: Mon, 26 Jan 2026 05:47:02 -0800
From: Breno Leitao <leitao@...ian.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>, 
	Pasha Tatashin <pasha.tatashin@...een.com>, linux-kernel@...r.kernel.org, kexec@...ts.infradead.org, 
	linux-mm@...ck.org, usamaarif642@...il.com, rmikey@...a.com, clm@...com, 
	riel@...riel.com, kernel-team@...a.com, SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain

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