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30d2839a7

Denys Vlasenko

2026-09-24

SUCCESS Duration: 183s

Subject

fatlabel: new applet

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Architecture & Compiler Flags

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

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Commit Diff GitHub →

include/libbb.h +2 -0 util-linux/fatlabel.c +258 -0 util-linux/mkfs_vfat.c +5 -5 util-linux/swaplabel.c +1 -2

include/libbb.h+2 -0

@@ -2609,6 +2609,8 @@ do { \
	BUG_wrong_field_size(); \
	} while (0)

+	#define FETCH_LE16(field) \
+	(sizeof(field) == 2 ? SWAP_LE16(field) : BUG_wrong_field_size())
	#define FETCH_LE32(field) \
	(sizeof(field) == 4 ? SWAP_LE32(field) : BUG_wrong_field_size())

util-linux/fatlabel.c+258 -0

@@ -0,0 +1,258 @@
+	/* vi: set sw=4 ts=4: */
+	/*
+	* Copyright 2026 Denys Vlasenko <[email protected]>
+	*
+	* Licensed under GPLv2, see file LICENSE in this source tree.
+	*/
+	//config:config FATLABEL
+	//config: bool "fatlabel (1 kb)"
+	//config: default y
+	//config: help
+	//config: fatlabel shows or sets FAT33 label.
+
+	//applet:IF_FATLABEL(APPLET(fatlabel, BB_DIR_SBIN, BB_SUID_DROP))
+
+	//kbuild:lib-$(CONFIG_FATLABEL) += fatlabel.o
+
+	//usage:#define fatlabel_trivial_usage
+	//usage: "BLOCKDEV [LABEL]"
+	//usage:#define fatlabel_full_usage "\n\n"
+	//usage: "Show or set FAT32 label (11 bytes at offset 0x47) on BLOCKDEV"
+
+	#include "libbb.h"
+	#include "common_bufsiz.h"
+
+	#define SECTOR_SIZE 512
+
+	#define ATTR_VOLUME 8
+
+	#define NUM_FATS 2
+
+	// FAT32 filesystem looks like this:
+	// sector -nn...-1: "hidden" sectors, all sectors before this partition
+	// (-h hidden-sectors sets it. Useful only for boot loaders,
+	// they need to know _disk_ offset in order to be able to correctly
+	// address sectors relative to start of disk)
+	// sector 0: boot sector
+	// sector 1: info sector
+	// sector 2: set aside for boot code which didn't fit into sector 0
+	// ...(zero-filled sectors)...
+	// sector B: backup copy of sector 0 [B set by -b backup-boot-sector]
+	// sector B+1: backup copy of sector 1
+	// sector B+2: backup copy of sector 2
+	// ...(zero-filled sectors)...
+	// sector R: FAT#1 [R set by -R reserved-sectors]
+	// ...(FAT#1)...
+	// sector R+fat_size: FAT#2
+	// ...(FAT#2)...
+	// sector R+fat_size*2: cluster #2
+	// ...(cluster #2)...
+	// sector R+fat_size*2+clust_size: cluster #3
+	// ...(the rest is filled by clusters till the end)...
+
+	struct msdos_dir_entry {
+	char name[11]; // 000 name and extension
+	uint8_t attr; // 00b attribute bits
+	uint8_t lcase; // 00c case for base and extension
+	uint8_t ctime_cs; // 00d creation time, centiseconds (0-199)
+	uint16_t ctime; // 00e creation time
+	uint16_t cdate; // 010 creation date
+	uint16_t adate; // 012 last access date
+	uint16_t starthi; // 014 high 16 bits of cluster in FAT32
+	uint16_t time; // 016 time
+	uint16_t date; // 018 date
+	uint16_t start; // 01a first cluster
+	uint32_t size; // 01c file size in bytes
+	} PACKED;
+
+	/* Example of boot sector's beginning:
+	0000 eb 58 90 4d 53 57 49 4e 34 2e 31 00 02 08 26 00 \|...MSWIN4.1...&.\|
+	0010 02 00 00 00 00 f8 00 00 3f 00 ff 00 3f 00 00 00 \|........?...?...\|
+	0020 54 9b d0 00 0d 34 00 00 00 00 00 00 02 00 00 00 \|T....4..........\|
+	0030 01 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 \|................\|
+	0040 80 00 29 71 df 51 e0 4e 4f 20 4e 41 4d 45 20 20 \|..)q.Q.NO NAME \|
+	0050 20 20 46 41 54 33 32 20 20 20 33 c9 8e d1 bc f4 \| FAT32 3.....\|
+	*/
+	struct msdos_volume_info { // (offsets are relative to start of boot sector)
+	uint8_t drive_number; // 040 BIOS drive number
+	uint8_t reserved; // 041 unused
