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/[email protected]
>
> Signed-off-by: Sascha Hauer <[email protected]>
Reviewed-by: Ahmad Fatoum <[email protected]>
> ---
> 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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