/* todo:
* - see Documentation/filesystems/romfs.txt
* - use allocated, not stack memory for file names?
* - considering write access...
* - network (tftp) files?
* - merge back some _op tables
*/
/*
* Sorry about some optimizations and for some goto's. I just wanted
* to squeeze some more bytes out of this code.. :)
*/
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
//校验数据
static __s32
romfs_checksum(void *data, int size)
{
__s32 sum, *ptr;
sum = 0; ptr = data;
size>>=2;
while (size>0) {
sum += ntohl(*ptr++);
size--;
}
return sum;
}
static struct super_operations romfs_ops;
static struct super_block *
romfs_read_super(struct super_block *s, void *data, int silent)
{
struct buffer_head *bh;
kdev_t dev = s->s_dev;
struct romfs_super_block *rsb;
int sz;
/* I would parse the options here, but there are none.. :) */
set_blocksize(dev, ROMBSIZE);
s->s_blocksize = ROMBSIZE;
s->s_blocksize_bits = ROMBSBITS;
s->u.generic_sbp = (void *) 0;
s->s_maxbytes = 0xFFFFFFFF;
bh = sb_bread(s, 0);
if (!bh) {
/* XXX merge with other printk? */
printk ("romfs: unable to read superblock\n");
goto outnobh;
}
rsb = (struct romfs_super_block *)bh->b_data;
sz = ntohl(rsb->size);
if (rsb->word0 != ROMSB_WORD0 || rsb->word1 != ROMSB_WORD1
|| sz < ROMFH_SIZE) {
if (!silent)
printk ("VFS: Can't find a romfs filesystem on dev "
"%s.\n", kdevname(dev));
goto out;
}
if (romfs_checksum(rsb, min_t(int, sz, 512))) {
printk ("romfs: bad initial checksum on dev "
"%s.\n", kdevname(dev));
goto out;
}
s->s_magic = ROMFS_MAGIC;
//文件系统的最大容量
s->u.romfs_sb.s_maxsize = sz;
s->s_flags |= MS_RDONLY;
/* Find the start of the fs */
sz = (ROMFH_SIZE +
strnlen(rsb->name, ROMFS_MAXFN) + 1 + ROMFH_PAD)
& ROMFH_MASK;
brelse(bh);
s->s_op = &romfs_ops;
//iget 获取inode
//root节点的no实际上是offset，不是0开始的
//d_alloc_root 分配该inode对应的dentry.
s->s_root = d_alloc_root(iget(s, sz));
if (!s->s_root)
goto outnobh;
/* Ehrhm; sorry.. :) And thanks to Hans-Joachim Widmaier :) */
if (0) {
out:
brelse(bh);
outnobh:
s = NULL;
}
return s;
}
/* That's simple too. */
static int
romfs_statfs(struct super_block *sb, struct statfs *buf)
{
buf->f_type = ROMFS_MAGIC;
buf->f_bsize = ROMBSIZE;
buf->f_bfree = buf->f_bavail = buf->f_ffree;
buf->f_blocks = (sb->u.romfs_sb.s_maxsize+ROMBSIZE-1)>>ROMBSBITS;
buf->f_namelen = ROMFS_MAXFN;
return 0;
}
/* some helper routines */
//获取文件名长度，不超过128字节
static int
romfs_strnlen(struct inode *i, unsigned long offset, unsigned long count)
{
struct buffer_head *bh;
unsigned long avail, maxsize, res;
//文件系统越界检查
maxsize = i->i_sb->u.romfs_sb.s_maxsize;
if (offset >= maxsize)
return -1;
/* strnlen is almost always valid */
if (count > maxsize || offset+count > maxsize)
count = maxsize-offset;
//由于采用offset寻址，没有block的说法
//这里通过 offset>>ROMBSBITS (10bit)，强制将offset换算为
//对应的block。 bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
if (!bh)
return -1; /* error */
//计算第一个block中,offset位置距离block边界
//剩余的字节数
avail = ROMBSIZE - (offset & ROMBMASK);
maxsize = min_t(unsigned long, count, avail);
