Coder
分类: LINUX
2010-08-01 23:22:11
open()系统调用的服务例程为sys_open()函数,该函数接收的参数为:要打开的文件的路径名filename、访问模式的一些标志flags,以及如果该文件被创建所需要的许可位掩码mode。如果该系统调用成功,就返回一个文件描述符,也就是指向文件对象的指针数组current->
files-> fd_array或者current-> files-> fdtable.fd中新分配给文件的索引;否则,返回-1。
open()系统调用的所有标志
---------------------------------------------------------------------
include/asm-generic/fcntl.h
#define
O_ACCMODE 00000003
#define
O_RDONLY 00000000 /* 为只读而打开 */
#define
O_WRONLY 00000001 /* 为只写而打开 */
#define
O_RDWR 00000002 /* 为读和写而打开 */
#ifndef
O_CREAT /*如果文件不存在则创建它 */
#define
O_CREAT 00000100 /* not fcntl */
#endif
#ifndef
O_EXCL /* 对于O_CREAT标志,如果文件已经存在,则失败 */
#define
O_EXCL 00000200 /* not fcntl */
#endif
#ifndef
O_NOCTTY /* 从不把文件看作终端 */
#define
O_NOCTTY 00000400 /* not fcntl */
#endif
#ifndef
O_TRUNC /* 截断文件(删除所有的现有内容) */
#define
O_TRUNC 00001000 /* not fcntl */
#endif
#ifndef
O_APPEND /* 总是在文件末尾写 */
#define
O_APPEND 00002000
#endif
#ifndef
O_NONBLOCK /* 非阻塞打开 */
#define
O_NONBLOCK 00004000
#endif
#ifndef
O_DSYNC /* 同步写(阻塞,直到物理写终止) */
#define
O_DSYNC 00010000 /* used to be O_SYNC, see below */
#endif
#ifndef
FASYNC /* 通过信号发出I/O事件通知 */
#define
FASYNC 00020000 /* fcntl, for BSD compatibility */
#endif
#ifndef
O_DIRECT
#define
O_DIRECT 00040000 /* direct disk access hint */
#endif
/* 大型文件(文件长度大于off_t所能表示的范围但小于off64_t)*/
#ifndef
O_LARGEFILE
#define
O_LARGEFILE 00100000
#endif
#ifndef
O_DIRECTORY /* 如果文件不是一个目录,则失败 */
#define
O_DIRECTORY 00200000 /* must be a directory */
#endif
#ifndef
O_NOFOLLOW /* 不解析路径名尾部的符号链接 */
#define
O_NOFOLLOW 00400000 /* don't follow links */
#endif
#ifndef
O_NOATIME /*不更新索引节点的访问时间。*/
#define
O_NOATIME 01000000
#endif
#ifndef
O_CLOEXEC
#define
O_CLOEXEC 02000000 /* set close_on_exec */
#endif
---------------------------------------------------------------------
有一些标志的定义是因体系结构而异的。
sys_open()定义如下:
---------------------------------------------------------------------
fs/open.c
SYSCALL_DEFINE3(open,
const char __user *, filename, int, flags, int, mode)
{
long ret;
if (force_o_largefile())
flags |= O_LARGEFILE;
ret = do_sys_open(AT_FDCWD, filename, flags,
mode);
/* avoid REGPARM breakage on x86: */
asmlinkage_protect(3, ret, filename, flags,
mode);
return ret;
}
---------------------------------------------------------------------
这个函数的操作如下:
首先,调用force_o_largefile()来判断是否支持大文件,若是,则设置标志的O_LARGEFILE位。force_o_largefile()其实是一个宏。这个宏也是因体系结构而异的。
其次,调用do_sys_open(AT_FDCWD, filename, flags, mode)来完成实际的打开文件的任务。下面有更详细说明。
最后,调用asmlinkage_protect()以使系统调用正确返回。它也是一个宏,为了防止编译器错误而设。其他平台为空,只有x86平台有定义,为:
---------------------------------------------------------------------
arch/x86/include/asm/linkage.h
/*
* Make sure the compiler doesn't do anything
stupid with the
* arguments on the stack - they are owned by
the *caller*, not
* the callee. This just fools gcc into not
spilling into them,
* and keeps it from doing tailcall recursion
and/or using the
* stack slots for temporaries, since they are
live and "used"
* all the way to the end of the function.
