Messages in this thread | | | Date | Fri, 10 Jul 2020 14:19:23 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] doc:kmsg: explictly state the return value in case of SEEK_CUR |
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On Thu 2020-07-09 12:54:15, Bruno Meneguele wrote: > 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 expected. > > --- a/Documentation/ABI/testing/dev-kmsg > +++ b/Documentation/ABI/testing/dev-kmsg > @@ -56,6 +56,11 @@ 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. > > + Considering that the seek operation is supported but has > + special meaning to the device, any attempt to seek specific > + positions on the buffer (i.e. using SEEK_CUR) results in an > + -EINVAL error returned to userspace.
Sigh, I see that devkmsg_llseek() returns -ESPIPE when offset is not zero. This is a real mess.
I wonder if we could afford to switch this one to -EINVAL and reduce the mess.
Anyway, for a random reader, it might be pretty unclear what is exactly special about /dev/kmsg. I wonder if the following might be more explanatory and strightforward:
Other seek operations or offsets are not supported because of the special behavior. The device allows to read or write only whole variable lenght messages that are stored in a ring buffer.
Because of the non-standard behavior also the error values are non-standardand. -ESPIPE is returned for non-zero offset. -EINVAL is returned for other operations, e.g. SEEK_CUR. This behavior is historical and could not be modified wihtout the risk of breaking userspace.
Finally, only few people read documentation. We should add some warning also to the code. I think about a something like:
/* * Be careful when modifying this function!!! * * Only few operations are supported because the device works only with * the entire variable length messages. Non-standard error values are * returned in the other cases. User space applications might depend * on this behavior. */
Best Regards, Petr
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