linux-kernel - [PATCH v2 3/3] pidfs: implement file handle support

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Message-Id: <20241113-pidfs_fh-v2-3-9a4d28155a37@e43.eu>
Date: Wed, 13 Nov 2024 17:55:25 +0000
From: Erin Shepherd <erin.shepherd@....eu>
To: Christian Brauner <brauner@...nel.org>, 
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
 Chuck Lever <chuck.lever@...cle.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Jeff Layton <jlayton@...nel.org>, Amir Goldstein <amir73il@...il.com>, 
 linux-nfs@...r.kernel.org, Erin Shepherd <erin.shepherd@....eu>
Subject: [PATCH v2 3/3] pidfs: implement file handle support

On 64-bit platforms, userspace can read the pidfd's inode in order to
get a never-repeated PID identifier. On 32-bit platforms this identifier
is not exposed, as inodes are limited to 32 bits. Instead expose the
identifier via export_fh, which makes it available to userspace via
name_to_handle_at

In addition we implement fh_to_dentry, which allows userspace to
recover a pidfd from a PID file handle.

We stash the process' PID in the root pid namespace inside the handle,
and use that to recover the pid (validating that pid->ino matches the
value in the handle, i.e. that the pid has not been reused).

We use the root namespace in order to ensure that file handles can be
moved across namespaces; however, we validate that the PID exists in
the current namespace before returning the inode.

Reviewed-by: Amir Goldstein <amir73il@...il.com>
Signed-off-by: Erin Shepherd <erin.shepherd@....eu>
---
 fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/anon_inodes.h>
+#include <linux/exportfs.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/magic.h>
@@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
 	.d_prune	= stashed_dentry_prune,
 };
 
+#define PIDFD_FID_LEN 3
+
+struct pidfd_fid {
+	u64 ino;
+	s32 pid;
+} __packed;
+
+static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
+			   struct inode *parent)
+{
+	struct pid *pid = inode->i_private;
+	struct pidfd_fid *fid = (struct pidfd_fid *)fh;
+
+	if (*max_len < PIDFD_FID_LEN) {
+		*max_len = PIDFD_FID_LEN;
+		return FILEID_INVALID;
+	}
+
+	fid->ino = pid->ino;
+	fid->pid = pid_nr(pid);
+	*max_len = PIDFD_FID_LEN;
+	return FILEID_INO64_GEN;
+}
+
+static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
+					 struct fid *gen_fid,
+					 int fh_len, int fh_type)
+{
+	int ret;
+	struct path path;
+	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
+	struct pid *pid;
+
+	if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
+		return NULL;
+
+	scoped_guard(rcu) {
+		pid = find_pid_ns(fid->pid, &init_pid_ns);
+		if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
+			return NULL;
+
+		pid = get_pid(pid);
+	}
+
+	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	mntput(path.mnt);
+	return path.dentry;
+}
+
+static const struct export_operations pidfs_export_operations = {
+	.encode_fh = pidfs_encode_fh,
+	.fh_to_dentry = pidfs_fh_to_dentry,
+	.flags = EXPORT_OP_UNRESTRICTED_OPEN,
+};
+
 static int pidfs_init_inode(struct inode *inode, void *data)
 {
 	inode->i_private = data;
 	inode->i_flags |= S_PRIVATE;
-	inode->i_mode |= S_IRWXU;
+	inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;
 	inode->i_op = &pidfs_inode_operations;
 	inode->i_fop = &pidfs_file_operations;
 	/*
@@ -382,6 +441,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 		return -ENOMEM;
 
 	ctx->ops = &pidfs_sops;
+	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;
 	fc->s_fs_info = (void *)&pidfs_stashed_ops;
 	return 0;

-- 
2.46.1