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Fix non-aligned disk reads in bootloader

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This commit is contained in:
vmttmv
2026-01-06 00:39:29 +02:00
parent 9eae2e1005
commit 35ebd5fd72
2 changed files with 115 additions and 119 deletions
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81
bootloader/stage2.asm
View File

@@ -24,84 +24,3 @@ _start:
call load_kernel
jmp eax
; ----------------------------------------------------------------------------
; ATA read sectors (LBA mode)
;
; sysv32 abi signature:
; void ata_lba_read(uint32_t lba, uint8_t nsect, void *addr);
; ----------------------------------------------------------------------------
ata_lba_read:
push ebp
mov ebp, esp
push ebx
push ecx
push edx
push edi
; Wait BSY=0 before proceeding to write the regs
.wait_rdy:
mov edx, 0x1F7
in al, dx
test al, 0x80
jnz .wait_rdy
mov eax, [ebp+8] ; arg #1 = LBA
mov cl, [ebp+12] ; arg #2 = # of sectors
mov edi, [ebp+16] ; arg #3 = buffer address
and eax, 0x0FFFFFFF
mov ebx, eax ; Save LBA in RBX
mov edx, 0x01F6 ; Port to send drive and bit 24 - 27 of LBA
shr eax, 24 ; Get bit 24 - 27 in al
or al, 11100000b ; Set bit 6 in al for LBA mode
out dx, al
mov edx, 0x01F2 ; Port to send number of sectors
mov al, cl ; Get number of sectors from CL
out dx, al
mov edx, 0x1F3 ; Port to send bit 0 - 7 of LBA
mov eax, ebx ; Get LBA from EBX
out dx, al
mov edx, 0x1F4 ; Port to send bit 8 - 15 of LBA
mov eax, ebx ; Get LBA from EBX
shr eax, 8 ; Get bit 8 - 15 in AL
out dx, al
mov edx, 0x1F5 ; Port to send bit 16 - 23 of LBA
mov eax, ebx ; Get LBA from EBX
shr eax, 16 ; Get bit 16 - 23 in AL
out dx, al
mov edx, 0x1F7 ; Command port
mov al, 0x20 ; Read with retry.
out dx, al
mov bl, cl ; Save # of sectors in BL
.wait_rdy2:
mov edx, 0x1F7
.do_wait_rdy2:
in al, dx
test al, 0x80 ; BSY?
jnz .do_wait_rdy2
test al, 0x8 ; DRQ?
jz .do_wait_rdy2
mov edx, 0x1F0 ; Data port, in and out
mov ecx, 256
rep insw ; in to [RDI]
dec bl ; are we...
jnz .wait_rdy2 ; ...done?
pop edi
pop edx
pop ecx
pop ebx
pop ebp
ret

