b68024b2b
Denys Vlasenko
2026-10-04
Subject
fatlabel: do not require the 55,AA "bootable" signature
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util-linux/fatlabel.c+12 -3
| @@ -80,6 +80,12 @@ struct msdos_volume_info { // (offsets are relative to start of boot sector) | |
| uint32_t volume_id32; // 043 volume ID number | |
| char volume_label[11];// 047 volume label | |
| char fs_type[8]; // 052 typically "FATnn " | |
| + | // Wikipedia: "If both total logical sectors entries at offset 0x020 and 0x013 |
| + | // are 0 on volumes using a FAT32 EBPB with signature 0x29, volumes with more |
| + | // than 4,294,967,295 (232-1) sectors (f.e. some DR-DOS volumes with 32-bit |
| + | // cluster entries) can use this entry as 64-bit total logical sectors entry |
| + | // instead. In this case, the OEM label at sector offset 0x003 may be retrieved |
| + | // as new-style file system type instead." |
| } PACKED; // 05a end. Total size 26 (0x1a) bytes | |
| struct msdos_boot_sector { | |
| @@ -109,7 +115,7 @@ struct msdos_boot_sector { | |
| struct msdos_volume_info vi; // 040 | |
| char boot_code[0x200 - 0x5a - 2]; // 05a | |
| #define BOOT_SIGN 0xAA55 | |
| - | uint16_t boot_sign; // 1fe |
| + | uint16_t boot_sign; // 1fe 55,aa here means "bootable". Offset is the same even for sectors > 512 bytes |
| } PACKED; | |
| // Microsoft's EFI FAT32 specification states that any FAT file system with | |
| @@ -173,11 +179,14 @@ static int bad_fat32(const struct msdos_boot_sector *boot_blk) | |
| return ((uint8_t)(boot_blk->fats - 1) > 1 // 0 or >2 | |
| || boot_blk->fat16_sect_per_fat != 0 | |
| || boot_blk->dir_entries != 0 | |
| - | || boot_blk->vi.ext_boot_sign != 0x29 |
| - | || boot_blk->boot_sign != SWAP_LE16(BOOT_SIGN) |
| + | || boot_blk->vi.ext_boot_sign != 0x29 // can be 0x28 in valid FAT32, but then it has no volume_label[] at offset 0x47 |
| || bytes_per_sect16 != SWAP_LE16(SECTOR_SIZE) | |
| || boot_blk->fat32_info_sector != SWAP_LE16(0x0001) | |
| + | //maybe? || boot_blk->fat32_root_cluster < 2 |
| //WRONG: || boot_blk->fat32_backup_boot != SWAP_LE16(0x0006) | |
| + | //WRONG: || boot_blk->boot_sign != SWAP_LE16(BOOT_SIGN) |
| + | // ^^^^ non-bootable FAT32 partitions may well NOT have this signature. |
| + | // Linux does not check it on mount. |
| ); | |
| } | |