28.5 UEFI Device Paths

The technique of using the EDK II library BaseLib functions to perform unaligned reads and writes is functional, but can become tedious if a large number of fields in data structures need to be accessed. In these cases, it may be necessary to copy a data structure from an unaligned source location to an aligned destination location so that the fields of the data structure can be accessed without generating an alignment fault. Two examples of this scenario are parsing UEFI device path nodes and parsing network packets.

The device path nodes in a UEFI device path are packed together so they take up as little space as possible when they are stored in environment variables such as ConIn, ConOut, StdErr, Boot####, and Driver####. As a result, individual device path nodes may not be aligned on a 64-bit boundary. UEFI device paths and UEFI device paths nodes may be passed around as opaque data structures, but whenever the fields of a UEFI device path node are accessed, the device path node must be copied to a location that is guaranteed to be on a 64-bit boundary. Likewise, network packets are packed so they take up as little space as possible. As each layer of a network packet is examined, the packet may need to be copied to a 64-bit aligned location before the individual fields of the packet are examined.

The following example shows an example of a function that parses a UEFI device path and extracts the 32-bit HID and UID from an ACPI device path node. This example generates an alignment fault if DevicePath is not aligned on a 32-bit boundary.

Example 247-UEFI device path node alignment fault

#include <Uefi.h>
#include <Protocol/DevicePath.h>
VOID
EFIAPI
GetAcpiHidUid (
EFI_DEVICE_PATH_PROTOCOL *DevicePath,
UINT32 *Hid,
UINT32 *Uid
)
{
ACPI_HID_DEVICE_PATH *AcpiDevicePath;
AcpiDevicePath = (ACPI_HID_DEVICE_PATH *)DevicePath;
//
// Wrong. May cause an alignment fault.
//
*Hid = AcpiDevicePath->HID;
//
// Wrong. May cause an alignment fault.
//
*Uid = AcpiDevicePath->UID;
}

Example 248, below, shows the corrected version of Example 247, above. Because the alignment of DevicePath cannot be guaranteed, the solution is to copy the ACPI device path node from DevicePath into an ACPI device path node structure that is declared as the local variable AcpiDevicePath. A structure declared as a local variable is guaranteed to be on a 64-bit boundary on IPF platforms. The fields of the ACPI device path node can then be safely accessed without generating an alignment fault.

Example 248-Corrected UEFI device path node alignment fault

#include <Uefi.h>
#include <Protocol/DevicePath.h>
#include <Library/BaseMemoryLib.h>
VOID
EFIAPI
GetAcpiHidUid (
EFI_DEVICE_PATH_PROTOCOL *DevicePath,
UINT32 *Hid,
UINT32 *Uid
)
{
ACPI_HID_DEVICE_PATH AcpiDevicePath;
CopyMem (&AcpiDevicePath, DevicePath, sizeof (ACPI_HID_DEVICE_PATH));
//
// Correct. Guaranteed not to generate an alignment fault.
//
*Hid = AcpiDevicePath.HID;
//
// Correct. Guaranteed not to generate an alignment fault.
//
*Uid = AcpiDevicePath.UID;
}