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[PATCH v4] efidisk: pass buffers with higher alignment

From: Stefan Agner
Subject: [PATCH v4] efidisk: pass buffers with higher alignment
Date: Tue, 31 May 2022 17:53:43 +0200

Some devices report IoAlign values but seem to require buffers with
higher alignment.

The UEFI specification is saying: "IoAlign values of 0 and 1 mean that
the buffer can be placed anywhere in memory. Otherwise, IoAlign must
be a power of 2, and the requirement is that the start address of a
buffer must be evenly divisible by IoAlign with no remainder."

Some devices report IoAlign of 2, however seem to require 4 bytes
aligned buffers. It seems that this got misinterpreted by some vendors
assuming IoAlign is 2^IoAlign. There is also such a hint in an example
in earlier versions of the Driver Writer's Guide:
ScsiPassThruMode.IoAlign = 2; // Data must be alligned on 4-byte boundary

Some devices report no alignment requirements at all but seem to read
corrupted data or report read errors when passing unaligned buffers.

Work around by using an alignment of at least BlockSize (typically 512
bytes) in any case. If IoAlign (interpreted as per UEFI specification)
requests a higher alignment than BlockSize, follow IoAlign still.

Note: The problem has only noticed with compressed squashfs. It seems
that ext4 (and presumably other file system drivers) pass buffers with
a higher alignment already.

Signed-off-by: Stefan Agner <>
This latest revision seems to work fine for Home Assistant OS 8.1.

Heinrich, since the logic changed a bit, I removed your Acked-by again.


 grub-core/disk/efi/efidisk.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/grub-core/disk/efi/efidisk.c b/grub-core/disk/efi/efidisk.c
index 562543b35..8ada2c45a 100644
--- a/grub-core/disk/efi/efidisk.c
+++ b/grub-core/disk/efi/efidisk.c
@@ -553,8 +553,19 @@ grub_efidisk_readwrite (struct grub_disk *disk, 
grub_disk_addr_t sector,
   d = disk->data;
   bio = d->block_io;
-  /* Set alignment to 1 if 0 specified */
-  io_align = bio->media->io_align ? bio->media->io_align : 1;
+  /*
+   * If IoAlign is > 1, it should represent the required alignment. However,
+   * some UEFI implementations seem to report IoAlign=2 but require 2^IoAlign.
+   * Some implementation seem to require alignment despite not reporting any
+   * specific requirements.
+   *
+   * Make sure to use buffers which are at least aligned to block size.
+   */
+  if (bio->media->io_align < bio->media->block_size)
+    io_align = bio->media->block_size;
+  else
+    io_align = bio->media->io_align;
   num_bytes = size << disk->log_sector_size;
   if ((grub_addr_t) buf & (io_align - 1))

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