+	uint8_t ext_boot_sign; // 042 0x29 if fields below exist (DOS 3.3+)
+	uint32_t volume_id32; // 043 volume ID number
+	char volume_label[11];// 047 volume label
+	char fs_type[8]; // 052 typically "FATnn"
+	} PACKED; // 05a end. Total size 26 (0x1a) bytes
+
+	struct msdos_boot_sector {
+	// We use strcpy to fill both, and gcc-4.4.x complains if they are separate
+	char boot_jump_and_sys_id[3+8]; //000 short or near jump instruction
+	/char system_id[8];/ // 003 name - can be used to special case partition manager volumes
+	uint16_t bytes_per_sect; // 00b bytes per logical sector
+	uint8_t sect_per_clust; // 00d sectors/cluster
+	uint16_t reserved_sect; // 00e reserved sectors (sector offset of 1st FAT relative to volume start)
+	uint8_t fats; // 010 number of FATs
+	uint16_t dir_entries; // 011 root directory entries
+	uint16_t volume_size_sect; // 013 volume size in sectors
+	uint8_t media_byte; // 015 media code
+	uint16_t fat16_sect_per_fat; // 016 sectors/FAT, must be 0 for FAT32
+	uint16_t sect_per_track; // 018 sectors per track
+	uint16_t heads; // 01a number of heads
+	uint32_t hidden; // 01c hidden sectors (sector offset of volume within physical disk)
+	uint32_t fat32_volume_size_sect; // 020 volume size in sectors (if volume_size_sect == 0)
+	uint32_t fat32_sect_per_fat; // 024 sectors/FAT
+	uint16_t fat32_flags; // 028 bit 8: fat mirroring, low 4: active fat
+	uint8_t fat32_version[2]; // 02a major, minor filesystem version (I see 0,0)
+	uint32_t fat32_root_cluster; // 02c first cluster in root directory
+	uint16_t fat32_info_sector; // 030 filesystem info sector (usually 1)
+	uint16_t fat32_backup_boot; // 032 backup boot sector (usually 6)
+	uint32_t reserved2[3]; // 034 unused
+	struct msdos_volume_info vi; // 040
+	char boot_code[0x200 - 0x5a - 2]; // 05a
+	#define BOOT_SIGN 0xAA55
+	uint16_t boot_sign; // 1fe
+	} PACKED;
+
+	#define FAT_FSINFO_SIG1 0x41615252
+	#define FAT_FSINFO_SIG2 0x61417272
+	struct fat32_fsinfo {
+	uint32_t signature1; // 0x52,0x52,0x41,0x61, "RRaA"
+	uint32_t reserved1[128 - 8];
+	uint32_t signature2; // 0x72,0x72,0x61,0x41, "rrAa"
+	uint32_t free_clusters; // free cluster count. -1 if unknown
+	uint32_t next_cluster; // most recently allocated cluster
+	uint32_t reserved2[3];
+	uint16_t reserved3; // 1fc
+	uint16_t boot_sign; // 1fe
+	} PACKED;
+
+	struct bug_check {
+	char BUG1[sizeof(struct msdos_dir_entry ) == 0x20 ? 1 : -1];
+	char BUG2[sizeof(struct msdos_volume_info) == 0x1a ? 1 : -1];
+	char BUG3[sizeof(struct msdos_boot_sector) == 0x200 ? 1 : -1];
+	char BUG4[sizeof(struct fat32_fsinfo ) == 0x200 ? 1 : -1];
+	};
+
+	// compat: dosfstools-4.2
+	// $ fatlabel --version
+	// fatlabel 4.2 (2021-01-31)
+	// $ fatlabel --help
+	// Usage: fatlabel [OPTIONS] DEVICE [NEW]
+	// Change the FAT filesystem label or serial on DEVICE to NEW or display the
+	// existing label or serial if NEW is not given.
+	// Options:
+	// -i, --volume-id Work on serial number instead of label
+	// -r, --reset Remove label or generate new serial number
+	// -c N, --codepage=N use DOS codepage N to encode/decode label (default: 850)
+	// -V, --version Show version number and terminate
+	// -h, --help Print this message and terminate
+	static int bad_fat32(const struct msdos_boot_sector *boot_blk)
+	{
+	uint16_t bytes_per_sect16;
+
+	move_from_unaligned16(bytes_per_sect16, &boot_blk->bytes_per_sect);
+	return ((uint8_t)(boot_blk->fats - 1) > 1 // 0 or >2
+	\|\| boot_blk->fat16_sect_per_fat != 0
+	// maybe also check boot_blk->dir_entries == 0? It's only for FAT16/12
+	\|\| boot_blk->vi.ext_boot_sign != 0x29
+	\|\| boot_blk->boot_sign != SWAP_LE16(BOOT_SIGN)
+	\|\| bytes_per_sect16 != SWAP_LE16(SECTOR_SIZE)