//为了得到文件名的长度
res = strnlen(((char *)bh->b_data)+(offset&ROMBMASK), maxsize);
brelse(bh);
if (res < maxsize)
return res; /* found all of it */
while (res < count) {
offset += maxsize;
bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
if (!bh)
return -1;
maxsize = min_t(unsigned long, count - res, ROMBSIZE);
avail = strnlen(bh->b_data, maxsize);
res += avail;
brelse(bh);
if (avail < maxsize)
return res;
}
return res;
}
/*
从offset位置，拷贝count数据，返回到dest
count 的值不一定小于1k，可以一次读取多个block
*/
static int
romfs_copyfrom(struct inode *i, void *dest, unsigned long offset, unsigned long count)
{
struct buffer_head *bh;
unsigned long avail, maxsize, res;
//文件系统越界检查
maxsize = i->i_sb->u.romfs_sb.s_maxsize;
if (offset >= maxsize || count > maxsize || offset+count>maxsize)
return -1;
//由于采用offset寻址，没有block的说法
//这里通过 offset>>ROMBSBITS (10bit)，强制将offset换算为
//对应的block。
bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
if (!bh)
return -1; /* error */
//计算第一个block中,offset位置距离block边界
//剩余的字节数
avail = ROMBSIZE - (offset & ROMBMASK);
//求count,和avail的最小值
maxsize = min_t(unsigned long, count, avail);
//拷贝数据
memcpy(dest, ((char *)bh->b_data) + (offset & ROMBMASK), maxsize);
brelse(bh);
//处理完第一个block
//完成maxsize个数据拷贝
res = maxsize; /* all of it */
//若count的数据无法从第一个block中全部获取
//执行下面的循环
while (res < count) {
//偏移指针
offset += maxsize;
dest += maxsize;
bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
if (!bh)
return -1;
//判断剩余的字节count-res是否超过一个block的大小
maxsize = min_t(unsigned long, count - res, ROMBSIZE);
memcpy(dest, bh->b_data, maxsize);
brelse(bh);
res += maxsize;
}
return res;
}
static unsigned char romfs_dtype_table[] = {
DT_UNKNOWN, DT_DIR, DT_REG, DT_LNK, DT_BLK, DT_CHR, DT_SOCK, DT_FIFO
};
//文件系统对目录的读操作
static int
romfs_readdir(struct file *filp, void *dirent, filldir_t filldir)
{
struct inode *i = filp->f_dentry->d_inode;
struct romfs_inode ri;
unsigned long offset, maxoff;
int j, ino, nextfh;
int stored = 0;
char fsname[ROMFS_MAXFN]; /* XXX dynamic? */
//文件系统越界检查
maxoff = i->i_sb->u.romfs_sb.s_maxsize;
offset = filp->f_pos;
if (!offset) {
//获取目录节点偏移
offset = i->i_ino & ROMFH_MASK;
//读取目录节点信息
if (romfs_copyfrom(i, &ri, offset, ROMFH_SIZE) <= 0)
return stored;
//找到目录对应的偏移位置
offset = ntohl(ri.spec) & ROMFH_MASK;
}
/* Not really failsafe, but we are read-only... */
for(;;) {
if (!offset || offset >= maxoff) {
offset = maxoff;
filp->f_pos = offset;
return stored;
}
filp->f_pos = offset;
/* Fetch inode info */
if (romfs_copyfrom(i, &ri, offset, ROMFH_SIZE) <= 0)
//错误返回
return stored;
j = romfs_strnlen(i, offset+ROMFH_SIZE, sizeof(fsname)-1);
if (j < 0)
return stored;
fsname[j]=0;
//获取文件名
romfs_copyfrom(i, fsname, offset+ROMFH_SIZE, j);
ino = offset;
nextfh = ntohl(ri.next);
//处理硬链接
if ((nextfh & ROMFH_TYPE) == ROMFH_HRD)
ino = ntohl(ri.spec);
//返回用户区，文件名信息
if (filldir(dirent, fsname, j, offset, ino,
romfs_dtype_table[nextfh & ROMFH_TYPE]) < 0) {
//错误返回
return stored;
}
//目录里面的元素个数
stored++;
offset = nextfh & ROMFH_MASK;
}
}
//目录节点查找函数
//可以查找该目录下面的文件，目录等。。
