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659644d8d

Denys Vlasenko
Denys Vlasenko
2026-09-28
SUCCESS Duration: 205s

Subject

fdisk: explain how extended partitions work on-disk

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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 →

util-linux/fdisk.c +156 -105util-linux/fdisk_gpt.c +1 -1
util-linux/fdisk.c+156 -105
@@ -389,10 +389,16 @@ struct globals {
char *line_ptr;
const char *disk_device;
- int g_partitions; // = 4; /* maximum partition + 1 */
+ // NB: g_partitions is never less than 4.
+ // If some primary partitions are not populated, they still "exist".
+ int g_partitions; // = 4; // maximum partition + 1
unsigned units_per_sector; // = 1;
unsigned sector_size; // = DEFAULT_SECTOR_SIZE;
- unsigned sector_offset; // = 1;
+ // How much to advance after MBR or extended partition link
+ // before starting next partition? Minimum is 1,
+ // "DOS compatible" way is to skip to the end of track
+ // (set it to g_sectors).
+ unsigned offset_after_MBR_and_ext; // = 1;
unsigned g_heads, g_sectors, g_cylinders;
smallint /* enum label_type */ current_label_type;
#if ENABLE_FEATURE_OSF_LABEL
@@ -406,11 +412,11 @@ struct globals {
unsigned xbsd_part_index;
# endif
#endif
- int ext_index; /* the prime extended partition */
unsigned user_cylinders, user_heads, user_sectors;
unsigned pt_heads, pt_sectors;
unsigned kern_heads, kern_sectors;
- sector_t extended_offset; /* offset of link pointers */
+ int ext_index; // the prime extended partition (0-3)
+ sector_t extended_offset; // offset of extended partition start
sector_t total_number_of_sectors;
const char *opt_t;
@@ -435,7 +441,7 @@ struct globals {
#define g_partitions (G.g_partitions )
#define units_per_sector (G.units_per_sector )
#define sector_size (G.sector_size )
-#define sector_offset (G.sector_offset )
+#define offset_after_MBR_and_ext (G.offset_after_MBR_and_ext)
#define g_heads (G.g_heads )
#define g_sectors (G.g_sectors )
#define g_cylinders (G.g_cylinders )
@@ -459,7 +465,7 @@ struct globals {
#define INIT_G() do { \
SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); \
sector_size = DEFAULT_SECTOR_SIZE; \
- sector_offset = 1; \
+ offset_after_MBR_and_ext = 1; \
g_partitions = 4; \
units_per_sector = 1; \
dos_compatible_flag = 1; \
@@ -599,7 +605,7 @@ str_units(void)
}
static int
-valid_part_table_flag(const uint8_t *mbuffer)
+valid_55AA_signature(const uint8_t *mbuffer)
{
return (mbuffer[510] == 0x55 && mbuffer[511] == 0xaa);
}
@@ -649,7 +655,7 @@ set_changed(int i)
}
static ALWAYS_INLINE void
-write_part_table_flag(uint8_t *b)
+write_55AA_signature(uint8_t *b)
{
b[510] = 0x55;
b[511] = 0xaa;
@@ -862,49 +868,7 @@ list_types(const char *const *sys)
} while (done < last[0]);
bb_putchar('\n');
}
-
-#define set_hsc(h, s, c, sector) do \
-{ \
- s = sector % g_sectors + 1; \
- sector /= g_sectors; \
- h = sector % g_heads; \
- sector /= g_heads; \
- c = sector & 0xff; \
- s |= (sector >> 2) & 0xc0; \
-} while (0)
-
-static void set_hsc_start_end(struct dos_partition *p, sector_t start, sector_t stop)
-{
- if (dos_compatible_flag && (start / (g_sectors * g_heads) > 1023))
- start = g_heads * g_sectors * 1024 - 1;
- set_hsc(p->head, p->sector, p->cyl, start);
-
- if (dos_compatible_flag && (stop / (g_sectors * g_heads) > 1023))
- stop = g_heads * g_sectors * 1024 - 1;
- set_hsc(p->end_head, p->end_sector, p->end_cyl, stop);
-}
-
-static void
-set_partition(int i, int doext, sector_t start, sector_t stop, int sysid)
-{
- struct dos_partition *p;
- sector_t offset;
-
- if (doext) {
- p = ptes[i].ext_pointer;
- offset = extended_offset;
- } else {
- p = ptes[i].part_table;
- offset = ptes[i].offset_from_dev_start;
- }
- p->boot_ind = 0;
- p->sys_ind = sysid;
- set_start_sect(p, start - offset);
- set_nr_sects(p, stop - start + 1);
- set_hsc_start_end(p, start, stop);
- ptes[i].changed = 1;
-}
-#endif
+#endif // WRITABLE
static int
warn_geometry(void)
@@ -954,24 +918,58 @@ warn_cylinders(void)
}
#endif
+// linux/block/partitions/msdos.c:
+// ...