+	\|\| boot_blk->fat32_info_sector != SWAP_LE16(0x0001)
+	//WRONG: \|\| boot_blk->fat32_backup_boot != SWAP_LE16(0x0006)
+	);
+	}
+
+	int fatlabel_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
+	int fatlabel_main(int argc UNUSED_PARAM, char **argv)
+	{
+	char sector1[1 * SECTOR_SIZE];
+	char sector2[1 * SECTOR_SIZE];
+	struct msdos_boot_sector boot_blk = (void)sector1;
+	unsigned backup_boot_sect;
+	int fd;
+
+	getopt32(argv, "^" "\0" "-1?2"/* 1 or 2 arg2*/);
+	argv += optind;
+
+	fd = xopen(argv[0], argv[1] ? O_RDWR : O_RDONLY);
+
+	xread(fd, sector1, sizeof(sector1));
+	backup_boot_sect = FETCH_LE16(boot_blk->fat32_backup_boot);
+	#if 0
+	fprintf(stderr,
+	"0x003: sys_id:'%.8s'\n"
+	"0x00b: bytes_per_sect: 0x%04x\n" // 0
+	"0x00d: sect_per_clust: 0x%02x\n" // 0
+	"0x010: fats: 0x%02x\n" // 1 or 2
+	"0x011: dir_entries: 0x%04x\n" // always 0 for FAT32?
+	"0x015: media_byte: 0x%02x\n" // 0xf8 for "hard disk"
+	"0x016: fat16_sect_per_fat: 0x%04x\n" // 0
+	"0x028: fat32_flags: 0x%04x\n" // 0,0
+	"0x02a: fat32_version: 0x%02x,0x%02x\n" // 0,0
+	"0x030: fat32_info_sector: 0x%04x\n" // always 1?
+	"0x032: fat32_backup_boot: 0x%04x\n" // usually 6 by microsoft, but our own mkfs.vfat uses 3
+	"0x041: mounted: 0x%02x\n" // 0/1
+	"0x042: ext_boot_sign: 0x%02x\n" // 0x29
+	"0x043: volume_id32: 0x%08x\n"
+	"0x047: volume_label: '%.11s'\n"
+	"0x052: fs_type: '%.8s'\n"
+	"0x1fe: boot_sign: 0x%02x,0x%02x\n" // 0x55,0xaa
+	, 3 + boot_blk->boot_jump_and_sys_id
+	, FETCH_LE16(boot_blk->bytes_per_sect) // FIXME: unaligned fetch
+	, boot_blk->sect_per_clust
+	, boot_blk->fats
+	, FETCH_LE16(boot_blk->dir_entries) // FIXME: unaligned fetch
+	, boot_blk->media_byte
+	, FETCH_LE16(boot_blk->fat16_sect_per_fat)
+	, FETCH_LE16(boot_blk->fat32_flags)
+	, boot_blk->fat32_version[0], boot_blk->fat32_version[1]
+	, FETCH_LE16(boot_blk->fat32_info_sector)
+	, backup_boot_sect
+	, boot_blk->vi.reserved // Linux uses this as "mounted now" bit, some WinNT too?
+	, boot_blk->vi.ext_boot_sign
+	, FETCH_LE32(boot_blk->vi.volume_id32) // FIXME: unaligned fetch
+	, boot_blk->vi.volume_label
+	, boot_blk->vi.fs_type
+	, ((uint8_t*)&boot_blk->boot_sign)[0]
+	, ((uint8_t*)&boot_blk->boot_sign)[1]
+	);
+	#endif
+	if (bad_fat32(boot_blk))
+	bb_error_msg_and_die("'%s' is not FAT32", argv[0]);
+
+	// backup_boot_sect can be 0 or 0xffff for 'disabled'.
+	// 1 is almost certainly bogus (we require that fat32_info_sector is 1).
+	// Let's also not accept suspiciously large values.
+	if (backup_boot_sect > 1 && backup_boot_sect < 0x100) {
+	xlseek(fd, backup_boot_sect * SECTOR_SIZE, SEEK_SET);
+	xread(fd, sector2, sizeof(sector2));
+	if (boot_blk->vi.reserved == 1) // "mounted now"?
+	boot_blk->vi.reserved = 0;
+	if (memcmp(sector1, sector2, sizeof(sector1)) != 0)
+	bb_error_msg_and_die("'%s' backup sector %d mismatch", argv[0], backup_boot_sect);
+	} else {
+	backup_boot_sect = 0;
+	}
+
+	if (argv[1]) {
+	// dosfstools-4.2 warns:
+	// "fatlabel: warning - lowercase labels might not work properly on some systems"
+	sprintf(boot_blk->vi.volume_label, "%-11.11s", argv[1]);
+	xlseek(fd, 0x047, SEEK_SET);
+	xwrite(fd, boot_blk->vi.volume_label, 11);
+	// Redundant is backup_boot_sect == 0, but no harm done:
+	xlseek(fd, 0x047 + backup_boot_sect * SECTOR_SIZE, SEEK_SET);
+	xwrite(fd, boot_blk->vi.volume_label, 11);
+	} else {
+	// Mimic dosfstools-4.2
+	// But do not special-case "NO NAME" label as 'label not set'.
+	int sz = strnlen(boot_blk->vi.volume_label, 11);
+	while (sz != 0 && boot_blk->vi.volume_label[sz - 1] == ' ')
+	sz--;
+	if (sz != 0)
+	printf("%.*s\n", sz, boot_blk->vi.volume_label);
+	}
+
+	if (ENABLE_FEATURE_CLEAN_UP)
+	close(fd);
+
+	fflush_stdout_and_exit_SUCCESS();
+	}