static struct dentry *
romfs_lookup(struct inode *dir, struct dentry *dentry)
{
unsigned long offset, maxoff;
int fslen, res;
struct inode *inode;
char fsname[ROMFS_MAXFN]; /* XXX dynamic? */
struct romfs_inode ri;
const char *name; /* got from dentry */
int len;
res = -EACCES; /* placeholder for "no data here" */
//目录节点 在文件系统的offset
//可以通过ino换算
offset = dir->i_ino & ROMFH_MASK;
//从文件系统读取对应目录文件头数据16字节
if (romfs_copyfrom(dir, &ri, offset, ROMFH_SIZE) <= 0)
goto out;
//maxoff表示文件系统的大小
maxoff = dir->i_sb->u.romfs_sb.s_maxsize;
//目录的偏移起始地址
offset = ntohl(ri.spec) & ROMFH_MASK;
/* OK, now find the file whose name is in "dentry" in the
* directory specified by "dir". */
name = dentry->d_name.name;
len = dentry->d_name.len;
//根据dentry查找对应的inode
for(;;) {
//越界检查
if (!offset || offset >= maxoff)
goto out0;
if (romfs_copyfrom(dir, &ri, offset, ROMFH_SIZE) <= 0)
goto out;
/* try to match the first 16 bytes of name */
//获取文件名长度
fslen = romfs_strnlen(dir, offset+ROMFH_SIZE, ROMFH_SIZE);
if (len < ROMFH_SIZE) { //小于128字节
if (len == fslen) {
/* both are shorter, and same size */
romfs_copyfrom(dir, fsname, offset+ROMFH_SIZE, len+1);
//判断是否一致，正确则退出for
if (strncmp (name, fsname, len) == 0)
break;
}
} else if (fslen >= ROMFH_SIZE) {
/* both are longer; XXX optimize max size */
fslen = romfs_strnlen(dir, offset+ROMFH_SIZE, sizeof(fsname)-1);
if (len == fslen) {
romfs_copyfrom(dir, fsname, offset+ROMFH_SIZE, len+1);
if (strncmp(name, fsname, len) == 0)
break;
}
}
//无法匹配则继续下一个
/* next entry */
offset = ntohl(ri.next) & ROMFH_MASK;
}
/* Hard link handling */
if ((ntohl(ri.next) & ROMFH_TYPE) == ROMFH_HRD)
offset = ntohl(ri.spec) & ROMFH_MASK;
if ((inode = iget(dir->i_sb, offset)))
goto outi;
/*
* it's a bit funky, _lookup needs to return an error code
* (negative) or a NULL, both as a dentry. ENOENT should not
* be returned, instead we need to create a negative dentry by
* d_add(dentry, NULL); and return 0 as no error.
* (Although as I see, it only matters on writable file
* systems).
*/
out0: inode = NULL;
outi: res = 0;
d_add (dentry, inode);
out: return ERR_PTR(res);
}
/*
* Ok, we do readpage, to be able to execute programs. Unfortunately,
* we can't use bmap, since we may have looser alignments.
*/
static int
romfs_readpage(struct file *file, struct page * page)
{
struct inode *inode = page->mapping->host;
unsigned long offset, avail, readlen;
void *buf;
int result = -EIO;
page_cache_get(page);
lock_kernel();
buf = kmap(page);
if (!buf)
goto err_out;
/* 32 bit warning -- but not for us :) */
offset = page->index << PAGE_CACHE_SHIFT;
//inode->i_size只有普通文件＆符号链接文件
if (offset < inode->i_size) {
//avail表明剩余的字节数
avail = inode->i_size-offset;
//读取一页还是avail
readlen = min_t(unsigned long, avail, PAGE_SIZE);
if (romfs_copyfrom(inode, buf, inode->u.romfs_i.i_dataoffset+offset, readlen) == readlen) {
//页面尾部填充零
if (readlen < PAGE_SIZE) {
memset(buf + readlen,0,PAGE_SIZE-readlen);
}
SetPageUptodate(page);
result = 0;
}
}
//错误处理
if (result) {
memset(buf, 0, PAGE_SIZE);