+// The logical partitions form a linked list, with each entry being
+// a partition table with two entries. The first entry
+// is the real data partition (with a start relative to the partition
+// table start). The second is a pointer to the next logical partition
+// (with a start **relative to the entire extended partition**).
+// We do not create a Linux partition for the partition tables, but
+// only for the actual data partitions.
+// ...
+// Usually, the first entry is the real data partition,
+// the 2nd entry is the next extended partition, or empty,
+// and the 3rd and 4th entries are unused.
+// However, DRDOS sometimes has the extended partition as
+// the first entry (when the data partition is empty),
+// and OS/2 **seems to use all four entries**.
+//
+// Due to the above, there can be up to four data partitions
+// in a link of this "extended partition chain".
+// ^^^ we do not handle this case
+//
+// However, kernel code uses only the first "next extended partition" entry
+// it finds in the current link, and ignores any other "extended partitions":
+// "tree of extended partitions" is not a thing.
+//
+// Kernel code requires that the links have nr_sects != 0
+// (otherwise it is not considered to be "extended partition entry").
+// The starting link, visible as one of first 4 partitions,
+// is a real block device (!) but its size is capped to 2 sectors
+// regardless of how large ns_sects field is.
+// Kernel does not complain if this tiny partition intrudes into first sector
+// of the next partition (!!!) which happens to start in the very next sector.
static void
-read_extended(int ext)
+read_extended_chain(int ext)
{
int i;
struct pte *pex;
- struct dos_partition *p, *q;
+ struct dos_partition *p;
ext_index = ext;
pex = &ptes[ext];
pex->ext_pointer = pex->part_table;
p = pex->part_table;
- if (!get_start_sect(p)) {
- puts("Bad offset in primary extended partition");
+ if (get_start_sect(p) == 0) {
+ puts("Zero offset in primary extended partition");
return;
}
+ //TODO: check nr_sect != 0 too?
while (IS_EXTENDED(p->sys_ind)) {
+ struct dos_partition *pt0;
struct pte *pe = &ptes[g_partitions];
if (g_partitions >= MAXIMUM_PARTS) {
@@ -989,41 +987,48 @@ read_extended(int ext)
}
read_pte(pe, extended_offset + get_start_sect(p));
+ // ^^^ does not return on failure to seek or read
- if (!extended_offset)
+ if (extended_offset == 0) // remember base ofs of whole extended partition
extended_offset = get_start_sect(p);
- q = p = pt_offset(pe->sectorbuffer, 0);
- for (i = 0; i < 4; i++, p++) if (get_nr_sects(p)) {
+ pt0 = p = pt_offset(pe->sectorbuffer, 0);
+ for (i = 0; i < 4; i++, p++) {
+ if (get_nr_sects(p) == 0)
+ continue;
if (IS_EXTENDED(p->sys_ind)) {
if (pe->ext_pointer)
- printf("Warning: extra link "
- "pointer in partition table"
- " %u\n", g_partitions + 1);
+ printf("Warning: extra %s "
+ "pointer in ext.partition chain"
+ " %u\n", "link", g_partitions + 1);
else
pe->ext_pointer = p;
- } else if (p->sys_ind) {
+ } else if (p->sys_ind != 0) {
if (pe->part_table)
- printf("Warning: ignoring extra "
- "data in partition table"
- " %u\n", g_partitions + 1);
+ printf("Warning: extra %s "
+ "pointer in ext.partition chain"
+ " %u\n", "data", g_partitions + 1);
else
pe->part_table = p;
}
}
- /* very strange code here... */
+ // Make sure that if not found, data/link pointers
+ // do terminate - point them to invalid entries
+ // (hopefully? Imagine a link with 4 "next ext" ptrs:
+ // none of them have manifestly invalid form!!!)