util-linux/mkfs_vfat.c+5 -5

@@ -142,7 +142,7 @@ struct msdos_boot_sector {
	uint16_t dir_entries; /* 011 root directory entries */
	uint16_t volume_size_sect; /* 013 volume size in sectors */
	uint8_t media_byte; /* 015 media code */
-	uint16_t sect_per_fat; /* 016 sectors/FAT */
+	uint16_t fat16_sect_per_fat; /* 016 sectors/FAT, must be 0 for FAT32 */
	uint16_t sect_per_track; /* 018 sectors per track */
	uint16_t heads; /* 01a number of heads */
	uint32_t hidden; /* 01c hidden sectors (sector offset of volume within physical disk) */
@@ -489,7 +489,7 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
	strcpy(boot_blk->boot_jump_and_sys_id, "\xeb\x58\x90" "mkdosfs");
	STORE_LE(boot_blk->bytes_per_sect, bytes_per_sect);
	STORE_LE(boot_blk->sect_per_clust, sect_per_clust);
-	// cast in needed on big endian to suppress a warning
+	// cast is needed on big endian to suppress a warning
	STORE_LE(boot_blk->reserved_sect, (uint16_t)reserved_sect);
	STORE_LE(boot_blk->fats, 2);
	//STORE_LE(boot_blk->dir_entries, 0); // for FAT32, stays 0
@@ -497,10 +497,10 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
	STORE_LE(boot_blk->volume_size_sect, volume_size_sect);
	STORE_LE(boot_blk->media_byte, media_byte);
	// wrong: this would make Linux think that it's fat12/16:
-	//if (sect_per_fat <= 0xffff)
-	// STORE_LE(boot_blk->sect_per_fat, sect_per_fat);
+	//if (fat16_sect_per_fat <= 0xffff)
+	// STORE_LE(boot_blk->fat16_sect_per_fat, sect_per_fat);
	// works:
-	//STORE_LE(boot_blk->sect_per_fat, 0);
+	//STORE_LE(boot_blk->fat16_sect_per_fat, 0);
	STORE_LE(boot_blk->sect_per_track, sect_per_track);
	STORE_LE(boot_blk->heads, heads);
	//STORE_LE(boot_blk->hidden, 0);

util-linux/swaplabel.c+1 -2

@@ -21,7 +21,6 @@
	//usage: "\n -L LBL Label"

	#include "libbb.h"
-	#include "common_bufsiz.h"

	/* from Linux 2.6.23 */
	/*
@@ -79,5 +78,5 @@ int swaplabel_main(int argc UNUSED_PARAM, char **argv)
	if (ENABLE_FEATURE_CLEAN_UP)
	close(fd);

-	return 0;
+	fflush_stdout_and_exit_SUCCESS();
	}