*
* NOTE! On x86-64, all the arguments are in
registers, so this
* only matters on a 32-bit kernel.
*/#define asmlinkage_protect(n, ret, args...)
\
__asmlinkage_protect##n(ret, ##args)
#define
__asmlinkage_protect_n(ret, args...) \
__asm__ __volatile__ ("" :
"=r" (ret) : "0" (ret), ##args)
#define
__asmlinkage_protect0(ret) \
__asmlinkage_protect_n(ret)
#define
__asmlinkage_protect1(ret, arg1) \
__asmlinkage_protect_n(ret, "g"
(arg1))
#define
__asmlinkage_protect2(ret, arg1, arg2) \
__asmlinkage_protect_n(ret, "g"
(arg1), "g" (arg2))
#define
__asmlinkage_protect3(ret, arg1, arg2, arg3) \
__asmlinkage_protect_n(ret, "g"
(arg1), "g" (arg2), "g" (arg3))
---------------------------------------------------------------------
do_sys_open()函数定义如下:
---------------------------------------------------------------------
fs/open.c
long
do_sys_open(int dfd, const char __user *filename, int flags, int mode)
{
char *tmp = getname(filename);
int fd = PTR_ERR(tmp);
if (!IS_ERR(tmp)) {
fd = get_unused_fd_flags(flags);
if (fd >= 0) {
struct file *f = do_filp_open(dfd,
tmp, flags, mode, 0);
if (IS_ERR(f)) {
put_unused_fd(fd);
fd = PTR_ERR(f);
} else {
fsnotify_open(f->f_path.dentry);
fd_install(fd, f);
}
}
putname(tmp);
}
return fd;
}
---------------------------------------------------------------------
第一个参数是多么的眼熟啊,查找路径名的do_path_lookup()函数的第一个参数也是它,用于说明查找路径名的基目录。关于它,前面路径名查找已有说明了。其余参数则是sys_open()将传进来的参数传给了它。
这个函数执行如下操作:
1、调用getname(filename)从进程地址空间读取文件的路径名,将路径名的地址存放在局部变量tmp。getname(filename)本质上,首先从名为"names_cache"路径名slab缓存中分配内存区,然后将路径名从用户空间复制到该内存区中。
2、调用get_unused_fd_flags (flags)从当前进程的文件描述符表中找一个空位。其定义为:
---------------------------------------------------------------------
include/linux/file.h
#define
get_unused_fd_flags(flags) alloc_fd(0, (flags))
---------------------------------------------------------------------
这是一个宏,仅仅是对alloc_fd()函数特殊参数下调用的别名,对alloc_fd()函数定义如下:
---------------------------------------------------------------------
fs/file.c
/*
* allocate a file descriptor, mark it busy.
*/
int
alloc_fd(unsigned start, unsigned flags)
{
struct files_struct *files =
current->files;
unsigned int fd;
int error;
struct fdtable *fdt;
spin_lock(&files->file_lock);
repeat:
fdt = files_fdtable(files);
fd = start;
if (fd < files->next_fd)
fd = files->next_fd;
if (fd < fdt->max_fds)
fd = find_next_zero_bit(fdt->open_fds->fds_bits,
fdt->max_fds, fd);
error = expand_files(files, fd);
if (error < 0)
goto out;
/*
* If
we needed to expand the fs array we
*
might have blocked - try again.
