[PATCH v4 3/3] proc: Disable cancellation of subset=pid option

[Alexey Gladkov <gladkov.alexey@gmail.com>]

There is no way to remount procfs mountpoint with subset=pid option
without it. This is done in order not to make visible what was hidden
since some checks occur during mount.

This patch makes this limitation explicit and demonstrates the error.

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
 fs/proc/root.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 0ab90e24d9ae..d4a91f48c430 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -145,7 +145,7 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	return 0;
 }
 
-static void proc_apply_options(struct super_block *s,
+static int proc_apply_options(struct super_block *s,
 			       struct fs_context *fc,
 			       struct user_namespace *user_ns)
 {
@@ -159,8 +159,11 @@ static void proc_apply_options(struct super_block *s,
 	if (ctx->mask & (1 << Opt_subset)) {
 		if (ctx->pidonly == PROC_PIDONLY_ON)
 			s->s_iflags |= SB_I_DYNAMIC;
+		else if (fs_info->pidonly == PROC_PIDONLY_ON)
+			return invalf(fc, "proc: subset=pid cannot be unset\n");
 		fs_info->pidonly = ctx->pidonly;
 	}
+	return 0;
 }
 
 static int proc_fill_super(struct super_block *s, struct fs_context *fc)
@@ -187,7 +190,10 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
 	fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
 	fs_info->mounter_cred = get_cred(fc->cred);
 
-	proc_apply_options(s, fc, current_user_ns());
+	ret = proc_apply_options(s, fc, current_user_ns());
+	if (ret) {
+		return ret;
+	}
 
 	/*
 	 * procfs isn't actually a stacking filesystem; however, there is
@@ -229,8 +235,7 @@ static int proc_reconfigure(struct fs_context *fc)
 	put_cred(fs_info->mounter_cred);
 	fs_info->mounter_cred = get_cred(fc->cred);
 
-	proc_apply_options(sb, fc, current_user_ns());
-	return 0;
+	return proc_apply_options(sb, fc, current_user_ns());
 }
 
 static int proc_get_tree(struct fs_context *fc)
-- 
2.25.4