Read Files Using UEFI Simple File System Protocol

Topic Overview

UEFI file I/O

The Simple File System protocol exposes FAT volumes on the EFI System Partition. Open the volume with OpenVolume, then Open a file with EFI_FILE_MODE_READ, and use Read in a loop until bytes returned is zero. For example, loading \\\\kernel.elf means wide-character paths like L"\\\\kernel.elf" on the root directory handle.

Read workflow

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• Handles: Device → volume → file hierarchy via boot services
• Paths: Backslashes and UTF-16 names, not POSIX forward slashes
• Attributes: Check EFI_FILE_INFO for size before allocating buffers
• After ExitBootServices, file protocols are invalid

Protocol locating

Locate the Simple File System protocol on the handle that represents the ESP volume, not the disk handle. OpenRoot on the loaded image's device handle is a common shortcut in small loaders. Always close file and volume handles to avoid leaking pool memory before exit boot services. Status EFI_BUFFER_TOO_SMALL on GetInfo tells you how large the file info buffer must be.

Why for this exercise

The file metadata behind every GetInfo call lives in 32-byte FAT directory entries on the ESP. You will decode one raw entry exactly as the Simple File System does: extract the 8.3 name, attribute byte, little-endian 32-bit first cluster (high word at offset 20, low word at 26) and file size (offset 28), and recognize deleted/LFN/end markers. This exercise requires reading the on-disk structure the protocol wraps.

Your Task

UEFI's Simple File System protocol reads FAT, where every file is a 32-byte directory entry. Decode one entry.

Check the markers first, in this order, and stop at the first that matches:

Otherwise it is a file or directory:

• Name: bytes 0-7 with trailing spaces removed. If any of bytes 8-10 is not a space, add . and the extension with trailing spaces removed: KERNEL ELF becomes KERNEL.ELF, and EFI plus three spaces stays EFI.
• Attributes: byte 11. Bit 0x10 marks a directory.
• First cluster: (bytes 20-21) << 16 | (bytes 26-27), each half little-endian.
• Size: bytes 28-31, little-endian.

Input

32 bytes in hexadecimal.

Output

For a file or directory:

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with an extra line directory after attr when bit 0x10 is set. The attribute is 2 uppercase hex digits. For a marker, only its single line.