*/
if (error)
goto repeat;
if (start <= files->next_fd)
files->next_fd = fd + 1;
FD_SET(fd, fdt->open_fds);
if (flags & O_CLOEXEC)
FD_SET(fd, fdt->close_on_exec);
else
FD_CLR(fd, fdt->close_on_exec);
error = fd;
#if
1
/* Sanity check */
if (rcu_dereference_raw(fdt->fd[fd]) !=
NULL) {
printk(KERN_WARNING "alloc_fd: slot
%d not NULL!\n", fd);
rcu_assign_pointer(fdt->fd[fd], NULL);
}
#endif
out:
spin_unlock(&files->file_lock);
return error;
}
---------------------------------------------------------------------
这个函数执行如下操作:
a.将current->files-> next_fd字段赋值给局部变量。
b.调用find_next_zero_bit(fdt->open_fds->fds_bits,
fdt->max_fds, fd)来在文件描述符表中寻找下一个可以分配的文件描述符。这个函数也是因体系结构而异,系统中通用的函数定义为:
---------------------------------------------------------------------
lib/find_next_bit.c
unsigned
long find_next_zero_bit(const unsigned long *addr,
unsigned long size, unsigned long offset)
{
const unsigned long *p = addr +
BITOP_WORD(offset);
unsigned long result = offset &
~(BITS_PER_LONG-1);
unsigned long tmp;
if (offset >= size)
return
size;
size -= result;
offset %= BITS_PER_LONG;
if (offset) {
tmp = *(p++);
tmp |= ~0UL >> (BITS_PER_LONG -
offset);
if (size < BITS_PER_LONG)
goto found_first;
if (~tmp)
goto found_middle;
size -= BITS_PER_LONG;
result += BITS_PER_LONG;
}
while (size & ~(BITS_PER_LONG-1)) {
if (~(tmp = *(p++)))
goto found_middle;
result += BITS_PER_LONG;
size -= BITS_PER_LONG;
}
if (!size)
return result;
tmp = *p;
found_first:
tmp |= ~0UL << size;
if (tmp == ~0UL) /* Are any bits zero? */
return result + size; /* Nope. */
found_middle:
return result + ffz(tmp);
}
---------------------------------------------------------------------
(1)、上面的BITOP_WORD(offset)清一色定义为:
#define
BITOP_WORD(nr) ((nr) / BITS_PER_LONG)
首先求得第一个要查找的long的位置。
(2)、offset &
~(BITS_PER_LONG-1)等价于offset除以32再乘以32,以此来求得所要查找的第一个字第0位在表中的位置,只不过会比除法运算效率高很多。
(3)、如果传递的起始查找位置甚至大于最大可能值,则返回最大可能值。
(4)、查找第一个0位所在的long型值的位置。而局部变量tmp中会保存该long型量的值。
(5)、返回查找到的第一个0位的位置或可能的最大值。
c.调用expand_files(files,
fd),来扩展文件描述符表,files_struct的fd_array数组成员可以在打开的文件较少时使用,但当打开的文件较多时,就会对文件描述符表进行扩展。
d.更新files->next_fd字段,将分配的文件描述符添加进fdt->open_fds,如果设置了O_CLOEXEC则将文件描述符添加进fdt->close_on_exec,若没有,则清除fdt->close_on_exec中的相应位。
e.返回文件描述符。
3、调用do_filp_open(dfd,
tmp, flags, mode, 0)函数,传递给它的参数依次为查找路径名的基目录、文件路径名、访问模式标志以及许可权位掩码、访问模式位。这个函数定义为:
---------------------------------------------------------------------
fs/namei.c
1761
/*
1762 * Note that the low bits of the passed in
"open_flag"
1763 * are not the same as in the local variable
"flag". See
1764 * open_to_namei_flags() for more details.
1765 */
1766
struct file *do_filp_open(int dfd, const char *pathname,
1767 int open_flag, int mode, int
acc_mode)
1768
{
1769 struct file *filp;
1770 struct nameidata nd;
1771 int error;
1772 struct path path;
1773 int count = 0;
1774 int flag =
open_to_namei_flags(open_flag);
1775 int force_reval = 0;
1776
1777 if (!(open_flag & O_CREAT))
1778 mode = 0;
1779
1780 /*
1781 * O_SYNC is implemented as
__O_SYNC|O_DSYNC. As many places only
1782 * check for O_DSYNC if the need any
syncing at all we enforce it's
1783 * always set instead of having to
deal with possibly weird behaviour
1784 * for malicious applications setting
only __O_SYNC.
1785 */
1786 if (open_flag & __O_SYNC)
1787 open_flag |= O_DSYNC;
1788
1789 if (!acc_mode)
1790 acc_mode = MAY_OPEN |
ACC_MODE(open_flag);
1791
1792 /* O_TRUNC implies we need access
checks for write permissions */
1793 if (open_flag & O_TRUNC)
1794 acc_mode |= MAY_WRITE;
1795
1796 /* Allow the LSM permission hook to
distinguish append
1797 access from general write access.
*/
1798 if (open_flag & O_APPEND)
1799 acc_mode |= MAY_APPEND;
1800
1801 /* find the parent */
1802
reval:
1803 error = path_init(dfd, pathname,
LOOKUP_PARENT, &nd);
1804 if (error)
1805 return ERR_PTR(error);
1806 if (force_reval)
1807 nd.flags |= LOOKUP_REVAL;
1808
1809 current->total_link_count = 0;
1810 error = link_path_walk(pathname,
&nd);
1811 if (error) {
1812 filp = ERR_PTR(error);
1813 goto out;
1814 }
1815 if (unlikely(!audit_dummy_context())
&& (open_flag & O_CREAT))
1816 audit_inode(pathname,
nd.path.dentry);
1817
1818 /*
1819 * We have the parent and last
component.
