2019-09-26 00:44:44

by He Zhe

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

From: He Zhe <[email protected]>

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 <[email protected]>
---
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

2019-09-26 09:00:33

by John Ogness

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

On 2019-09-24, <[email protected]> wrote:
> From: He Zhe <[email protected]>
>
> 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 <[email protected]>

Signed-off-by: John Ogness <[email protected]>

> ---
> 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];