+//FIXME: allow them to dangle?
if (!pe->part_table) {
- if (q != pe->ext_pointer)
- pe->part_table = q;
+ if (pe->ext_pointer != pt0)
+ pe->part_table = pt0;
else
- pe->part_table = q + 1;
+ pe->part_table = pt0 + 1;
}
if (!pe->ext_pointer) {
- if (q != pe->part_table)
- pe->ext_pointer = q;
+ if (pe->part_table != pt0)
+ pe->ext_pointer = pt0;
else
- pe->ext_pointer = q + 1;
+ pe->ext_pointer = pt0 + 1;
}
p = pe->ext_pointer;
@@ -1061,7 +1066,7 @@ create_doslabel(void)
g_partitions = 4;
memset(&MBRbuffer[510 - 4*16], 0, 4*16);
- write_part_table_flag(MBRbuffer);
+ write_55AA_signature(MBRbuffer);
extended_offset = 0;
set_all_unchanged();
set_changed(0);
@@ -1103,7 +1108,7 @@ get_partition_table_geometry(void)
int first = 1;
int bad = 0;
- if (!(valid_part_table_flag(bufp)))
+ if (!(valid_55AA_signature(bufp)))
return;
hh = ss = 0;
@@ -1147,9 +1152,9 @@ get_geometry(void)
kern_sectors ? kern_sectors : 63;
total_number_of_sectors = bb_getsize_in_sectors(dev_fd);
- sector_offset = 1;
+ offset_after_MBR_and_ext = 1;
if (dos_compatible_flag)
- sector_offset = g_sectors;
+ offset_after_MBR_and_ext = g_sectors;
g_cylinders = total_number_of_sectors / (g_heads * g_sectors);
//TODO? if (total_number_of_sectors % (g_heads * g_sectors) != 0) g_cylinders++;
@@ -1218,11 +1223,11 @@ static int get_boot(void)
fd = open(disk_device, O_RDONLY);
if (fd < 0)
return 1;
- if (512 != full_read(fd, MBRbuffer, 512)) {
- close(fd);
+ xmove_fd(fd, dev_fd);
+ if (512 != full_read(dev_fd, MBRbuffer, 512)) {
+ close_dev_fd();
return 1;
}
- xmove_fd(fd, dev_fd);
#endif
get_geometry();
@@ -1239,7 +1244,7 @@ static int get_boot(void)
#if ENABLE_FEATURE_OSF_LABEL
if (check_osf_label()) {
possibly_osf_label = 1;
- if (!valid_part_table_flag(MBRbuffer)) {
+ if (!valid_55AA_signature(MBRbuffer)) {
current_label_type = LABEL_OSF;
return 0;
}
@@ -1249,10 +1254,10 @@ static int get_boot(void)
#endif
#if !ENABLE_FEATURE_FDISK_WRITABLE
- if (!valid_part_table_flag(MBRbuffer))
+ if (!valid_55AA_signature(MBRbuffer))
return -1;
#else
- if (!valid_part_table_flag(MBRbuffer)) {
+ if (!valid_55AA_signature(MBRbuffer)) {
if (what == TRY_ONLY)
return -1;
// OPEN_MAIN:
@@ -1271,19 +1276,22 @@ static int get_boot(void)
#endif /* FEATURE_FDISK_WRITABLE */
warn_geometry();
+ // We saw 55AA signature in MBR, this is considered enough to conclude
+ // partition table exists (for fdisk -l. Without -l, the requirement
+ // is that there are at least 512 bytes in this disk, that's all).
