kern/119993: ddb capture buffer too small for textdumps

[Scot Hetzel]

>Number:         119993
>Category:       kern
>Synopsis:       ddb capture buffer too small for textdumps
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Jan 26 03:30:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator:     Scot Hetzel
>Release:        8.0-CURRENT
>Organization:
>Environment:
FreeBSD hp010.hetzel.org 8.0-CURRENT FreeBSD 8.0-CURRENT #0: Sat Jan 19 00:03:54 CST 2008     root at hp010.hetzel.org:/usr/src/sys/amd64/compile/DV8135NR  amd64

>Description:
While trying to debug a kernel panic, I decided to try the new textdump feature and scripted ddb.  I increased debug.ddb.capture.bufsize to it's maximum size (512K).  When the kerne paniced, the kdb.enter.panic script ran but had its output truncated to 512K.

debug.ddb.capture.bufsize: 49152
debug.ddb.capture.maxbufsize: 524288

The problem is that sys/ddb/db_capture.c has fixed DB_CAPTURE_MAXBUFSIZE to 512K.  I was able to get the full output by increasing DB_CAPTURE_MAXBUFSIZE to 5M (3M would have been enough, as ddb.txt was only 2.7M).
>How-To-Repeat:
Add the following to the kernel config file:

options         DEBUG_VFS_LOCKS
options         KTR
options         KTR_COMPILE=(KTR_SPARE2)
options         KTR_MASK=(KTR_SPARE2)
options         KTR_ENTRIES=32768

and then build/install the kernel.  After the reboot, use ddb to add the following scripts:

/sbin/ddb script lockinfo="show locks; show alllocks; show lockedvnods"

/sbin/ddb script kdb.enter.panic="textdump set; capture on; show ktr ; run lockinfo ; show pcpu; bt; ps; alltrace; capture off; call doadump; reset"

sysctl debug.ddb.capture.bufsize=524288

Then do something that causes the kernel to panic.
>Fix:
Apply the attached patch which adds two kernel options to allow the capture buffer size to be changed in the kernel config file:

options         DB_CAPTURE_DEFAULTBUFSIZE=2*1024*1024
options         DB_CAPTURE_MAXBUFSIZE=5*1024*1024

Should DB_CAPTURE* be changed to DDB_CAPTURE*?  Will require a change in ddb/db_capture.c also.

Note: DB_CAPTURE_DEFAULTBUFSIZE can also be set by sysctl.


Patch attached with submission follows:

Index: conf/options
===================================================================
RCS file: /home/ncvs/src/sys/conf/options,v
retrieving revision 1.615
diff -u -r1.615 options
--- conf/options	7 Jan 2008 21:40:09 -0000	1.615
+++ conf/options	25 Jan 2008 23:52:31 -0000
@@ -49,6 +49,8 @@
 # Debugging options.
 DDB
 DDB_NUMSYM	opt_ddb.h
+DB_CAPTURE_DEFAULTBUFSIZE	opt_ddb.h
+DB_CAPTURE_MAXBUFSIZE	opt_ddb.h
 GDB
 KDB		opt_global.h
 KDB_TRACE	opt_kdb.h
Index: ddb/db_capture.c
===================================================================
RCS file: /home/ncvs/src/sys/ddb/db_capture.c,v
retrieving revision 1.2
diff -u -r1.2 db_capture.c
--- ddb/db_capture.c	26 Dec 2007 11:32:32 -0000	1.2
+++ ddb/db_capture.c	25 Jan 2008 23:50:46 -0000
@@ -46,6 +46,8 @@
 #include <ddb/ddb.h>
 #include <ddb/db_lex.h>
 
+#include <opt_ddb.h>
+
 /*
  * While it would be desirable to use a small block-sized buffer and dump
  * incrementally to disk in fixed-size blocks, it's not possible to enter
@@ -55,8 +57,12 @@
  */
 static MALLOC_DEFINE(M_DB_CAPTURE, "db_capture", "DDB capture buffer");
 
+#ifndef DB_CAPTURE_DEFAULTBUFSIZE
 #define	DB_CAPTURE_DEFAULTBUFSIZE	48*1024
+#endif
+#ifndef DB_CAPTURE_MAXBUFSIZE
 #define	DB_CAPTURE_MAXBUFSIZE	512*1024
+#endif
 #define	DB_CAPTURE_FILENAME	"ddb.txt"	/* Captured DDB output. */
 
 static char *db_capture_buf;


>Release-Note:
>Audit-Trail:
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