[PATCH 6.18.y 1/2] pidfs: prepare to drop exit_info pointer

[Sasha Levin <sashal@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

[ Upstream commit ad6e3ea683ef4086cdf3d2ec40cb3f6e62edb945 ]

There will likely be more info that we need to store in struct
pidfs_attr. We need to make sure that some of the information such as
exit info or coredump info that consists of multiple bits is either
available completely or not at all, but never partially. Currently we
use a pointer that we assign to. That doesn't scale. We can't waste a
pointer for each mulit-part information struct we want to expose. Use a
bitmask instead.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-6-ca449b7b7aa0@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: 172658cf6009 ("pidfs: don't let PIDFD_GET_INFO miss a reaped task")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/pidfs.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 4341492b91e12..d2afc85b9df37 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -49,7 +49,12 @@ struct pidfs_exit_info {
 	__u32 coredump_mask;
 };
 
+enum pidfs_attr_mask_bits {
+	PIDFS_ATTR_BIT_EXIT	= 0,
+};
+
 struct pidfs_attr {
+	unsigned long attr_mask;
 	struct simple_xattrs *xattrs;
 	struct pidfs_exit_info __pei;
 	struct pidfs_exit_info *exit_info;
@@ -325,8 +330,8 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 
 	attr = READ_ONCE(pid->attr);
 	if (mask & PIDFD_INFO_EXIT) {
-		exit_info = READ_ONCE(attr->exit_info);
-		if (exit_info) {
+		if (test_bit(PIDFS_ATTR_BIT_EXIT, &attr->attr_mask)) {
+			smp_rmb();
 			kinfo.mask |= PIDFD_INFO_EXIT;
 #ifdef CONFIG_CGROUPS
 			kinfo.cgroupid = exit_info->cgroupid;
@@ -681,7 +686,8 @@ void pidfs_exit(struct task_struct *tsk)
 	exit_info->exit_code = tsk->exit_code;
 
 	/* Ensure that PIDFD_GET_INFO sees either all or nothing. */
-	smp_store_release(&attr->exit_info, &attr->__pei);
+	smp_wmb();
+	set_bit(PIDFS_ATTR_BIT_EXIT, &attr->attr_mask);
 }
 
 #ifdef CONFIG_COREDUMP
-- 
2.53.0