[Crash-utility] [Patch-v2] Fix handling the ring buffer for 'log' command.

[Ken'ichi Ohmichi]

Hi Dave, Jeff,

Thank you for comments.

Jeff Moyer wrote:
> > Dave Anderson <anderson at redhat.com> writes:
> > 
>> >> Good question -- I'm not sure.
>> >>
>> >> If the buffer were "cleared" by the administrator, the logical "end" of
>> >> the buffer would not be the last thing displayed by the "log" command.
>> >> But that's really not a problem, given that the relevant kernel-crash-related
>> >> data is still available to be examined.  
>> >>
>> >> On the other hand, even though the administrator has "cleared" the log
>> >> buffer, the data is still there.  My concern would be what if there is
>> >> crucial data in the log buffer that the administrator "cleared" out of
>> >> convenience?  Like for example, I've often done a "dmesg -c" to clear
>> >> the buffer so that subsequent dmesg commands just dump the latest 
>> >> information.  But then I've gone back with the crash utility and
>> >> re-examined the log buffer data that still remains in memory -- which
>> >> can be still be seen with the "log" command.

Good comment, I see.


>> >> So my initial leaning would be to continue to show what's actually there.
>> >> I trust myself as a crash analyzer more than I trust the administrator. 
>> >>
>> >> But I could be convinced otherwise.
>> >>
>> >> What do others on the list think about this?
> > 
> > I would much rather see all of the contents of the log buffer.

OK, I attach a new patch for outputing all of the contents of the log
buffer. How about this patch ?


By the way, I'm implementing a new option '--dump-dmesg' of makedumpfile
command with Neil Horman. The option is similar to 'log' command of the
crash utility, so the option extracts log data from /proc/vmcore and dumps
it to a file. I will implement the option for outputting all of the contents
of the log buffer like this patch.


---
[Patch-v2] Fix handling the ring buffer for 'log' command.

Changelog of v2:
 - Extending the output range of 'log' command to all the ring buffer.

I found there is a 'log' command problem related to handling the ring
buffer, and this patch fixes it.

The ring buffer can be cleared by klogctl(2) from a user process, but
current crash utility does not consider this case. The following output
of current crash utility is example of this case. You see there are some
messages after the panic message. These messages are the oldest log data,
so they should be output first.

  crash > log
  [snip]
  exception[8291] trap divide error rip:4004c0 rsp:7fffb56cdf60 error:0
  exception[8293] trap divide error rip:4004c0 rsp:7fffbb0628f0 error:0
  SysRq : Trigger a crashdump
   14 15) *0, disabled.
  ACPI: PCI Interrupt Link [LNKF] (IRQs 4 5 6 7 10 14 15) *0, disabled.
  ACPI: PCI Interrupt Link [LNKG] (IRQs 3 4 5 6 7 10 14 15) *0, disabled.
  [snip]
  crash >

This patch fixes this invalid output to the folloing:

  crash > log
   14 15) *0, disabled.
  ACPI: PCI Interrupt Link [LNKF] (IRQs 4 5 6 7 10 14 15) *0, disabled.
  ACPI: PCI Interrupt Link [LNKG] (IRQs 3 4 5 6 7 10 14 15) *0, disabled.
  [snip]
  exception[8291] trap divide error rip:4004c0 rsp:7fffb56cdf60 error:0
  exception[8293] trap divide error rip:4004c0 rsp:7fffbb0628f0 error:0
  SysRq : Trigger a crashdump
  crash>


Thanks
Ken'ichi Ohmichi

Signed-off-by: Ken'ichi Ohmichi <oomichi at mxs.nes.nec.co.jp>
---
--- crash-4.0-7.7/kernel.c.orig	2009-02-06 01:13:43.000000000 +0900
+++ crash-4.0-7.7/kernel.c	2009-02-18 10:42:29.000000000 +0900
@@ -3365,7 +3365,7 @@ void 
 dump_log(int msg_level)
 {
 	int i;
-	ulong log_buf, logged_chars;
+	ulong log_buf, logged_chars, log_end;
 	char *buf;
 	char last;
 	ulong index;
@@ -3393,17 +3393,16 @@ dump_log(int msg_level)
 	buf = GETBUF(log_buf_len);
 	log_wrap = FALSE;
 	get_symbol_data("logged_chars", sizeof(ulong), &logged_chars);
+	get_symbol_data("log_end", sizeof(ulong), &log_end);
         readmem(log_buf, KVADDR, buf,
         	log_buf_len, "log_buf contents", FAULT_ON_ERROR);
 
-	if (logged_chars < log_buf_len) {
+	if (log_end < log_buf_len)
 		index = 0;
-	} else {
-		get_symbol_data("log_end", sizeof(ulong), &index);
-		index &= log_buf_len-1;
-	} 
+	else
+		index = log_end & (log_buf_len - 1);
 
-	if ((logged_chars < log_buf_len) && (index == 0) && (buf[index] == '<'))
+	if ((log_end < log_buf_len) && (index == 0) && (buf[index] == '<'))
 		loglevel = TRUE;
 	else
 		loglevel = FALSE;