Re: [PATCH v4 01/19] liveupdate, kho: Remove __packed from KHO, LUO and memfd ABI structures

["Pasha Tatashin" <pasha.tatashin@soleen.com>]

On Tue Nov 10, 2026 at 12:01 AM EST, Tarun Sahu wrote:
> All structures in include/linux/kho/abi/block.h (struct
> kho_block_header_ser), include/linux/kho/abi/luo.h (struct luo_ser,
> struct luo_file_ser, struct luo_file_set_ser, struct luo_session_ser,
> struct luo_flb_header_ser, struct luo_flb_ser) and
> include/linux/kho/abi/memfd.h (struct memfd_luo_folio_ser, struct
> memfd_luo_ser) have naturally aligned fields on 64-bit architectures,
> with no padding holes between fields or tail padding. Assert both the
> structure size and 8-byte alignment under CONFIG_64BIT at compile time.
> Any future unaligned field addition should come with reserved field to
> keep the structure aligned.

Just state:

Remove __packed, as it may break natural alignment. Instead, ensure that 
all padding holes are explicitly covered by reserved or similar fields, 
as is already done for KHO/LUO serialized data, so no functional changes 
are intended by this change.

>
> Signed-off-by: Tarun Sahu <tarunsahu@google.com>
> ---
>  include/linux/kho/abi/block.h | 10 +++++++--
>  include/linux/kho/abi/luo.h   | 38 +++++++++++++++++++++++++++++------
>  include/linux/kho/abi/memfd.h | 16 ++++++++++++---
>  3 files changed, 53 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/kho/abi/block.h b/include/linux/kho/abi/block.h
> index d06d64b963be..36bdbaf9b3b8 100644
> --- a/include/linux/kho/abi/block.h
> +++ b/include/linux/kho/abi/block.h
> @@ -12,7 +12,7 @@
>   * pre-update kernel to a post-update kernel.
>   *
>   * This interface is a contract. Any modification to the structure fields,
> - * compatible strings, or the layout of the `__packed` serialization
> + * compatible strings, or the layout of the serialization
>   * structures defined here constitutes a breaking change. Such changes require
>   * incrementing the version number in the `KHO_FDT_COMPATIBLE` string to
>   * prevent a new kernel from misinterpreting data from an old kernel.
> @@ -26,6 +26,8 @@
>  #define _LINUX_KHO_ABI_BLOCK_H
>  
>  #include <asm/page.h>
> +#include <linux/build_bug.h>
> +#include <linux/kconfig.h>
>  #include <linux/types.h>
>  
>  /**
> @@ -49,6 +51,10 @@
>  struct kho_block_header_ser {
>  	u64 next;
>  	u64 count;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct kho_block_header_ser) == 16);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct kho_block_header_ser) == 8);
>  
>  #endif /* _LINUX_KHO_ABI_BLOCK_H */
> diff --git a/include/linux/kho/abi/luo.h b/include/linux/kho/abi/luo.h
> index 288076de6d4a..721949374fa1 100644
> --- a/include/linux/kho/abi/luo.h
> +++ b/include/linux/kho/abi/luo.h
> @@ -58,6 +58,8 @@
>  #define _LINUX_KHO_ABI_LUO_H
>  
>  #include <linux/align.h>
> +#include <linux/build_bug.h>
> +#include <linux/kconfig.h>
>  #include <linux/kho/abi/block.h>
>  #include <uapi/linux/liveupdate.h>
>  
> @@ -82,7 +84,11 @@ struct luo_ser {
>  	u64 liveupdate_num;
>  	u64 sessions_pa;
>  	u64 flbs_pa;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_ser) == 32);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_ser) == 8);

Please remove the two assertions above throughout this patch, as they 
are replaced later in the series. In general, introducing add -> remove 
-> add churn across a patch series is discouraged, as it indicates a 
lack of planning.