1820 */
1821
1822 error = -ENFILE;
1823 filp = get_empty_filp();
1824 if (filp == NULL)
1825 goto exit_parent;
1826 nd.intent.open.file = filp;
1827 filp->f_flags = open_flag;
1828 nd.intent.open.flags = flag;
1829 nd.intent.open.create_mode = mode;
1830 nd.flags &= ~LOOKUP_PARENT;
1831 nd.flags |= LOOKUP_OPEN;
1832 if (open_flag & O_CREAT) {
1833 nd.flags |= LOOKUP_CREATE;
1834 if (open_flag & O_EXCL)
1835 nd.flags |=
LOOKUP_EXCL;
1836 }
1837 if (open_flag & O_DIRECTORY)
1838 nd.flags |= LOOKUP_DIRECTORY;
1839 if (!(open_flag & O_NOFOLLOW))
1840 nd.flags |= LOOKUP_FOLLOW;
1841 filp = do_last(&nd, &path,
open_flag, acc_mode, mode, pathname);
1842 while (unlikely(!filp)) { /* trailing
symlink */
1843 struct path holder;
1844 struct inode *inode =
path.dentry->d_inode;
1845 void *cookie;
1846 error = -ELOOP;
1847 /* S_ISDIR part is a temporary
automount kludge */
1848 if (!(nd.flags &
LOOKUP_FOLLOW) && !S_ISDIR(inode->i_mode))
1849 goto exit_dput;
1850 if (count++ == 32)
1851 goto exit_dput;
1852 /*
1853 * This is subtle. Instead of
calling do_follow_link() we do
1854 * the thing by hands. The
reason is that this way we have zero
1855 * link_count and path_walk()
(called from ->follow_link)
1856 * honoring
LOOKUP_PARENT. After that we have the
parent and
1857 * last component, i.e. we are
in the same situation as after
1858 * the first path_walk(). Well, almost - if the last component
1859 * is normal we get its copy
stored in nd->last.name and we will
1860 * have to putname() it when
we are done. Procfs-like symlinks
1861 * just set LAST_BIND.
1862 */
1863 nd.flags |= LOOKUP_PARENT;
1864 error =
security_inode_follow_link(path.dentry, &nd);
1865 if (error)
1866 goto exit_dput;
1867 error =
__do_follow_link(&path, &nd, &cookie);
1868 if (unlikely(error)) {
1869 /* nd.path had been
dropped */
1870 if (!IS_ERR(cookie)
&& inode->i_op->put_link)
1871
inode->i_op->put_link(path.dentry, &nd, cookie);
1872 path_put(&path);
1873
release_open_intent(&nd);
1874 filp = ERR_PTR(error);
1875 goto out;
1876 }
1877 holder = path;
1878 nd.flags &=
~LOOKUP_PARENT;
1879 filp = do_last(&nd,
&path, open_flag, acc_mode, mode, pathname);
1880 if
(inode->i_op->put_link)
1881
inode->i_op->put_link(holder.dentry, &nd, cookie);
1882 path_put(&holder);
1883 }
1884
out:
1885 if (nd.root.mnt)
1886 path_put(&nd.root);
1887 if (filp == ERR_PTR(-ESTALE)
&& !force_reval) {
1888 force_reval = 1;
1889 goto reval;
1890 }
1891 return filp;
1892
1893
exit_dput:
1894 path_put_conditional(&path,
&nd);
1895 if (!IS_ERR(nd.intent.open.file))
1896 release_open_intent(&nd);
1897
exit_parent:
1898
path_put(&nd.path);
1899 filp = ERR_PTR(error);
1900 goto out;
1901
} ---------------------------------------------------------------------
这个函数一次执行下列步骤:
a.把访问模式拷贝到flag标志中,但是,用特殊的格式对方问模式标志O_RDONLY、O_WRONLY和O_RDWR进行编码。由函数open_to_namei_flags()完成,其定义如下:
---------------------------------------------------------------------
fs/namei.c
/*
* Note that while the flag value (low two
bits) for sys_open means:
* 00 -
read-only
* 01 -
write-only
* 10 -
read-write
* 11 -
special
* it is changed into
* 00 -
no permissions needed
* 01 -
read-permission
* 10 -
write-permission
* 11 -
read-write
* for the internal routines (ie
open_namei()/follow_link() etc)
* This is more logical, and also allows the 00
"no perm needed"
* to be used for symlinks (where the
permissions are checked
* later).
