Re: [PATCH 04/16] printk: implement support for extended console drivers
From: Tejun Heo
Date: Mon Apr 27 2015 - 17:09:31 EST
Sorry about the delay.
On Mon, Apr 20, 2015 at 05:43:07PM +0200, Petr Mladek wrote:
> > * Extended message formatting for console drivers is enabled iff there
if and only if
> I was afraid that there might be a potential buffer overflow because
> the user-provided dict need not be limited by '\0'. But LOG_DICT_META
> is used only with the internal data that are safe. We might document
> this as well.
> BTW: Do you want to print the internal dict when calling this function
> in devkmsg_read()?
No, plesae see below.
> > + scnprintf(fragid_buf, sizeof(fragid_buf),
> > + ",fragid=%llu", fragid);
> > + return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
> > + (msg->facility << 3) | msg->level, seq, ts_usec, cont,
> > + fragid_buf);
> The above comment suggests that "cont" and "fragid_buf" are delimited
> by a comma but it is not used here. Is it by intention.
Hmm... how is it not? The fragid printf has preceding comma.
> > + dict_len = scnprintf(dict_buf, sizeof(dict_buf), "FRAGID=%llu",
> > + cont.fragid);
> > + log_store(cont.facility, cont.level,
> > + flags | LOG_NOCONS | LOG_DICT_META,
> > + cont.ts_nsec, dict_buf, dict_len, cont.buf, cont.len);
> I wonder if it would make sense to restart fragid here. Another line
> will get distinguished by "seqnum".
That'd assume that there can only ever be a single continuation
buffer, which is true now but it's possible that we may want to make
it per-cpu in the future.
> Sigh, the whole mess is caused by the fact that we could not extend
> struct printk_log easily. It would be much better if we could put
> fragid there. I finally understand why you need to reuse the dict.
I've been thinking about it and using dict area for internal metadata
is indeed quite messy. I think a better way to do it is declaring
dict_len as a union w/ fragid. This'd limit the fragid to 16bit but
that should be more than enough and we can do away with the string
formatting and reading back.
> Another solution would be to allow to disable the continuous buffer
> via some boot option or /sys/kernel/debug entry if you want to debug
> such a problem and have problem with the partial flushing.
It isn't really about debugging partial flushing itself but rather
always being able to push out the messages before the printk
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