>  
>  #define LIVEUPDATE_HNDL_COMPAT_LENGTH	48
>  
> @@ -98,7 +104,11 @@ struct luo_file_ser {
>  	char compatible[LIVEUPDATE_HNDL_COMPAT_LENGTH];
>  	u64 data;
>  	u64 token;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_file_ser) == 64);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_file_ser) == 8);
>  
>  /**
>   * struct luo_file_set_ser - Represents the serialized metadata for file set
> @@ -113,7 +123,11 @@ struct luo_file_ser {
>  struct luo_file_set_ser {
>  	u64 files;
>  	u64 count;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_file_set_ser) == 16);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_file_set_ser) == 8);
>  
>  /**
>   * struct luo_session_ser - Represents the serialized metadata for a LUO session.
> @@ -131,7 +145,11 @@ struct luo_file_set_ser {
>  struct luo_session_ser {
>  	char name[LIVEUPDATE_SESSION_NAME_LENGTH];
>  	struct luo_file_set_ser file_set_ser;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_session_ser) == 80);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_session_ser) == 8);
>  
>  /* The max size is set so it can be reliably used during in serialization */
>  #define LIVEUPDATE_FLB_COMPAT_LENGTH	48
> @@ -152,7 +170,11 @@ struct luo_session_ser {
>  struct luo_flb_header_ser {
>  	u64 pgcnt;
>  	u64 count;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_flb_header_ser) == 16);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_flb_header_ser) == 8);
>  
>  /**
>   * struct luo_flb_ser - Represents the serialized state of a single FLB object.
> @@ -175,7 +197,11 @@ struct luo_flb_ser {
>  	char name[LIVEUPDATE_FLB_COMPAT_LENGTH];
>  	u64 data;
>  	u64 count;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct luo_flb_ser) == 64);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct luo_flb_ser) == 8);
>  
>  /* Kernel Live Update Test ABI */
>  #ifdef CONFIG_LIVEUPDATE_TEST
> diff --git a/include/linux/kho/abi/memfd.h b/include/linux/kho/abi/memfd.h
> index 08b10fea2afc..376a724a1d8c 100644
> --- a/include/linux/kho/abi/memfd.h
> +++ b/include/linux/kho/abi/memfd.h
> @@ -11,6 +11,8 @@
>  #ifndef _LINUX_KHO_ABI_MEMFD_H
>  #define _LINUX_KHO_ABI_MEMFD_H
>  
> +#include <linux/build_bug.h>
> +#include <linux/kconfig.h>
>  #include <linux/types.h>
>  #include <linux/kho/abi/kexec_handover.h>
>  
> @@ -20,7 +22,7 @@
>   * memfd uses the ABI defined below for preserving its state across a kexec
>   * reboot using the LUO.
>   *
> - * The state is serialized into a packed structure `struct memfd_luo_ser`
> + * The state is serialized into `struct memfd_luo_ser`
>   * which is handed over to the next kernel via the KHO mechanism.
>   *
>   * This interface is a contract. Any modification to the structure layout
> @@ -54,7 +56,11 @@ struct memfd_luo_folio_ser {
>  	u64 pfn:52;
>  	u64 flags:12;
>  	u64 index;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct memfd_luo_folio_ser) == 16);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct memfd_luo_folio_ser) == 8);
>  
>  /*
>   * The set of seals this version supports preserving. If support for any new
> @@ -85,7 +91,11 @@ struct memfd_luo_ser {
>  	u32 flags;
>  	u64 nr_folios;
>  	struct kho_vmalloc folios;
> -} __packed;
> +};
> +
> +static_assert(sizeof(struct memfd_luo_ser) == 48);
> +static_assert(!IS_ENABLED(CONFIG_64BIT) ||
> +	      __alignof__(struct memfd_luo_ser) == 8);
>  
>  /* The compatibility string for memfd file handler */
>  #define MEMFD_LUO_FH_COMPATIBLE	"memfd-v2"

-- 
Pasha