*
*/
static
inline int open_to_namei_flags(int flag)
{
if ((flag+1) & O_ACCMODE)
flag++;
return flag;
}
---------------------------------------------------------------------
注释中说的已经很清楚了。
根据open_flag原来的值适当更新打开标志open_flag,根据open_flag适当更新访问模式acc_mode
b.调用path_init(dfd,
pathname, LOOKUP_PARENT, &nd),将查找的路径名的基路径找到,并赋给nd的path字段。注意在这个函数中设置了nd->flags的LOOKUP_PARENT,也就是要查找路径名最后一个分量的父目录。
c.设置current->total_link_count为0。
d.调用link_path_walk(pathname,
&nd)查找路径名最后一个分量的父母的路径path结构体,保存在nd.path中。因为,路径名的最后一个分量有可能是不存在的而需要创建。
e.调用get_empty_filp()函数从file的slab缓冲区filp_cachep中分配一个file结构,并初始化它的一些字段。如果返回值为NULL,则返回错误码-ENFILE的指针形式。
f.设置nd.intent.open.file为上一步分配的file结构的地址filp,设置filp的打开文件时所制定的标志f_flags为传递进来并经过适当修改的打开标志open_flag。设置nd.intent.open.flags为访问模式flag,设置nd.intent.open.create_mode为创建模式mode。清除nd.flags的LOOKUP_PARENT标志,设置其LOOKUP_OPEN标志。如果在打开标志中设置了O_CREAT,则设置nd.flags的LOOKUP_CREATE标志,若同时设置了打开标志的O_EXCL,则同时设置nd.flags的OOKUP_EXCL。若设置了打开标志的O_DIRECTORY,则设置nd.flags的LOOKUP_DIRECTORY。若没有设置打开标志的O_NOFOLLOW位,则设置nd.flags的LOOKUP_FOLLOW。
即是根据打开标志来设置nd.flags的相应位。这些标志似乎都只与查找的路径名的最后一个分量有关。
g.调用do_last()来完成路径名最后一个分量的处理。又是一个非常长的函数,它接受六个参数,nd为前面查找的路径名最后一个分量的父目录的nameidata结构,path为一个path结构的局部变量,打开标志,acc_mode,创建模式(如果需要的话)mode和路径名。该函数定义如下:
---------------------------------------------------------------------
fs/namei.c
1617
static struct file *do_last(struct nameidata *nd, struct path *path,
1618 int open_flag, int acc_mode,
1619 int mode, const
char *pathname)
1620
{
1621 struct dentry *dir =
nd->path.dentry;
1622 struct file *filp;
1623 int error = -EISDIR;
1624
1625 switch (nd->last_type) {
1626 case LAST_DOTDOT:
1627 follow_dotdot(nd);
1628 dir = nd->path.dentry;
1629 case LAST_DOT:
1630 if
(nd->path.mnt->mnt_sb->s_type->fs_flags & FS_REVAL_DOT) {
1631 if
(!dir->d_op->d_revalidate(dir, nd)) {
1632 error =
-ESTALE;
1633 goto exit;
1634 }
1635 }
1636 /* fallthrough */
1637 case LAST_ROOT:
1638 if (open_flag & O_CREAT)
1639 goto exit;
1640 /* fallthrough */
1641 case LAST_BIND:
1642 audit_inode(pathname, dir);
1643 goto ok;
1644 }
1645
1646 /* trailing slashes? */
1647 if (nd->last.name[nd->last.len])
{
1648 if (open_flag & O_CREAT)
1649 goto exit;
1650 nd->flags |=
LOOKUP_DIRECTORY | LOOKUP_FOLLOW;
1651 }
1652
1653 /* just plain open? */
1654 if (!(open_flag & O_CREAT)) {
1655 error = do_lookup(nd,
&nd->last, path);
1656 if (error)
1657 goto exit;
1658 error = -ENOENT;
1659 if
(!path->dentry->d_inode)
1660 goto exit_dput;
1661 if
(path->dentry->d_inode->i_op->follow_link)
1662 return NULL;
1663 error = -ENOTDIR;
1664 if (nd->flags &
LOOKUP_DIRECTORY) {
1665 if
(!path->dentry->d_inode->i_op->lookup)
1666 goto
exit_dput;
1667 }
1668 path_to_nameidata(path, nd);
1669 audit_inode(pathname,
nd->path.dentry);
1670 goto ok;
1671 }
1672
1673 /* OK, it's O_CREAT */