SetPageError(page);
}
flush_dcache_page(page);
UnlockPage(page);
kunmap(page);
err_out:
page_cache_release(page);
unlock_kernel();
return result;
}
/* Mapping from our types to the kernel */
static struct address_space_operations romfs_aops = {
readpage: romfs_readpage
};
static struct file_operations romfs_dir_operations = {
read: generic_read_dir,
readdir: romfs_readdir,
};
static struct inode_operations romfs_dir_inode_operations = {
lookup: romfs_lookup,
};
//权限映射表
static mode_t romfs_modemap[] =
{
0, S_IFDIR+0644, S_IFREG+0644, S_IFLNK+0777,
S_IFBLK+0600, S_IFCHR+0600, S_IFSOCK+0644, S_IFIFO+0644
};
static void
romfs_read_inode(struct inode *i)
{
int nextfh, ino;
struct romfs_inode ri;
ino = i->i_ino & ROMFH_MASK;
i->i_mode = 0;
/* Loop for finding the real hard link */
for(;;) {
//根据ino作为offset获取16字节的文件头
if (romfs_copyfrom(i, &ri, ino, ROMFH_SIZE) <= 0) {
printk("romfs: read error for inode 0x%x\n", ino);
return;
}
/* XXX: do romfs_checksum here too (with name) */
nextfh = ntohl(ri.next);
//当前文件节点是否是硬链接
if ((nextfh & ROMFH_TYPE) != ROMFH_HRD)
break;
//如果是硬链接，spec.info保存对应的地址
//由于ino和offset一致，所以重新读取节点
ino = ntohl(ri.spec) & ROMFH_MASK;
}
//到此只是得到了文件头信息
i->i_nlink = 1; /* Hard to decide.. */
i->i_size = ntohl(ri.size);
i->i_mtime = i->i_atime = i->i_ctime = 0;
i->i_uid = i->i_gid = 0;
/* Precalculate the data offset */
//获取文件名的长度
ino = romfs_strnlen(i, ino+ROMFH_SIZE, ROMFS_MAXFN);
//ino非负，表明没有错误
if (ino >= 0)
//计算从offset位置到数据区的偏移ino=16+name+pad
//仍然16字节对其
ino = ((ROMFH_SIZE+ino+1+ROMFH_PAD)&ROMFH_MASK);
else
ino = 0;
//metasize就是该文件除数据区外的信息长度
i->u.romfs_i.i_metasize = ino;
//dataoffset就是offset+偏移ino,表明数据区的位置
i->u.romfs_i.i_dataoffset = ino+(i->i_ino&ROMFH_MASK);
/* Compute permissions */
//判断节点类型，文件?目录 ?....
//赋予相应的权限
ino = romfs_modemap[nextfh & ROMFH_TYPE];
/* only "normal" files have ops */
switch (nextfh & ROMFH_TYPE) {
case 1: //目录
i->i_size = i->u.romfs_i.i_metasize;
//目录节点的操作函数
i->i_op = &romfs_dir_inode_operations;
i->i_fop = &romfs_dir_operations;
//是否有执行权限
if (nextfh & ROMFH_EXEC)
ino |= S_IXUGO;
i->i_mode = ino;
break;
case 2: //普通文件
i->i_fop = &generic_ro_fops;
i->i_data.a_ops = &romfs_aops;
//是否有执行权限
if (nextfh & ROMFH_EXEC)
ino |= S_IXUGO;
i->i_mode = ino;
break;
case 3://符号链接
i->i_op = &page_symlink_inode_operations;
i->i_data.a_ops = &romfs_aops;
i->i_mode = ino | S_IRWXUGO;
break;
default: //其他由内核处理
/* depending on MBZ for sock/fifos */
nextfh = ntohl(ri.spec);
nextfh = kdev_t_to_nr(MKDEV(nextfh>>16,nextfh&0xffff));
init_special_inode(i, ino, nextfh);
}
}
static struct super_operations romfs_ops = {
read_inode: romfs_read_inode,
statfs: romfs_statfs,
};
static DECLARE_FSTYPE_DEV(romfs_fs_type, "romfs", romfs_read_super);
static int __init init_romfs_fs(void)
{
return register_filesystem(&romfs_fs_type);
}
static void __exit exit_romfs_fs(void)
{
unregister_filesystem(&romfs_fs_type);
}
/* Yes, works even as a module... :) */
EXPORT_NO_SYMBOLS;
module_init(init_romfs_fs)
module_exit(exit_romfs_fs)
MODULE_LICENSE("GPL");