[PATCH] kernel/printk: add kmsg SEEK_CUR handling

[Bruno Meneguele <bmeneg@redhat.com>]

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
guidance.

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>
---
 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;
 		break;
+	case SEEK_CUR:
+		/* return the default errno for invalid seek */
+		ret = -ESPIPE;
+		break;
 	default:
 		ret = -EINVAL;
 	}
-- 
2.24.1