1674
mutex_lock(&dir->d_inode->i_mutex);
1675
1676 path->dentry = lookup_hash(nd);
1677 path->mnt = nd->path.mnt;
1678
1679 error = PTR_ERR(path->dentry);
1680 if (IS_ERR(path->dentry)) {
1681
mutex_unlock(&dir->d_inode->i_mutex);
1682 goto exit;
1683 }
1684
1685 if (IS_ERR(nd->intent.open.file)) {
1686 error =
PTR_ERR(nd->intent.open.file);
1687 goto exit_mutex_unlock;
1688 }
1689
1690 /* Negative dentry, just create the
file */
1691 if (!path->dentry->d_inode) {
1692 /*
1693 * This write is needed to
ensure that a
1694 * ro->rw transition does
not occur between
1695 * the time when the file is
created and when
1696 * a permanent write count is
taken through
1697 * the 'struct file' in
nameidata_to_filp().
1698 */
1699 error =
mnt_want_write(nd->path.mnt);
1700 if (error)
1701 goto exit_mutex_unlock;
1702 error =
__open_namei_create(nd, path, open_flag, mode);
1703 if (error) {
1704
mnt_drop_write(nd->path.mnt);
1705 goto exit;
1706 }
1707 filp = nameidata_to_filp(nd);
1708
mnt_drop_write(nd->path.mnt);
1709 if (!IS_ERR(filp)) {
1710 error =
ima_file_check(filp, acc_mode);
1711 if (error) {
1712 fput(filp);
1713 filp =
ERR_PTR(error);
1714 }
1715 }
1716 return filp;
1717 }
1718
1719 /*
1720 * It already exists.
1721 */
1722
mutex_unlock(&dir->d_inode->i_mutex);
1723 audit_inode(pathname,
path->dentry);
1724
1725 error = -EEXIST;
1726 if (open_flag & O_EXCL)
1727 goto exit_dput;
1728
1729 if (__follow_mount(path)) {
1730 error = -ELOOP;
1731 if (open_flag &
O_NOFOLLOW)
1732 goto exit_dput;
1733 }
1734
1735 error = -ENOENT;
1736 if (!path->dentry->d_inode)
1737 goto exit_dput;
1738
1739 if
(path->dentry->d_inode->i_op->follow_link)
1740 return NULL;
1741
1742 path_to_nameidata(path, nd);
1743 error = -EISDIR;
1744 if
(S_ISDIR(path->dentry->d_inode->i_mode))
1745 goto exit;
1746
ok:
1747 filp = finish_open(nd, open_flag,
acc_mode);
1748 return filp;
1749
1750
exit_mutex_unlock:
1751
mutex_unlock(&dir->d_inode->i_mutex);
1752
exit_dput:
1753 path_put_conditional(path, nd);
1754
exit:
1755 if (!IS_ERR(nd->intent.open.file))
1756 release_open_intent(nd);
1757 path_put(&nd->path);
1758 return ERR_PTR(error);
1759
}
---------------------------------------------------------------------
再对这些参数进行以下说明,nd指向的nameidata的path中存放的是路径名最后一个分量的父目录的路径path,其last字段中存放的是路径名最后一个分量的名字的信息qstr结构
(1)这个函数首先根据路径名的最后一个分量的名字信息,来采取一些动作。
如果最后一个分量是“..”则调用follow_dotdot(nd)返回上一级目录,并设置局部变量dir为nd->path.dentry。
若最后一个分量是“.”,则检查nd->path.mnt->mnt_sb->s_type->fs_flags的FS_REVAL_DOT,若设置了该标志,则调用目录项的dir->d_op->d_revalidate(dir,
nd)方法,若该方法失败,则释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-ESTALE。
若最后一个分量为根目录。若设置了打开标志为O_CREAT,则释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-EISDIR。
若为符号链接,则调用finish_open()来完成最后的打开文件操作,并返回file结构指针filp。finish_open()函数稍后解释。
