diff options
| -rw-r--r-- | Documentation/ABI/testing/dev-kmsg | 11 | ||||
| -rw-r--r-- | kernel/printk/printk.c | 8 |
2 files changed, 19 insertions, 0 deletions
diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg index 1e6c28b1942b..a917efc289a2 100644 --- a/Documentation/ABI/testing/dev-kmsg +++ b/Documentation/ABI/testing/dev-kmsg @@ -61,6 +61,17 @@ Description: The /dev/kmsg character device node provides userspace access SEEK_CUR is not supported, returning -ESPIPE (invalid seek) to errno whenever requested. + Other seek operations or offsets are not supported because of + the special behavior this device has. The device allows to read + or write only whole variable length messages (records) that are + stored in a ring buffer. + + Because of the non-standard behavior also the error values are + non-standard. -ESPIPE is returned for non-zero offset. -EINVAL + is returned for other operations, e.g. SEEK_CUR. This behavior + and values are historical and could not be modified without the + risk of breaking userspace. + 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 8c14835be46c..5f6eca65dd9a 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -943,6 +943,14 @@ out: return ret; } +/* + * Be careful when modifying this function!!! + * + * Only few operations are supported because the device works only with the + * entire variable length messages (records). Non-standard values are + * returned in the other cases and has been this way for quite some time. + * User space applications might depend on this behavior. + */ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) { struct devkmsg_user *user = file->private_data; |