fk98/src/dosfs.c

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "dosfs.h"
#include "debug.h"

static unsigned int *read_fat_chain(dosfs_t *, unsigned int, int *);

/*
 * Opens an image a la open(2). Returns 0 on success, or
 * some error number on failure.
 */
int
open_image(const char *path, int flags, dosfs_t *fsd)
{
    int res, ifd;

    if (fsd == NULL)
        return EINVAL;

    ifd = open(path, flags);
    if (ifd == -1)
        return errno;

    memset(&fsd->ib, 0, sizeof(struct bpb));

    res = read(ifd, &fsd->ib, sizeof(struct bpb));
    if (res == -1) {
        close(ifd);
        return errno;
    }

    if (fsd->ib.jmp[0] != 0xEB || fsd->ib.jmp[2] != 0x90) {
        DPRINTF(("hm... doesn't look like a FAT image?\n"));
        close(ifd);
        return EOPNOTSUPP;
    }

    if (fsd->ib.sect_ct == 0 || fsd->ib.fat_size == 0) {
        DPRINTF(("sect_ct or fat_size == 0, likely fat>32 (unsupported)\n"));
        close(ifd);
        return EOPNOTSUPP;
    }

    fsd->root_size = ((fsd->ib.dirent_ct * 32) + (fsd->ib.sect_size - 1)) /
        fsd->ib.sect_size;

    fsd->data_size = fsd->ib.sect_ct - (fsd->ib.resv_sect +
        (fsd->ib.fat_ct * fsd->ib.fat_size) + fsd->root_size);

    if (fsd->data_size < FAT12_MAX_CLUSTERS) {
        fsd->fs_ver = 12;
    } else if (fsd->data_size < FAT16_MAX_CLUSTERS) {
        fsd->fs_ver = 16;
    } else {
        DPRINTF(("data_size >= FAT12_MAX_CLUSTERS, likely fat>32 (unsupported)\n"));
        close(ifd);
        return EOPNOTSUPP;
    }

    fsd->ifd = ifd;
    fsd->root_loc = (fsd->ib.resv_sect +
        (fsd->ib.fat_ct * fsd->ib.fat_size)) * fsd->ib.sect_size;
    fsd->data_loc = fsd->root_loc + (fsd->root_size * fsd->ib.sect_size);

    return 0;
}

/*
 * Frees the file descriptor for a given image.
 */
void
close_image(dosfs_t *fsd)
{
    close(fsd->ifd);
    fsd->ifd = -1;
}

/*
 * Reads directory listing at given offset. Returns a pointer
 * to a dosfile_t on success, or NULL on failure (errno set).
 */
dosfile_t *
dos_listdir(dosfs_t *fsd, unsigned int offset)
{
    dosfile_t *start, *cur;
    struct dos_dirent *dirs = NULL;
    int i, j, res, root_ents;

    start = malloc(sizeof(dosfile_t));
    if (start == NULL) {
        DPRINTF(("malloc failed\n"));
        return NULL;
    }

    /*
     * Not sure if we should be trusting the bpb for anything,
     * but relying on it here will be better than reading every
     * single entry one at a time.
     */
    root_ents = (fsd->root_size * fsd->ib.sect_size) /
        sizeof(struct dos_dirent);

    res = reallocarr(&dirs, root_ents, sizeof(struct dos_dirent));
    if (res == -1) {
        DPRINTF(("reallocarr failed\n"));
        return NULL;
    }

    res = pread(fsd->ifd, dirs, (root_ents * sizeof(struct dos_dirent)),
        offset);
    if (res == -1) {
        DPRINTF(("pread failed\n"));
        return NULL;
    }

    /* iterate through every dir entry */
    cur = start;
    for (i = 0; i < root_ents; i++) {
        if (dirs[i].filename[0] == 0) {
            break;
        }

        if (i > 0) {
            cur->next = malloc(sizeof(dosfile_t));
            if (cur->next == NULL) {
                DPRINTF(("inner malloc failed!?\n"));
                res = -1;
                break;
            }

            cur = cur->next;
        }

        for (j = 0; j < 8; j++) {
            if (dirs[i].filename[j] == ' ') {
                cur->fname[j] = 0;
                break;
            } else {
                cur->fname[j] = dirs[i].filename[j];
            }
        }
        cur->fname[8] = 0;

        for (j = 0; j < 3; j++) {
            cur->fext[j] = dirs[i].filename[j+8];
        }
        cur->fext[4] = 0;

        (void)memcpy(&cur->ent, &dirs[i], sizeof(struct dos_dirent));

        /* useful for get_byte_offset() */
        cur->fat_chain = read_fat_chain(fsd, cur->ent.low_loc,
            &cur->fc_len);

        res = 0;
    }

    free(dirs);

    /* cleanup if something went wrong */
    if (res != 0) {
        dos_freedir(start);
        return NULL;
    }

    return start;
}

/*
 * Frees a directory listing.
 */
void
dos_freedir(dosfile_t *dirs)
{
    dosfile_t *cur, *prev;

    cur = dirs;
    while (cur != NULL) {
        prev = cur;
        cur = cur->next;
        free(prev);
    }
}

static unsigned int *
read_fat_chain(dosfs_t *fsd, unsigned int first, int *length)
{
    unsigned int cur, foff, byte_off, *chain = NULL;
    uint16_t next;
    int len = 0, res;

    cur = first;
    for (;;) {
        len++;
        res = reallocarr(&chain, len, sizeof(unsigned int));
        if (res == -1) {
            DPRINTF(("reallocarr failed\n"));
            free(chain);
            return NULL;
        }

        foff = (fsd->fs_ver == 12) ? (cur + (cur / 2)) : /* fat12 */
            (cur * 2); /* fat16 */
        byte_off = (fsd->ib.resv_sect * fsd->ib.sect_size) + foff;

        /* XXX: endianness */
        res = pread(fsd->ifd, &next, 2, byte_off);
        if (res == -1) {
            DPRINTF(("pread failed\n"));
            free(chain);
            return NULL;
        }

        if (fsd->fs_ver == 12) {
            if (cur & 0x0001) {
                next = next >> 4;
            } else {
                next = next & 0x0FFF;
            }
            if (next == 0x0FFF) {
                break;
            }
        } else {
            if (next == 0xFFFF) {
                break;
            }
        }

        chain[len-1] = next;
    }

    *length = len;
    return chain;
}

/*
 * Converts the byte offset of a file into the "real"
 * offset, i.e. one that can be used with lseek()/pread().
 */
int
get_byte_offset(dosfs_t *fsd, dosfile_t *file, unsigned int f_offset)
{
    int coff, csz;

    if (f_offset > file->ent.size) {
        DPRINTF(("offset requested is out of bounds!\n"));
        return -EINVAL;
    }

    /* size in bytes of a cluster */
    csz = fsd->ib.sect_per_cluster * fsd->ib.sect_size;

    /* get the offset in the fat chain */
    coff = f_offset / csz;

    if (coff > file->fc_len) {
        DPRINTF(("offset is within filesize, but not enough clusters??\n"));
        return -EINVAL;
    }

    return (file->fat_chain[coff] * csz) + (f_offset % csz);
}