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Message-ID: <20250507032926.377076-2-chenlinxuan@uniontech.com>
Date: Wed, 7 May 2025 11:29:26 +0800
From: Chen Linxuan <chenlinxuan@...ontech.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Chen Linxuan <chenlinxuan@...ontech.com>,
Amir Goldstein <amir73il@...il.com>,
linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [RFC PATCH] fs: fuse: add backing_files control file
Add a new FUSE control file "/sys/fs/fuse/connections/*/backing_files"
that exposes the paths of all backing files currently being used in
FUSE mount points. This is particularly valuable for tracking and
debugging files used in FUSE passthrough mode.
This approach is similar to how fixed files in io_uring expose their
status through fdinfo, providing administrators with visibility into
backing file usage. By making backing files visible through the FUSE
control filesystem, administrators can monitor which files are being
used for passthrough operations and can force-close them if needed by
aborting the connection.
This exposure of backing files information is an important step towards
potentially relaxing CAP_SYS_ADMIN requirements for certain passthrough
operations in the future, allowing for better security analysis of
passthrough usage patterns.
The control file is implemented using the seq_file interface for
efficient handling of potentially large numbers of backing files.
Access permissions are set to read-only (0400) as this is an
informational interface.
FUSE_CTL_NUM_DENTRIES has been increased from 5 to 6 to accommodate the
additional control file.
Some related discussions can be found at:
Link: https://lore.kernel.org/all/4b64a41c-6167-4c02-8bae-3021270ca519@fastmail.fm/T/#mc73e04df56b8830b1d7b06b5d9f22e594fba423e
Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxhAY1m7ubJ3p-A3rSufw_53WuDRMT1Zqe_OC0bP_Fb3Zw@mail.gmail.com/
Cc: Amir Goldstein <amir73il@...il.com>
Signed-off-by: Chen Linxuan <chenlinxuan@...ontech.com>
---
Please review this patch carefully. I am new to kernel development and
I am not quite sure if I have followed the best practices, especially
in terms of seq_file, error handling and locking. I would appreciate
any feedback.
I have do some simply testing using libfuse example [1]. It seems to
work well.
[1]: https://github.com/libfuse/libfuse/blob/master/example/passthrough_hp.cc
---
fs/fuse/control.c | 129 +++++++++++++++++++++++++++++++++++++++++++++-
fs/fuse/fuse_i.h | 2 +-
2 files changed, 129 insertions(+), 2 deletions(-)
diff --git a/fs/fuse/control.c b/fs/fuse/control.c
index 2a730d88cc3bd..4d1e0acc5030f 100644
--- a/fs/fuse/control.c
+++ b/fs/fuse/control.c
@@ -11,6 +11,7 @@
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs_context.h>
+#include <linux/seq_file.h>
#define FUSE_CTL_SUPER_MAGIC 0x65735543
@@ -180,6 +181,129 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
return ret;
}
+struct fuse_backing_files_seq_state {
+ struct fuse_conn *fc;
+ int pos;
+};
+
+static void *fuse_backing_files_seq_start(struct seq_file *seq, loff_t *pos)
+{
+ struct fuse_backing_files_seq_state *state = seq->private;
+ struct fuse_conn *fc = state->fc;
+
+ if (!fc)
+ return NULL;
+
+ spin_lock(&fc->lock);
+
+ if (*pos > idr_get_cursor(&fc->backing_files_map)) {
+ spin_unlock(&fc->lock);
+ return NULL;
+ }
+
+ state->pos = *pos;
+ return state;
+}
+
+static void *fuse_backing_files_seq_next(struct seq_file *seq, void *v,
+ loff_t *pos)
+{
+ struct fuse_backing_files_seq_state *state = seq->private;
+
+ (*pos)++;
+ state->pos = *pos;
+
+ if (state->pos > idr_get_cursor(&state->fc->backing_files_map)) {
+ spin_unlock(&state->fc->lock);
+ return NULL;
+ }
+
+ return state;
+}
+
+static int fuse_backing_files_seq_show(struct seq_file *seq, void *v)
+{
+ struct fuse_backing_files_seq_state *state = seq->private;
+ struct fuse_conn *fc = state->fc;
+ struct fuse_backing *fb;
+
+ fb = idr_find(&fc->backing_files_map, state->pos);
+ if (!fb || !fb->file)
+ return 0;
+
+ seq_file_path(seq, fb->file, " \t\n\\");
+ seq_puts(seq, "\n");
+
+ return 0;
+}
+
+static void fuse_backing_files_seq_stop(struct seq_file *seq, void *v)
+{
+ struct fuse_backing_files_seq_state *state = seq->private;
+
+ if (v)
+ spin_unlock(&state->fc->lock);
+}
+
+static const struct seq_operations fuse_backing_files_seq_ops = {
+ .start = fuse_backing_files_seq_start,
+ .next = fuse_backing_files_seq_next,
+ .stop = fuse_backing_files_seq_stop,
+ .show = fuse_backing_files_seq_show,
+};
+
+static int fuse_backing_files_seq_open(struct inode *inode, struct file *file)
+{
+ struct fuse_conn *fc;
+ struct fuse_backing_files_seq_state *state;
+ int err;
+
+ fc = fuse_ctl_file_conn_get(file);
+ if (!fc)
+ return -ENOTCONN;
+
+ err = seq_open(file, &fuse_backing_files_seq_ops);
+ if (err) {
+ fuse_conn_put(fc);
+ return err;
+ }
+
+ state = kmalloc(sizeof(*state), GFP_KERNEL);
+ if (!state) {
+ seq_release(file->f_inode, file);
+ fuse_conn_put(fc);
+ return -ENOMEM;
+ }
+
+ state->fc = fc;
+ state->pos = 0;
+ ((struct seq_file *)file->private_data)->private = state;
+
+ return 0;
+}
+
+static int fuse_backing_files_seq_release(struct inode *inode,
+ struct file *file)
+{
+ struct seq_file *seq = file->private_data;
+ struct fuse_backing_files_seq_state *state = seq->private;
+
+ if (state) {
+ fuse_conn_put(state->fc);
+ kfree(state);
+ seq->private = NULL;
+ }
+
+ return seq_release(inode, file);
+}
+
+static const struct file_operations fuse_conn_passthrough_backing_files_ops = {
+ .open = fuse_backing_files_seq_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = fuse_backing_files_seq_release,
+};
+
static const struct file_operations fuse_ctl_abort_ops = {
.open = nonseekable_open,
.write = fuse_conn_abort_write,
@@ -270,7 +394,10 @@ int fuse_ctl_add_conn(struct fuse_conn *fc)
1, NULL, &fuse_conn_max_background_ops) ||
!fuse_ctl_add_dentry(parent, fc, "congestion_threshold",
S_IFREG | 0600, 1, NULL,
- &fuse_conn_congestion_threshold_ops))
+ &fuse_conn_congestion_threshold_ops) ||
+ !fuse_ctl_add_dentry(parent, fc, "backing_files", S_IFREG | 0400, 1,
+ NULL,
+ &fuse_conn_passthrough_backing_files_ops))
goto err;
return 0;
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index d56d4fd956db9..2830b05bb0928 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -46,7 +46,7 @@
#define FUSE_NAME_MAX (PATH_MAX - 1)
/** Number of dentries for each connection in the control filesystem */
-#define FUSE_CTL_NUM_DENTRIES 5
+#define FUSE_CTL_NUM_DENTRIES 6
/* Frequency (in seconds) of request timeout checks, if opted into */
#define FUSE_TIMEOUT_TIMER_FREQ 15
--
2.43.0
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