fs/ext2/io.c

/*
 * Copyright (c) 2007 Travis Geiselbrecht
 *
 * Use of this source code is governed by a MIT-style
 * license that can be found in the LICENSE file or at
 * https://opensource.org/licenses/MIT
 */

#include <string.h>
#include <stdlib.h>
#include <lk/debug.h>
#include <lk/trace.h>
#include "ext2_priv.h"

#define LOCAL_TRACE 0

int ext2_read_block(ext2_t *ext2, void *buf, blocknum_t bnum) {
    return bcache_read_block(ext2->cache, buf, bnum);
}

int ext2_get_block(ext2_t *ext2, void **ptr, blocknum_t bnum) {
    return bcache_get_block(ext2->cache, ptr, bnum);
}

int ext2_put_block(ext2_t *ext2, blocknum_t bnum) {
    return bcache_put_block(ext2->cache, bnum);
}

static int ext2_calculate_block_pointer_pos(ext2_t *ext2, blocknum_t block_to_find, uint32_t *level, uint32_t pos[]) {
    uint32_t block_ptr_per_block, block_ptr_per_2nd_block;

    // XXX optimize this

    // See if it's in the direct blocks
    if (block_to_find < EXT2_NDIR_BLOCKS) {
        *level = 0;
        pos[0] = block_to_find;
        return 0;
    }

    block_ptr_per_block = EXT2_ADDR_PER_BLOCK(ext2->sb);
    block_to_find -= EXT2_NDIR_BLOCKS;
    // See if it's in the first indirect block
    if (block_to_find < block_ptr_per_block) {
        *level = 1;
        pos[0] = EXT2_IND_BLOCK;
        pos[1] = block_to_find;
        return 0;
    }

    block_to_find -= block_ptr_per_block;
    block_ptr_per_2nd_block = block_ptr_per_block * block_ptr_per_block;
    // See if it's in the second indirect block
    if (block_to_find < (block_ptr_per_2nd_block)) {
        *level = 2;
        pos[0] = EXT2_DIND_BLOCK;
        pos[1] = block_to_find / block_ptr_per_block;
        pos[2] = block_to_find % block_ptr_per_block;
        return 0;
    }

    block_to_find -= block_ptr_per_2nd_block;
    // See if it's in the third indirect block
    if (block_to_find < (block_ptr_per_2nd_block * block_ptr_per_block)) {
        *level = 3;
        pos[0] = EXT2_TIND_BLOCK;
        pos[1] = block_to_find / block_ptr_per_2nd_block;
        pos[2] = (block_to_find % block_ptr_per_2nd_block) / block_ptr_per_block;
        pos[3] = (block_to_find % block_ptr_per_2nd_block) % block_ptr_per_block;
        return 0;
    }

    // The block requested must be too big.
    return -1;
}

// This function returns a pointer to the cache block that corresponds to the indirect block pointer.
static int ext2_get_indirect_block_pointer_cache_block(ext2_t *ext2, struct ext2_inode *inode,
        blocknum_t **cache_block, uint32_t level, uint32_t pos[], uint *block_loaded) {
    uint32_t current_level = 0;
    uint current_block = 0, last_block;
    blocknum_t *block = NULL;
    int err;

    if ((level > 3) || (level == 0)) {
        err = -1;
        goto error;
    }

    // Dig down into the indirect blocks. When done, current_block should point to the target.
    while (current_level < level) {
        if (current_level == 0) {
            // read the direct block, simulates a prior loop
            current_block = LE32(inode->i_block[pos[0]]);
        }

        if (current_block == 0) {
            err = -1;
            goto error;
        }

        last_block = current_block;
        current_level++;
        *block_loaded = current_block;

        err = ext2_get_block(ext2, (void **)(void *)&block, current_block);
        if (err < 0) {
            goto error;
        }

        if (current_level < level) {
            current_block = LE32(block[pos[current_level]]);
            ext2_put_block(ext2, last_block);
        }
    }

    *cache_block = block;
    return 0;

error:
    *cache_block = NULL;
    *block_loaded = 0;
    return err;
}

/* translate a file block to a physical block */
static blocknum_t file_block_to_fs_block(ext2_t *ext2, struct ext2_inode *inode, uint fileblock) {
    int err;
    blocknum_t block;

