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List:       linux-kernel
Subject:    flink(2) patch, O_ANONYMOUS flag and struct inode_operations ideas
From:       Remi Turk <remi () abcweb ! nl>
Date:       2000-07-31 20:19:01
[Download RAW message or body]

Hi,

Attached is a patch against 2.4.0-test5 which implements
flink(fd, newname)
It allows you to link a file with inode->nlink == 0 back into the
filesystem, so it won't be garbage-collected when your process exits.

% echo 'Hello, world!' > old
% ll -i
total 0
  32619 -rw-r--r--   1 remi     users          14 Jul 31 15:44 old
% exec 5<old
% rm old
% ll -i
total 0
% ~/src/flink/flink 5 new
% ll -i 
total 0
  32619 -rw-r--r--   1 remi     users          14 Jul 31 15:44 new
% cat new
Hello, world!
%

My second idea is related and even crazier:
Temporary files are often create(2)ed and then unlink(2)ed.
create(2) creates an inode and a hard-link. You then use unlink(2)
to remove the hard-link.
So I've been thinking about an O_ANONYMOUS or O_NONAME flag for open(2)
to specify you don't want any name for it.
(So create(2) with O_ANONYMOUS + flink(2) would be the same (except
non-atomic) as create(2))

However, the ->create method of struct inode_operations already creates
the first name for the file. Which brings me to my next idea:
What if ->create only creates the inode, not the first hard-link?
If the O_ANONYMOUS flag is not given, the VFS could then call the ->link
method for the creation of the first hardlink.
(I'm afraid this would cause trouble for filesystems without support
for nlink > 1 like FAT.)

P.S. flink(2) is my first patch, so please don't laugh too hard ;-).

--
Linux localhost.localdomain 2.4.0-test5 #1 Mon Jul 31 13:21:59 CEST 2000
["flink.c" (text/plain)]

#include <linux/unistd.h>
#include <stdlib.h>
#include <stdio.h>

_syscall2(long, flink, unsigned int, fd, const char *, newname)

int main(int argc, char **argv)
{
	if (argc != 3) {
		fprintf(stderr, "Usage: flink fd newname\n");
		return 1;
	}
	if (flink(atoi(argv[1]), argv[2]) < 0) {
		perror("flink");
		return 1;
	}
	return 0;
}
















["flink-2.4.0-test5.patch" (text/plain)]

diff -urN linux-2.4.0-test5-std/arch/i386/kernel/entry.S linux-2.4.0-test5-flink/arch/i386/kernel/entry.S
--- linux-2.4.0-test5-std/arch/i386/kernel/entry.S	Sun Jul 30 12:48:23 2000
+++ linux-2.4.0-test5-flink/arch/i386/kernel/entry.S	Mon Jul 31 13:22:35 2000
@@ -641,6 +641,7 @@
 	.long SYMBOL_NAME(sys_pivot_root)
 	.long SYMBOL_NAME(sys_mincore)
 	.long SYMBOL_NAME(sys_madvise)
+	.long SYMBOL_NAME(sys_flink)
 
 
 	/*
@@ -649,6 +650,6 @@
 	 * entries. Don't panic if you notice that this hasn't
 	 * been shrunk every time we add a new system call.
 	 */
-	.rept NR_syscalls-219
+	.rept NR_syscalls-220
 		.long SYMBOL_NAME(sys_ni_syscall)
 	.endr
diff -urN linux-2.4.0-test5-std/fs/namei.c linux-2.4.0-test5-flink/fs/namei.c
--- linux-2.4.0-test5-std/fs/namei.c	Sun Jul 30 12:48:40 2000
+++ linux-2.4.0-test5-flink/fs/namei.c	Mon Jul 31 21:54:09 2000
@@ -21,6 +21,7 @@
 #include <linux/quotaops.h>
 #include <linux/pagemap.h>
 #include <linux/dcache.h>
+#include <linux/file.h>
 
 #include <asm/uaccess.h>
 #include <asm/unaligned.h>
@@ -1598,6 +1599,62 @@
 	}
 	putname(from);
 
+	return error;
+}
+
+asmlinkage long sys_flink(unsigned int fd, const char *newname)
+{
+	int error;
+	struct file *file;
+	struct dentry *dentry;
+	const char *to;
+
+	error = -EBADF;
+	file = fget(fd);
+	if (!file)
+		goto exit;
+
+	to = getname(newname);
+	error = PTR_ERR(to);
+	if (!IS_ERR(to)) {
+
+		struct dentry *new_dentry;
+		struct nameidata nd;
+		
+		error = 0;
+
+		dentry = file->f_dentry;
+
+		/* Where (and why) should I put this (un)lock_kernel() */
+		lock_kernel();
+		
+		if (path_init(to, LOOKUP_PARENT, &nd))
+			error = path_walk(to, &nd);
+		if (error)
+			goto out;
+
+		error = -EXDEV;
+		if (file->f_vfsmnt != nd.mnt)
+			goto out;
+
+		new_dentry = lookup_create(&nd, 0);
+		error = PTR_ERR(new_dentry);
+		if (!IS_ERR(new_dentry)) {
+			error = vfs_link(dentry, nd.dentry->d_inode, new_dentry);
+			/* Where is the corresponding dget? */
+			dput(new_dentry);
+		}
+
+		/* lookup_create() down()s it */
+		up(&nd.dentry->d_inode->i_sem);
+		
+		path_release(&nd);
+	}
+out:	
+	unlock_kernel();
+	putname(to);
+	fput(file);
+exit:
 	return error;
 }
 
diff -urN linux-2.4.0-test5-std/include/asm-i386/unistd.h linux-2.4.0-test5-flink/include/asm-i386/unistd.h
--- linux-2.4.0-test5-std/include/asm-i386/unistd.h	Sun Apr 30 19:13:12 2000
+++ linux-2.4.0-test5-flink/include/asm-i386/unistd.h	Mon Jul 31 13:22:35 2000
@@ -225,6 +225,7 @@
 #define __NR_mincore		218
 #define __NR_madvise		219
 #define __NR_madvise1		219	/* delete when C lib stub is removed */
+#define __NR_flink		220
 
 /* user-visible error numbers are in the range -1 - -124: see <asm-i386/errno.h> */
 








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