linux-kernel - Re: Testing if two open descriptors refer to the same inode

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Message-ID: <vmjtzzz7sxctmf7qrf6mw5hdd653elsi423joiiusahei22bft@quvxy4kajtxt>
Date: Mon, 29 Jul 2024 12:50:47 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Florian Weimer <fweimer@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-api@...r.kernel.org, Dave Chinner <dchinner@...hat.com>
Subject: Re: Testing if two open descriptors refer to the same inode

On Mon, Jul 29, 2024 at 12:40:35PM +0200, Florian Weimer wrote:
> * Mateusz Guzik:
> 
> > On Mon, Jul 29, 2024 at 08:55:46AM +0200, Florian Weimer wrote:
> >> It was pointed out to me that inode numbers on Linux are no longer
> >> expected to be unique per file system, even for local file systems.
> >
> > I don't know if I'm parsing this correctly.
> >
> > Are you claiming on-disk inode numbers are not guaranteed unique per
> > filesystem? It sounds like utter breakage, with capital 'f'.
> 
> Yes, POSIX semantics and traditional Linux semantics for POSIX-like
> local file systems are different.
> 

Can you link me some threads about this?

> > While the above is not what's needed here, I guess it sets a precedent
> > for F_DUPINODE_QUERY (or whatever other name) to be added to handily
> > compare inode pointers. It may be worthwhile regardless of the above.
> > (or maybe kcmp could be extended?)
> 
> I looked at kcmp as well, but I think it's dependent on
> checkpoint/restore.  File sameness checks are much more basic than that.
> 

I had this in mind (untested modulo compilation):

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 300e5d9ad913..5723c3e82eac 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -343,6 +343,13 @@ static long f_dupfd_query(int fd, struct file *filp)
 	return f.file == filp;
 }
 
+static long f_dupfd_query_inode(int fd, struct file *filp)
+{
+	CLASS(fd_raw, f)(fd);
+
+	return f.file->f_inode == filp->f_inode;
+}
+
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
@@ -361,6 +368,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_DUPFD_QUERY:
 		err = f_dupfd_query(argi, filp);
 		break;
+	case F_DUPFD_QUERY_INODE:
+		err = f_dupfd_query_inode(argi, filp);
+		break;
 	case F_GETFD:
 		err = get_close_on_exec(fd) ? FD_CLOEXEC : 0;
 		break;
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index c0bcc185fa48..2e93dbdd8fd2 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -16,6 +16,8 @@
 
 #define F_DUPFD_QUERY	(F_LINUX_SPECIFIC_BASE + 3)
 
+#define F_DUPFD_QUERY_INODE (F_LINUX_SPECIFIC_BASE + 4)
+
 /*
  * Cancel a blocking posix lock; internal use only until we expose an
  * asynchronous lock api to userspace: