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

[Breno Leitao <leitao@debian.org>]

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.

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.

Link: https://lore.kernel.org/all/20260108-kho-v3-0-b1d6b7a89342@debian.org/ [1]

--breno