for (i = 0; i < 4; i++) {
if (IS_EXTENDED(ptes[i].part_table->sys_ind)) {
if (g_partitions != 4)
printf("Ignoring extra extended "
"partition %u\n", i + 1);
else
- read_extended(i);
+ read_extended_chain(i);
}
}
for (i = 3; i < g_partitions; i++) {
struct pte *pe = &ptes[i];
- if (!valid_part_table_flag(pe->sectorbuffer)) {
+ if (!valid_55AA_signature(pe->sectorbuffer)) {
printf("Warning: invalid flag 0x%02x,0x%02x of partition "
"table %u will be corrected by w(rite)\n",
pe->sectorbuffer[510],
@@ -1511,11 +1519,11 @@ toggle_dos_compatibility_flag(void)
{
dos_compatible_flag = 1 - dos_compatible_flag;
if (dos_compatible_flag) {
- sector_offset = g_sectors;
- printf("DOS Compatibility flag is %sset\n", "");
+ offset_after_MBR_and_ext = g_sectors;
+ printf("DOS Compatibility flag (%u sector gap) is %sset\n", offset_after_MBR_and_ext, "");
} else {
- sector_offset = 1;
- printf("DOS Compatibility flag is %sset\n", "not ");
+ offset_after_MBR_and_ext = 1;
+ printf("DOS Compatibility flag (%u sector gap) is %sset\n", offset_after_MBR_and_ext, "not ");
}
}
@@ -2251,6 +2259,47 @@ verify(void)
}
}
+static void
+set_hsc_start_end(struct dos_partition *p, sector_t start, sector_t stop)
+{
+#define SET_HEAD_SECT_CYL(h, s, c, sector) do { \
+ s = sector % g_sectors + 1; \
+ sector /= g_sectors; \
+ h = sector % g_heads; \
+ sector /= g_heads; \
+ c = sector & 0xff; \
+ s |= (sector >> 2) & 0xc0; \
+} while (0)
+ if (dos_compatible_flag && (start / (g_sectors * g_heads) > 1023))
+ start = g_heads * g_sectors * 1024 - 1;
+ SET_HEAD_SECT_CYL(p->head, p->sector, p->cyl, start);
+ if (dos_compatible_flag && (stop / (g_sectors * g_heads) > 1023))
+ stop = g_heads * g_sectors * 1024 - 1;
+ SET_HEAD_SECT_CYL(p->end_head, p->end_sector, p->end_cyl, stop);
+#undef SET_HEAD_SECT_CYL
+}
+
+// add_partition() helper
+static void
+set_partition(int i, int doext, sector_t start, sector_t stop, int sysid)
+{
+ struct dos_partition *p;
+ sector_t offset;
+
+ if (doext) {
+ p = ptes[i].ext_pointer;
+ offset = extended_offset;
+ } else {
+ p = ptes[i].part_table;
+ offset = ptes[i].offset_from_dev_start;
+ }
+ p->boot_ind = 0;
+ p->sys_ind = sysid;
+ set_start_sect(p, start - offset);
+ set_nr_sects(p, stop - start + 1);
+ set_hsc_start_end(p, start, stop);
+ ptes[i].changed = 1;
+}
static void
add_partition(int n, int sys)
{
@@ -2259,7 +2308,7 @@ add_partition(int n, int sys)
struct dos_partition *p = ptes[n].part_table;
struct dos_partition *q = ptes[ext_index].part_table;
sector_t limit, temp;
- sector_t start, stop = 0;
+ sector_t start, stop;
sector_t first[g_partitions], last[g_partitions];
if (p && p->sys_ind) {
@@ -2268,7 +2317,7 @@ add_partition(int n, int sys)
}
fill_bounds(first, last);
if (n < 4) {
- start = sector_offset;
+ start = offset_after_MBR_and_ext;
if (DISPLAY_IN_CYL_UNITS || !total_number_of_sectors)
limit = (sector_t) g_heads * g_sectors * g_cylinders - 1;
else
@@ -2279,7 +2328,7 @@ add_partition(int n, int sys)
get_nr_sects(q) - 1;
}
} else {
- start = extended_offset + sector_offset;
+ start = extended_offset + offset_after_MBR_and_ext;
limit = get_start_sect(q) + get_nr_sects(q) - 1;
}
if (DISPLAY_IN_CYL_UNITS)
@@ -2294,14 +2343,14 @@ add_partition(int n, int sys)
