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Subject[PATCH] kernel/printk: add kmsg SEEK_CUR handling
Userspace libraries, e.g. glibc's dprintf(), expect the default return value
for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the
current state of kernel code was returning the generic case of an -EINVAL.
Hence, userspace programs were not behaving as expected or documented.

With this patch we add SEEK_CUR case returning the expected value and also a
simple mention of it in kernel's documentation for those relying on that for

Signed-off-by: Bruno Meneguele <>
Documentation/ABI/testing/dev-kmsg | 2 ++
kernel/printk/printk.c | 4 ++++
2 files changed, 6 insertions(+)

diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
index f307506eb54c..8533d28e6fda 100644
--- a/Documentation/ABI/testing/dev-kmsg
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -56,6 +56,8 @@ Description: The /dev/kmsg character device node provides userspace access
seek after the last record available at the time
the last SYSLOG_ACTION_CLEAR was issued.

+ While SEEK_CUR sets -ESPIPE (invalid seek) to errno.
The output format consists of a prefix carrying the syslog
prefix including priority and facility, the 64 bit message
sequence number and the monotonic timestamp in microseconds,
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ad4606234545..d02606723d2d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -963,6 +963,10 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
user->idx = log_next_idx;
user->seq = log_next_seq;
+ case SEEK_CUR:
+ /* return the default errno for invalid seek */
+ ret = -ESPIPE;
+ break;
ret = -EINVAL;
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