这一步中处理那些最后一个分量的路径已经获得并保存在nd->path中或者最后一个分量为符号链接的情况。同时我们也可以看到,是可以直接使用open来打开目录的,但是不能创建目录。
(2)、若路径名的最后一个分量是以“/”结尾(这根据nd->last.name[nd->last.len]的值来判断,在link_path_walk()函数中求出nd->last值的相关部分可以看出,若已“/”结尾,则nd->last.name[nd->last.len]的值正是字符’ /’)的,则检查打开标志是否设置了O_CREAT,若是则释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-EISDIR;若没有则,设置nd查找标志nd->flags的LOOKUP_DIRECTORY
和LOOKUP_FOLLOW位。
(3)、若打开标志没有设置O_CREAT。则调用do_lookup(nd, &nd->last, path)来完成最路径名中最后一个分量路径的查找。
若返回错误码,则释放先前分配的file结构,减少nd->path的引用计数,并返回该错误码。
若查找的结果path->dentry->d_inode为NULL,则调用path_put_conditional(path, nd)来释放查找到的path->dentry,若最后一个分量表示的是挂载点则还要释放path->mnt。释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-ENOENT。
若最后一个分量表示的是符号链接,则返回NULL。
若设置了查找标志的LOOKUP_DIRECTORY位,则还有判断找到的是否为一个目录(通过检查path->dentry->d_inode->i_op->lookup),若不是目录,则调用path_put_conditional(path, nd)来释放查找到的path->dentry,若最后一个分量表示的是挂载点则还要释放path->mnt。释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-ENOTDIR。
调用path_to_nameidata(path, nd)将使得nd->path中保存有路径名最后一个分量的路径。完成审计信息记录。
调用finish_open()来完成最后的打开文件操作,并返回file结构指针filp。finish_open()函数稍后解释。
(4)、打开标志设置了O_CREAT,若文件不存在则要创建的情况。首先要对父目录的inode上锁(mutex_lock(&dir->d_inode->i_mutex)),调用lookup_hash(nd)在目录项缓存中查找或者创建路径名最后一个分量的目录项。其定义为:
---------------------------------------------------------------------
fs/namei.c
1122
static struct dentry *__lookup_hash(struct qstr *name,
1123 struct dentry *base, struct
nameidata *nd)
1124
{
1125 struct dentry *dentry;
1126 struct inode *inode;
1127 int err;
1128
1129 inode = base->d_inode;
1130
1131 /*
1132 * See if the low-level filesystem
might want
1133 * to use its own hash..
1134 */
1135 if (base->d_op &&
base->d_op->d_hash) {
1136 err =
base->d_op->d_hash(base, name);
1137 dentry = ERR_PTR(err);
1138 if (err < 0)
1139 goto out;
1140 }
1141
1142 dentry = __d_lookup(base, name);
1143
1144 /* lockess __d_lookup may fail due to
concurrent d_move()
1145 * in some unrelated directory, so try
with d_lookup
1146 */
1147 if (!dentry)
1148 dentry = d_lookup(base, name);
1149
1150 if (dentry && dentry->d_op
&& dentry->d_op->d_revalidate)
1151 dentry = do_revalidate(dentry,
nd);
1152
1153 if (!dentry) {
1154 struct dentry *new;
1155
1156 /* Don't create child dentry
for a dead directory. */
1157 dentry = ERR_PTR(-ENOENT);
1158 if (IS_DEADDIR(inode))
1159 goto out;
1160
1161 new = d_alloc(base, name);
1162 dentry = ERR_PTR(-ENOMEM);
1163 if (!new)
1164 goto out;
1165 dentry =
inode->i_op->lookup(inode, new, nd);
1166 if (!dentry)
1167 dentry = new;
1168 else
1169 dput(new);
1170 }
1171
out:
1172 return dentry;
1173
}
1175
/*
1176 * Restricted form of lookup. Doesn't follow
links, single-component only,
1177 * needs parent already locked. Doesn't follow
mounts.
1178 * SMP-safe.
