[PATCH RT] printk: devkmsg: read: Return EPIPE when the first message user-space wants has gone

[PATCH RT] printk: devkmsg: read: Return EPIPE when the first message user-space wants has gone

[zhe.he]

From: He Zhe <zhe.he@windriver.com>

When user-space wants to read the first message, that is when user->seq
is 0, and that message has gone, it currently automatically resets
user->seq to current first seq. This mis-aligns with mainline kernel.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/dev-kmsg#n39
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/printk/printk.c#n899

We should inform user-space that what it wants has gone by returning EPIPE
in such scenario.

Signed-off-by: He Zhe <zhe.he@windriver.com>
---
 kernel/printk/printk.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e3fa33f2e23c..58c545a528b3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -703,14 +703,10 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 		goto out;
 	}
 
-	if (user->seq == 0) {
-		user->seq = seq;
-	} else {
-		user->seq++;
-		if (user->seq < seq) {
-			ret = -EPIPE;
-			goto restore_out;
-		}
+	user->seq++;
+	if (user->seq < seq) {
+		ret = -EPIPE;
+		goto restore_out;
 	}
 
 	msg = (struct printk_log *)&user->msgbuf[0];
-- 
2.17.1

Re: [PATCH RT] printk: devkmsg: read: Return EPIPE when the first message user-space wants has gone

[John Ogness]

On 2019-09-24, <zhe.he@windriver.com> wrote:
> From: He Zhe <zhe.he@windriver.com>
>
> When user-space wants to read the first message, that is when user->seq
> is 0, and that message has gone, it currently automatically resets
> user->seq to current first seq. This mis-aligns with mainline kernel.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/dev-kmsg#n39
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/printk/printk.c#n899
>
> We should inform user-space that what it wants has gone by returning EPIPE
> in such scenario.
>
> Signed-off-by: He Zhe <zhe.he@windriver.com>

Signed-off-by: John Ogness <john.ogness@linutronix.de>

> ---
>  kernel/printk/printk.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e3fa33f2e23c..58c545a528b3 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -703,14 +703,10 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
>  		goto out;
>  	}
>  
> -	if (user->seq == 0) {
> -		user->seq = seq;
> -	} else {
> -		user->seq++;
> -		if (user->seq < seq) {
> -			ret = -EPIPE;
> -			goto restore_out;
> -		}
> +	user->seq++;
> +	if (user->seq < seq) {
> +		ret = -EPIPE;
> +		goto restore_out;
>  	}
>  
>  	msg = (struct printk_log *)&user->msgbuf[0];