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From: He Zhe <zhe.he@windriver.com>
Date: Tue, 24 Sep 2019 15:26:39 +0800
Subject: [PATCH] printk: devkmsg: read: Return EPIPE when the first
message user-space wants has gone
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.2/older/patches-5.2.17-rt9.tar.xz
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.
Link: https://lore.kernel.org/r/20190924072639.25986-1-zhe.he@windriver.com
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/printk/printk.c | 12 ++++--------
1 file changed, 4 insertions(+), 8 deletions(-)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -703,14 +703,10 @@ static ssize_t devkmsg_read(struct file
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];
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