From bc885f1ab6de0d38c6956a71b0126543b64875b0 Mon Sep 17 00:00:00 2001 From: Bruno Meneguele Date: Fri, 10 Jul 2020 14:44:23 -0300 Subject: doc:kmsg: explicitly state the return value in case of SEEK_CUR The commit 625d3449788f ("Revert "kernel/printk: add kmsg SEEK_CUR handling"") reverted a change done to the return value in case a SEEK_CUR operation was performed for kmsg buffer based on the fact that different userspace apps were handling the new return value (-ESPIPE) in different ways, breaking them. At the same time -ESPIPE was the wrong decision because kmsg /does support/ seek() but doesn't follow the "normal" behavior userspace is used to. Because of that and also considering the time -EINVAL has been used, it was decided to keep this way to avoid more userspace breakage. This patch adds an official statement to the kmsg documentation pointing to the current return value for SEEK_CUR, -EINVAL, thus userspace libraries and apps can refer to it for a definitive guide on what to expect. Signed-off-by: Bruno Meneguele Reviewed-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20200710174423.10480-1-bmeneg@redhat.com --- Documentation/ABI/testing/dev-kmsg | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'Documentation/ABI/testing/dev-kmsg') 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, -- cgit v1.2.3