int lastplusoff;
if (start == ptes[i].offset_from_dev_start)
- start += sector_offset;
- lastplusoff = last[i] + ((n < 4) ? 0 : sector_offset);
+ start += offset_after_MBR_and_ext;
+ lastplusoff = last[i] + ((n < 4) ? 0 : offset_after_MBR_and_ext);
if (start >= first[i] && start <= lastplusoff)
start = lastplusoff + 1;
}
if (start > limit)
break;
- if (start >= temp+units_per_sector && num_read) {
+ if (start >= temp + units_per_sector && num_read) {
printf("Sector %"SECT_FMT"u is already allocated\n", temp);
temp = start;
num_read = 0;
@@ -2319,13 +2368,14 @@ add_partition(int n, int sys)
num_read = 1;
}
} while (start != temp || !num_read);
+
if (n > 4) { /* NOT for fifth partition */
struct pte *pe = &ptes[n];
- pe->offset_from_dev_start = start - sector_offset;
+ pe->offset_from_dev_start = start - offset_after_MBR_and_ext;
if (pe->offset_from_dev_start == extended_offset) { /* must be corrected */
pe->offset_from_dev_start++;
- if (sector_offset == 1)
+ if (offset_after_MBR_and_ext == 1)
start++;
}
}
@@ -2354,14 +2404,14 @@ add_partition(int n, int sys)
stop = read_int(cround(start), cround(limit), cround(limit), cround(start), mesg);
if (DISPLAY_IN_CYL_UNITS) {
stop = stop * units_per_sector - 1;
- if (stop >limit)
+ if (stop > limit)
stop = limit;
}
}
- set_partition(n, 0, start, stop, sys);
+ set_partition(n, /* ext:*/ 0, start, stop, sys);
if (n > 4)
- set_partition(n - 1, 1, ptes[n].offset_from_dev_start, stop, EXTENDED);
+ set_partition(n - 1, /* ext:*/ 1, ptes[n].offset_from_dev_start, stop, EXTENDED);
if (IS_EXTENDED(sys)) {
struct pte *pe4 = &ptes[4];
@@ -2430,7 +2480,7 @@ new_partition(void)
" p primary partition (1-4)\n"
" %s\n",
(extended_offset ?
- "l logical (5 or over)" : "e extended"));
+ "l logical (5+)" : "e extended (1-4)"));
while (1) {
c = read_nonempty(line);
c |= 0x20; /* lowercase */
@@ -2486,10 +2536,11 @@ write_table_and_exit(void)
for (i = 0; i < 3; i++)
if (ptes[i].changed)
ptes[3].changed = 1;
+ // If needed, write MBR and extended partition sectors
for (i = 3; i < g_partitions; i++) {
struct pte *pe = &ptes[i];
if (pe->changed) {
- write_part_table_flag(pe->sectorbuffer);
+ write_55AA_signature(pe->sectorbuffer);
write_sector(pe->offset_from_dev_start, pe->sectorbuffer);
}
}
@@ -2576,7 +2627,7 @@ menu(void)
} else {
puts("a\ttoggle a bootable flag");
puts("b\tedit bsd disklabel");
- puts("c\ttoggle the dos compatibility flag");
+ printf("c\tturn o%s DOS sector gap flag\n", dos_compatible_flag ? "ff" : "n");
puts("d\tdelete a partition");
puts("l\tlist known partition types");
puts("n\tadd a new partition");
@@ -2681,9 +2732,9 @@ xselect(void)
case 's':
user_sectors = g_sectors = read_int(1, g_sectors, 63, 0, "Number of sectors");
if (dos_compatible_flag) {
- sector_offset = g_sectors;
- puts("Warning: setting sector offset for DOS "
- "compatibility");
+ offset_after_MBR_and_ext = g_sectors;
+ printf("Warning: setting sector offset %u for DOS "
+ "compatibility\n", g_sectors);
}
update_units();
break;
util-linux/fdisk_gpt.c+1 -1
@@ -153,7 +153,7 @@ check_gpt_label(void)
/* LBA 0 contains the legacy MBR */
- if (!valid_part_table_flag(MBRbuffer)
+ if (!valid_55AA_signature(MBRbuffer)
|| first->sys_ind != LEGACY_GPT_TYPE
) {
return 0;