Re: [PATCH 5/6] partitions: gpt: fix GPT restauration from alternate GPT

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 12/3/25 4:19 PM, Sascha Hauer wrote:
> We read GPT headers from the device and store it in epd->gpt. When
> writing a partition table we assume that epd->gpt is read from
> the primary GPT header: We only adjust the partition_entry_lba for
> the alternate header. This assumption is wrong, in case the primary
> GPT header is corrupted epd->gpt could also come from the alternate
> GPT header. When this happens we have to set partition_entry_lba
> correctly for the primary GPT header as well.
> 
> Unfortunately this is not trivial. While we could just use LBA2 for
> the partition entries, this might overwrite a bootloader stored there (see
> [1]). Instead the following logic is applied:
> 
> When the primary GPT header was good when reading it, keep the partition
> table entries where they are. If not, take the first usable LBA and
> substract the size we need for the partition entries.
> 
> [1] https://lore.barebox.org/20250602-createnv-v1-2-c3239ff875d5@pengutronix.de
> 
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

> ---
>  common/partitions/efi.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/common/partitions/efi.c b/common/partitions/efi.c
> index 1766f0123353d502065673d02e983e6cdd51b9af..0fd60c9fd0442c3c571c685a5b0b41b8f33db7bd 100644
> --- a/common/partitions/efi.c
> +++ b/common/partitions/efi.c
> @@ -762,7 +762,36 @@ static int __efi_partition_write(struct efi_partition_desc *epd, bool primary)
>  
>  	if (primary) {
>  		my_lba = 1;
> -		partition_entry_lba = le64_to_cpu(gpt->partition_entry_lba);
> +
> +		if (epd->good_pgpt) {
> +			/*
> +			 * The primary GPT was good, we can just leave the partition
> +			 * entries where they are.
> +			 */
> +			partition_entry_lba = le64_to_cpu(gpt->partition_entry_lba);
> +		} else {
> +			/*
> +			 * The primary GPT was bad. When this is not a new partition
> +			 * table, but instead read from the disk, the header is from the
> +			 * alternate GPT, meaning the partition_entry_lba also points
> +			 * to the alternate partitions. We have to find a place for the
> +			 * primary partition entries in this case. We could store them
> +			 * right after the GPT header, but this might destroy a bootloader
> +			 * stored there. Instead, substract the size we need from the
> +			 * first usable LBA.
> +			 */
> +			uint64_t first_usable_lba;
> +
> +			first_usable_lba = le64_to_cpu(gpt->first_usable_lba);
> +
> +			if (first_usable_lba < 32 + 1 + 1) {
> +				pr_err("First usable LBA is %llu which doesn't leave "
> +				       "enough space for partition entries\n",
> +					first_usable_lba);
> +				return -EINVAL;
> +			}
> +			partition_entry_lba = first_usable_lba - 32;
> +		}
>  		gpt->alternate_lba = cpu_to_le64(last_lba(blk));
>  	} else {
>  		my_lba = last_lba(blk);
> 

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