[PATCH printk-rework 07/14] printk: use atomic64_t for devkmsg_user.seq

John Ogness
2021-02-18T09:40:04Z

@user->seq is indirectly protected by @logbuf_lock. Once @logbuf_lock
is removed, @user->seq will be no longer safe from an atomicity point
of view.

In preparation for the removal of @logbuf_lock, change it to
atomic64_t to provide this safety.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index a71e0d41ccb5..20c21a25143d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -662,7 +662,7 @@ static ssize_t msg_print_ext_body(char *buf, size_t size,

/* /dev/kmsg - userspace message inject/listen interface */
struct devkmsg_user {
- u64 seq;
+ atomic64_t seq;
struct ratelimit_state rs;
struct mutex lock;
char buf[CONSOLE_EXT_LOG_MAX];
@@ -764,7 +764,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
return ret;

logbuf_lock_irq();
- if (!prb_read_valid(prb, user->seq, r)) {
+ if (!prb_read_valid(prb, atomic64_read(&user->seq), r)) {
if (file->f_flags & O_NONBLOCK) {
ret = -EAGAIN;
logbuf_unlock_irq();
@@ -773,15 +773,15 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,

logbuf_unlock_irq();
ret = wait_event_interruptible(log_wait,
- prb_read_valid(prb, user->seq, r));
+ prb_read_valid(prb, atomic64_read(&user->seq), r));
if (ret)
goto out;
logbuf_lock_irq();
}

- if (r->info->seq != user->seq) {
+ if (r->info->seq != atomic64_read(&user->seq)) {
/* our last seen message is gone, return error and reset */
- user->seq = r->info->seq;
+ atomic64_set(&user->seq, r->info->seq);
ret = -EPIPE;
logbuf_unlock_irq();
goto out;
@@ -792,7 +792,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
&r->text_buf[0], r->info->text_len,
&r->info->dev_info);

- user->seq = r->info->seq + 1;
+ atomic64_set(&user->seq, r->info->seq + 1);
logbuf_unlock_irq();

if (len > count) {
@@ -832,7 +832,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
switch (whence) {
case SEEK_SET:
/* the first record */
- user->seq = prb_first_valid_seq(prb);
+ atomic64_set(&user->seq, prb_first_valid_seq(prb));
break;
case SEEK_DATA:
/*
@@ -840,11 +840,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
* like issued by 'dmesg -c'. Reading /dev/kmsg itself
* changes no global state, and does not clear anything.
*/
- user->seq = latched_seq_read_nolock(&clear_seq);
+ atomic64_set(&user->seq, latched_seq_read_nolock(&clear_seq));
break;
case SEEK_END:
/* after the last record */
- user->seq = prb_next_seq(prb);
+ atomic64_set(&user->seq, prb_next_seq(prb));
break;
default:
ret = -EINVAL;
@@ -865,9 +865,9 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
poll_wait(file, &log_wait, wait);

logbuf_lock_irq();
- if (prb_read_valid_info(prb, user->seq, &info, NULL)) {
+ if (prb_read_valid(prb, atomic64_read(&user->seq), NULL)) {
/* return error when data has vanished underneath us */
- if (info.seq != user->seq)
+ if (info.seq != atomic64_read(&user->seq))
ret = EPOLLIN|EPOLLRDNORM|EPOLLERR|EPOLLPRI;
else
ret = EPOLLIN|EPOLLRDNORM;
@@ -906,7 +906,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
&user->text_buf[0], sizeof(user->text_buf));

logbuf_lock_irq();
- user->seq = prb_first_valid_seq(prb);
+ atomic64_set(&user->seq, prb_first_valid_seq(prb));
logbuf_unlock_irq();

file->private_data = user;
--
2.20.1