Re: [PATCH 2/3] uml: Create a private mount of proc for mconsole

On Fri, Feb 28, 2020 at 03:28:54PM -0600, Eric W. Biederman wrote:

Christian Brauner [off-list ref] writes:

quoted

On Fri, Feb 28, 2020 at 02:18:43PM -0600, Eric W. Biederman wrote:

quoted

The mconsole code only ever accesses proc for the initial pid
namespace.  Instead of depending upon the proc_mnt which is
for proc_flush_task have uml create it's own mount of proc
instead.

This allows proc_flush_task to evolve and remove the
need for having a proc_mnt to do it's job.

Cc: Jeff Dike <redacted>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Eric W. Biederman <redacted>
---
 arch/um/drivers/mconsole_kern.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index e8f5c81c2c6c..30575bd92975 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c

@@ -36,6 +36,8 @@
 #include "mconsole_kern.h"
 #include <os.h>
 
+static struct vfsmount *proc_mnt = NULL;
+
 static int do_unlink_socket(struct notifier_block *notifier,
 			    unsigned long what, void *data)
 {

@@ -123,7 +125,7 @@ void mconsole_log(struct mc_request *req)
 
 void mconsole_proc(struct mc_request *req)
 {
-	struct vfsmount *mnt = init_pid_ns.proc_mnt;
+	struct vfsmount *mnt = proc_mnt;
 	char *buf;
 	int len;
 	struct file *file;

@@ -134,6 +136,10 @@ void mconsole_proc(struct mc_request *req)
 	ptr += strlen("proc");
 	ptr = skip_spaces(ptr);
 
+	if (!mnt) {
+		mconsole_reply(req, "Proc not available", 1, 0);
+		goto out;
+	}
 	file = file_open_root(mnt->mnt_root, mnt, ptr, O_RDONLY, 0);
 	if (IS_ERR(file)) {
 		mconsole_reply(req, "Failed to open file", 1, 0);

@@ -683,6 +689,24 @@ void mconsole_stack(struct mc_request *req)
 	with_console(req, stack_proc, to);
 }
 
+static int __init mount_proc(void)
+{
+	struct file_system_type *proc_fs_type;
+	struct vfsmount *mnt;
+
+	proc_fs_type = get_fs_type("proc");
+	if (!proc_fs_type)
+		return -ENODEV;
+
+	mnt = kern_mount(proc_fs_type);
+	put_filesystem(proc_fs_type);
+	if (IS_ERR(mnt))
+		return PTR_ERR(mnt);
+
+	proc_mnt = mnt;
+	return 0;
+}
+
 /*
  * Changed by mconsole_setup, which is __setup, and called before SMP is
  * active.

@@ -696,6 +720,8 @@ static int __init mconsole_init(void)
 	int err;
 	char file[UNIX_PATH_MAX];
 
+	mount_proc();

Hm, either check the return value or make the mount_proc() void?
Probably worth logging something but moving on without proc.

I modified mconsole_proc (the only place that cares to see if
it has a valid proc_mnt).

So the code already does the moving on without mounting proc
and continues to work.

Ok, but then make mount_proc()

static void __init mount_proc(void)

and not

static int __init mount_proc(void)

like you have now. That was what I was getting it. Unless there's
another reason for this.

Christian