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SubjectRe: rm-ing files with open file descriptors
Date
In article <a2almg$vtl$1@cesium.transmeta.com>,
H. Peter Anvin <hpa@zytor.com> wrote:
>Followup to: <a2afsg$73g$2@ncc1701.cistron.net>
>By author: Miquel van Smoorenburg <miquels@cistron.nl>
>In newsgroup: linux.dev.kernel
>>
>> There is no way to recreate a file with a nlink count of 0,
>> well that is until someone adds flink(fd, newpath) to the kernel.
>
>This *might* work:
>
>link("/proc/self/fd/40", newpath);

I used the following perl script to test this, and alas this
doesn't work (yet):

open(FD, ">flink-test.txt") || die;
print FD "flink-test\n";
unlink "flink-test.txt"|| die;
link("/proc/self/fd/" . fileno(FD), "flink-test2.txt") || die;
print "Success.\n";

It results in:

link("/proc/self/fd/3", "flink-test2.txt") = -1 EXDEV (Invalid cross-device link)

This is probably because link() doesn't look up the target of the
symlink, it links the symlink itself. Linux allows symlinks with
a nlink count of 2:

% ln -s a b
% ln b c
ln: `b': warning: making a hard link to a symbolic link is not portable
% ls -l
lrwxrwxrwx 2 miquels staff 1 Jan 19 11:34 b -> a
lrwxrwxrwx 2 miquels staff 1 Jan 19 11:34 c -> a

This could be hacked around ofcourse in fs/namei.c, so I tried
it for fun. And indeed, with a minor correction it works:

% perl flink.pl
Success.

I now have a flink-test2.txt file. That is pretty cool ;)

Here's the patch:

--- linux-2.4.15-pre4/fs/namei.c.orig Wed Oct 17 23:46:29 2001
+++ linux-2.4.15-pre4/fs/namei.c Sat Jan 19 12:08:13 2002
@@ -22,6 +22,7 @@
#include <linux/dnotify.h>
#include <linux/smp_lock.h>
#include <linux/personality.h>
+#include <linux/proc_fs.h>

#include <asm/namei.h>
#include <asm/uaccess.h>
@@ -1640,6 +1641,16 @@
error = path_walk(from, &old_nd);
if (error)
goto exit;
+#if 1 /* flink()-like support */
+ if (old_nd.mnt->mnt_sb->s_magic == PROC_SUPER_MAGIC) {
+ path_release(&old_nd);
+ if (path_init(from, LOOKUP_POSITIVE|LOOKUP_FOLLOW,
+ &old_nd))
+ error = path_walk(from, &old_nd);
+ if (error)
+ goto exit;
+ }
+#endif
if (path_init(to, LOOKUP_PARENT, &nd))
error = path_walk(to, &nd);
if (error)


Mike.

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