147
bootloader/stage2_load.c
View File

@@ -1,5 +1,19 @@
#include <stdint.h>
// ATA IO Ports
#define ATA_PRIMARY_DATA 0x1F0
#define ATA_PRIMARY_ERR_FEATURES 0x1F1
#define ATA_PRIMARY_SEC_COUNT 0x1F2
#define ATA_PRIMARY_LBA_LOW 0x1F3
#define ATA_PRIMARY_LBA_MID 0x1F4
#define ATA_PRIMARY_LBA_HIGH 0x1F5
#define ATA_PRIMARY_DRIVE_SEL 0x1F6
#define ATA_PRIMARY_COMM_STAT 0x1F7
// ATA Commands
#define ATA_CMD_READ_PIO 0x20
#define ATA_CMD_WRITE_PIO 0x30
// ELF Ident indexes
#define EI_NIDENT 16
@@ -7,6 +21,14 @@
#define PT_NULL 0
#define PT_LOAD 1
// Disk sector size
#define SECTOR_SIZE 512
// Kernel start LBA
#define KERN_START_SECT 5
extern uint8_t read_buf[];
// ELF Header (32-bit)
typedef struct {
uint8_t e_ident[EI_NIDENT];
@@ -37,8 +59,84 @@ typedef struct {
uint32_t p_align;
} __attribute__((packed)) Elf32_Phdr;
static inline uint8_t inb(uint16_t port)
{
uint8_t ret;
__asm__ volatile ("inb %1, %0"
: "=a"(ret)
: "Nd"(port));
return ret;
}
static inline void outb(uint16_t port, uint8_t val)
{
__asm__ volatile ("outb %0, %1"
:
: "a"(val), "Nd"(port));
}
static inline uint16_t inw(uint16_t port)
{
uint16_t ret;
__asm__ volatile ("inw %1, %0"
: "=a"(ret)
: "Nd"(port));
return ret;
}
static inline void ata_wait_bsy() {
while (inb(ATA_PRIMARY_COMM_STAT) & 0x80);
}
static inline void ata_wait_drq() {
while (!(inb(ATA_PRIMARY_COMM_STAT) & 0x08));
}
static void ata_read_sector(void *addr, uint32_t lba) {
ata_wait_bsy();
outb(ATA_PRIMARY_DRIVE_SEL, 0xE0 | ((lba >> 24) & 0x0F));
outb(ATA_PRIMARY_SEC_COUNT, 1);
outb(ATA_PRIMARY_LBA_LOW, (uint8_t)lba);
outb(ATA_PRIMARY_LBA_MID, (uint8_t)(lba >> 8));
outb(ATA_PRIMARY_LBA_HIGH, (uint8_t)(lba >> 16));
outb(ATA_PRIMARY_COMM_STAT, ATA_CMD_READ_PIO);
uint16_t* ptr = (uint16_t*)addr;
ata_wait_bsy();
ata_wait_drq();
for (int i = 0; i < 256; i++) {
*ptr++ = inw(ATA_PRIMARY_DATA);
}
}
static void load_segment(uint8_t *addr, uint32_t offset, uint32_t size)
{
uint32_t lba = KERN_START_SECT + offset / SECTOR_SIZE;
uint32_t off = offset % 512;
uint8_t data[512];
while (size > 0) {
ata_read_sector(data, lba);
uint32_t copy = 512 - off;
if (copy > size) {
copy = size;
}
for (uint32_t i = 0; i < copy; i++) {
addr[i] = data[off + i];
}
addr += copy;
size -= copy;
lba++;
off = 0;
}
}
// Load an ELF executable into memory.
static int elf_load(const void* data, void (*load_segment)(uint8_t *vaddr, uint32_t src, uint32_t size)) {
static int elf_load(const void *data) {
const Elf32_Ehdr* header = (const Elf32_Ehdr*)data;
const Elf32_Phdr* ph = (const Elf32_Phdr*)((uint8_t*)data + header->e_phoff);
@@ -51,8 +149,6 @@ static int elf_load(const void* data, void (*load_segment)(uint8_t *vaddr, uint3
uint32_t filesz = ph[i].p_filesz;
uint32_t memsz = ph[i].p_memsz;
// Copy data segment
//load_segment((uint8_t *)vaddr, offset, filesz);
load_segment((uint8_t *)vaddr, offset, filesz);
// Zero remaining BSS (if any)
@@ -67,52 +163,33 @@ static int elf_load(const void* data, void (*load_segment)(uint8_t *vaddr, uint3
return header->e_entry;
}
#define KERN_START_SECT 5
#define MAX(a, b) ((a)>(b) ? (a) : (b))
extern void ata_lba_read(uint32_t lba, uint8_t nsect, void *addr);
extern uint8_t read_buf[];
static uint32_t
total_header_size(const Elf32_Ehdr *header) {
total_headers_size(const Elf32_Ehdr *header) {
uint32_t phend = header->e_phoff + header->e_phentsize*header->e_phnum;
// Align to 512
return (phend + 511) & ~511;
}
static void read_sectors(uint8_t *vaddr, uint32_t offset, uint32_t size) {
// # of sectors to read
uint32_t rem_nsect = ((size + 511) & ~511) / 512;
// Current lba address, offset by the first sector already read
uint32_t lba = KERN_START_SECT + offset / 512;
// Max 255 sectors at a time
while (rem_nsect) {
uint8_t nsect = rem_nsect > 255 ? 255 : rem_nsect;
ata_lba_read(lba, nsect, vaddr);
vaddr += nsect * 512;
rem_nsect -= nsect;
lba += nsect;
}
// Align to sector size
uint32_t a = SECTOR_SIZE-1;
return (phend + a) & ~a;
}
void *load_kernel(void) {
// Read the first sector
ata_lba_read(KERN_START_SECT, 1, read_buf);
ata_read_sector(read_buf, KERN_START_SECT);
const Elf32_Ehdr* header = (const Elf32_Ehdr*)read_buf;
// Remaining data size, subtract the first 512B already read
uint32_t rem = total_header_size(header) - 512;
uint32_t rem = total_headers_size(header) - SECTOR_SIZE;
// Read the rest if necessary
if (rem)
read_sectors(read_buf+512, 512, rem);
if (rem) {
uint8_t *dst = read_buf + SECTOR_SIZE;
for (uint32_t i = 0; i < rem / SECTOR_SIZE; i++, dst += 512) {
ata_read_sector(dst, KERN_START_SECT + i + 1);
}
}
elf_load(read_buf, read_sectors);
elf_load(read_buf);
return (void *)header->e_entry;
}