ce8d2cdf3d
Why do we need r/o bind mounts? This feature allows a read-only view into a read-write filesystem. In the process of doing that, it also provides infrastructure for keeping track of the number of writers to any given mount. This has a number of uses. It allows chroots to have parts of filesystems writable. It will be useful for containers in the future because users may have root inside a container, but should not be allowed to write to somefilesystems. This also replaces patches that vserver has had out of the tree for several years. It allows security enhancement by making sure that parts of your filesystem read-only (such as when you don't trust your FTP server), when you don't want to have entire new filesystems mounted, or when you want atime selectively updated. I've been using the following script to test that the feature is working as desired. It takes a directory and makes a regular bind and a r/o bind mount of it. It then performs some normal filesystem operations on the three directories, including ones that are expected to fail, like creating a file on the r/o mount. This patch: Some filesystems forego the vfs and may_open() and create their own 'struct file's. This patch creates a couple of helper functions which can be used by these filesystems, and will provide a unified place which the r/o bind mount code may patch. Also, rename an existing, static-scope init_file() to a less generic name. Signed-off-by: Dave Hansen <haveblue@us.ibm.com> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
145 lines
3.1 KiB
C
145 lines
3.1 KiB
C
/*
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* tiny-shmem.c: simple shmemfs and tmpfs using ramfs code
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*
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* Matt Mackall <mpm@selenic.com> January, 2004
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* derived from mm/shmem.c and fs/ramfs/inode.c
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*
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* This is intended for small system where the benefits of the full
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* shmem code (swap-backed and resource-limited) are outweighed by
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* their complexity. On systems without swap this code should be
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* effectively equivalent, but much lighter weight.
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*/
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#include <linux/fs.h>
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#include <linux/init.h>
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#include <linux/vfs.h>
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#include <linux/mount.h>
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#include <linux/file.h>
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/swap.h>
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#include <linux/ramfs.h>
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static struct file_system_type tmpfs_fs_type = {
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.name = "tmpfs",
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.get_sb = ramfs_get_sb,
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.kill_sb = kill_litter_super,
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};
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static struct vfsmount *shm_mnt;
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static int __init init_tmpfs(void)
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{
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BUG_ON(register_filesystem(&tmpfs_fs_type) != 0);
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shm_mnt = kern_mount(&tmpfs_fs_type);
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BUG_ON(IS_ERR(shm_mnt));
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return 0;
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}
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module_init(init_tmpfs)
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/*
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* shmem_file_setup - get an unlinked file living in tmpfs
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*
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* @name: name for dentry (to be seen in /proc/<pid>/maps
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* @size: size to be set for the file
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*
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*/
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struct file *shmem_file_setup(char *name, loff_t size, unsigned long flags)
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{
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int error;
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struct file *file;
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struct inode *inode;
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struct dentry *dentry, *root;
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struct qstr this;
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if (IS_ERR(shm_mnt))
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return (void *)shm_mnt;
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error = -ENOMEM;
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this.name = name;
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this.len = strlen(name);
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this.hash = 0; /* will go */
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root = shm_mnt->mnt_root;
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dentry = d_alloc(root, &this);
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if (!dentry)
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goto put_memory;
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error = -ENOSPC;
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inode = ramfs_get_inode(root->d_sb, S_IFREG | S_IRWXUGO, 0);
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if (!inode)
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goto put_dentry;
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d_instantiate(dentry, inode);
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error = -ENFILE;
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file = alloc_file(shm_mnt, dentry, FMODE_WRITE | FMODE_READ,
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&ramfs_file_operations);
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if (!file)
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goto put_dentry;
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inode->i_nlink = 0; /* It is unlinked */
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/* notify everyone as to the change of file size */
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error = do_truncate(dentry, size, 0, file);
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if (error < 0)
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goto close_file;
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return file;
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close_file:
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put_filp(file);
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put_dentry:
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dput(dentry);
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put_memory:
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return ERR_PTR(error);
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}
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/*
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* shmem_zero_setup - setup a shared anonymous mapping
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*
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* @vma: the vma to be mmapped is prepared by do_mmap_pgoff
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*/
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int shmem_zero_setup(struct vm_area_struct *vma)
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{
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struct file *file;
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loff_t size = vma->vm_end - vma->vm_start;
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file = shmem_file_setup("dev/zero", size, vma->vm_flags);
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if (IS_ERR(file))
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return PTR_ERR(file);
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if (vma->vm_file)
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fput(vma->vm_file);
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vma->vm_file = file;
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vma->vm_ops = &generic_file_vm_ops;
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return 0;
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}
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int shmem_unuse(swp_entry_t entry, struct page *page)
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{
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return 0;
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}
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#if 0
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int shmem_mmap(struct file *file, struct vm_area_struct *vma)
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{
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file_accessed(file);
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#ifndef CONFIG_MMU
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return ramfs_nommu_mmap(file, vma);
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#else
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return 0;
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#endif
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}
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#endif /* 0 */
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#ifndef CONFIG_MMU
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unsigned long shmem_get_unmapped_area(struct file *file,
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unsigned long addr,
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unsigned long len,
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unsigned long pgoff,
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unsigned long flags)
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{
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return ramfs_nommu_get_unmapped_area(file, addr, len, pgoff, flags);
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}
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#endif
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