1179 */
1180
static struct dentry *lookup_hash(struct nameidata *nd)
1181
{
1182 int err;
1183
1184 err =
exec_permission(nd->path.dentry->d_inode);
1185 if (err)
1186 return ERR_PTR(err);
1187 return __lookup_hash(&nd->last,
nd->path.dentry, nd);
1188
}
---------------------------------------------------------------------
lookup_hash(nd)函数当在目录项缓存中没有找到要找的目录项时,会分配目录项,并且会调用父目录的inode->i_op->lookup(inode, new, nd)方法来创建所要查找的文件的inode等信息,并设置目录项的适当字段。但是在目录中没有所要查找的文件时,lookup(inode,
new, nd)方法并不返回错误。
lookup_hash(nd)函数返回的结果被赋给path->dentry。path用来存放路径名最后一个分量的path结构。
初始化path->mnt为父目录的vfsmount对象。
(5)、检查path->dentry是否是一个错误码的指针形式,若是则对父目录的inode解锁(mutex_unlock(&dir->d_inode->i_mutex)),减少nd->path的引用计数,并返回该错误码。
(6)、检查nd->intent.open.file是否包含一个错误码,若是则首先对父目录的inode解锁,接着调用path_put_conditional(path, nd)来释放由lookup_hash(nd)查找到或创建的path->dentry,若最后一个分量表示的是挂载点则还要释放path->mnt。释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-ENOTDIR。
(7)、检查path->dentry->d_inode是否为NULL,若是,则说明要创建一个文件。
首先调用mnt_want_write(nd->path.mnt),来确保在创建文件和在nameidata_to_filp()中通过struct file取得固定的写计数之间不会发生ro -> rw的转换。这个函数本质上增加vfsmount对象的写着计数器mnt->mnt_writers。
调用__open_namei_create(nd, path, open_flag, mode)函数来创建一个文件,这个函数定义为:
---------------------------------------------------------------------
fs/namei.c
1502
/*
1503 * Be careful about ever adding any more
callers of this
1504 * function.
Its flags must be in the namei format, not
1505 * what get passed to sys_open().
1506 */
1507
static int __open_namei_create(struct nameidata *nd, struct path *path,
1508 int open_flag,
int mode)
1509
{
1510 int error;
1511 struct dentry *dir =
nd->path.dentry;
1512
1513 if (!IS_POSIXACL(dir->d_inode))
1514 mode &= ~current_umask();
1515 error =
security_path_mknod(&nd->path, path->dentry, mode, 0);
1516 if (error)
1517 goto out_unlock;
1518 error = vfs_create(dir->d_inode,
path->dentry, mode, nd);
1519
out_unlock:
1520
mutex_unlock(&dir->d_inode->i_mutex);
1521 dput(nd->path.dentry);
1522 nd->path.dentry = path->dentry;
1523 if (error)
1524 return error;
1525 /* Don't check for write permission,
don't truncate */
1526 return may_open(&nd->path, 0,
open_flag & ~O_TRUNC);
1527
}
---------------------------------------------------------------------
__open_namei_create()函数在执行了访问权限检查后,就调用父目录inode的create方法dir->i_op->create(dir, dentry,
mode, nd)来创建文件。之后,__open_namei_create()解除对于父目录inode的锁定,释放父目录目录项,并将路径名最后一个分量目录项path->dentry赋给nd->path.dentry。然后返回对may_open(&nd->path,
0, open_flag & ~O_TRUNC)调用的返回值。
调用nameidata_to_filp(nd)来将一个nameidata转换为一个打开的filp,这个函数本质上主要调用__dentry_open(nd->path.dentry,
nd->path.mnt, filp, NULL, cred)来根据当前进程的状态和获得的目录项来设置nd->intent.open.file所指向的file结构的各字段。
调用mnt_drop_write(nd->path.mnt)来减少nd->path.mnt写者计数器的值。
返回filp。
(8)、尽管设置了打开标志的O_CREAT,但是却找到了所需的文件。则首先对父目录inode解锁。检查打开标志是否设置了O_EXCL,若是,则调用path_put_conditional(path, nd)来释放查找到的path->dentry,若最后一个分量表示的是挂载点则还要释放path->mnt。释放先前分配的file结构,减少nd->path的引用计数,并返回错误码-EEXIST。
调用__follow_mount(path) 找到挂载在本路径上的文件系统,即vfsmount对象的地址和目录项对象地址。
调用finish_open()来完成打开草走。
(9)、返回filp。
h.若最后一个分量是一个符号链接,则追踪符号链接。
i、返回filp
4、将文件安装在fd数组中。
5、释放路径名所占用的临时内存空间tmp
6、返回文件描述符fd。