    LTRACEF("inode %p, fileblock %u\n", inode, fileblock);

    uint32_t pos[4];
    uint32_t level = 0;
    ext2_calculate_block_pointer_pos(ext2, fileblock, &level, pos);

    LTRACEF("level %d, pos 0x%x 0x%x 0x%x 0x%x\n", level, pos[0], pos[1], pos[2], pos[3]);

    if (level == 0) {
        /* direct block, just return it directly */
        block = LE32(inode->i_block[fileblock]);
    } else {
        /* at least one level of indirection, get a pointer to the final indirect block table and dereference it */
        blocknum_t *ind_table;
        blocknum_t phys_block;
        err = ext2_get_indirect_block_pointer_cache_block(ext2, inode, &ind_table, level, pos, &phys_block);
        if (err < 0)
            return 0;

        /* dereference the final entry in the final table */
        block = LE32(ind_table[pos[level]]);
        LTRACEF("block %u, indirect_block %u\n", block, phys_block);

        /* release the ref on the cache block */
        ext2_put_block(ext2, phys_block);
    }

    LTRACEF("returning %u\n", block);

    return block;
}

ssize_t ext2_read_inode(ext2_t *ext2, struct ext2_inode *inode, void *_buf, off_t offset, size_t len) {
    int err = 0;
    size_t bytes_read = 0;
    uint8_t *buf = _buf;

    /* calculate the file size */
    off_t file_size = ext2_file_len(ext2, inode);

    LTRACEF("inode %p, offset %lld, len %zd, file_size %lld\n", inode, offset, len, file_size);

    /* trim the read */
    if (offset > file_size)
        return 0;
    if ((off_t)(offset + len) >= file_size)
        len = file_size - offset;
    if (len == 0)
        return 0;

    /* calculate the starting file block */
    uint file_block = offset / EXT2_BLOCK_SIZE(ext2->sb);

    /* handle partial first block */
    if ((offset % EXT2_BLOCK_SIZE(ext2->sb)) != 0) {
        uint8_t temp[EXT2_BLOCK_SIZE(ext2->sb)];

        /* calculate the block and read it */
        blocknum_t phys_block = file_block_to_fs_block(ext2, inode, file_block);
        if (phys_block == 0) {
            memset(temp, 0, EXT2_BLOCK_SIZE(ext2->sb));
        } else {
            ext2_read_block(ext2, temp, phys_block);
        }

        /* copy out what we need */
        size_t block_offset = offset % EXT2_BLOCK_SIZE(ext2->sb);
        size_t tocopy = MIN(len, EXT2_BLOCK_SIZE(ext2->sb) - block_offset);
        memcpy(buf, temp + block_offset, tocopy);

        /* increment our stuff */
        file_block++;
        len -= tocopy;
        bytes_read += tocopy;
        buf += tocopy;
    }

    /* handle middle blocks */
    while (len >= EXT2_BLOCK_SIZE(ext2->sb)) {
        /* calculate the block and read it */
        blocknum_t phys_block = file_block_to_fs_block(ext2, inode, file_block);
        if (phys_block == 0) {
            memset(buf, 0, EXT2_BLOCK_SIZE(ext2->sb));
        } else {
            ext2_read_block(ext2, buf, phys_block);
        }

        /* increment our stuff */
        file_block++;
        len -= EXT2_BLOCK_SIZE(ext2->sb);
        bytes_read += EXT2_BLOCK_SIZE(ext2->sb);
        buf += EXT2_BLOCK_SIZE(ext2->sb);
    }

    /* handle partial last block */
    if (len > 0) {
        uint8_t temp[EXT2_BLOCK_SIZE(ext2->sb)];

        /* calculate the block and read it */
        blocknum_t phys_block = file_block_to_fs_block(ext2, inode, file_block);
        if (phys_block == 0) {
            memset(temp, 0, EXT2_BLOCK_SIZE(ext2->sb));
        } else {
            ext2_read_block(ext2, temp, phys_block);
        }

        /* copy out what we need */
        memcpy(buf, temp, len);

        /* increment our stuff */
        bytes_read += len;
    }

    LTRACEF("err %d, bytes_read %zu\n", err, bytes_read);

    return (err < 0) ? err : (ssize_t)bytes_read;
}