[Crash-utility] Viewing module symbols using crash

[Dave Anderson]

Durga Prasad wrote:
> Hi,
> 
> [1]:
> I need to debug a kernel module (a device driver).
> I added the desired module to crash using mod -S <path to module>. The 
> driver is compiled with -g option.
> I could not see the structures defined within the module.
> 
> Am I missing something?

Accessing data in kernel modules can be spotty at best,
but the embedded gdb module should typically be able
to pick up structure definitions in modules.

For example, these two structures are defined in ./fs/ext3/xattr.c
like so:

   struct ext3_xattr_info {
           int name_index;
           const char *name;
           const void *value;
           size_t value_len;
   };

   struct ext3_xattr_search {
           struct ext3_xattr_entry *first;
           void *base;
           void *end;
           struct ext3_xattr_entry *here;
           int not_found;
   };

Without the ext3 module debuginfo, crash (gdb) doesn't know what
they are:

   # crash -s
   crash> struct ext3_xattr_search
   struct: invalid data structure reference: ext3_xattr_search
   crash> struct ext3_xattr_info
   struct: invalid data structure reference: ext3_xattr_info
   crash>

Until after adding the module debuginfo data:

   crash> mod -s ext3
        MODULE       NAME               SIZE  OBJECT FILE
   ffffffff8806be00  ext3             168017  /lib/modules/2.6.18-102.el5.perfmon2/kernel/fs/ext3/ext3.ko
   crash> struct ext3_xattr_search
   struct ext3_xattr_search {
       struct ext3_xattr_entry *first;
       void *base;
       void *end;
       struct ext3_xattr_entry *here;
       int not_found;
   }
   SIZE: 40
   crash> struct ext3_xattr_info
   struct ext3_xattr_info {
       int name_index;
       const char *name;
       const void *value;
       size_t value_len;
   }
   SIZE: 32
   crash>

Then you may be able to "p" individual instances of the structures
if there are symbolic/static declaration of them.  Otherwise you
can always cast the address of the structure to the structure type,
as in:

   crash> struct ext3_xattr_info <address>

If you cannot get the structure declaration after using "mod -[sS]",
then you'll have to get primitive, and just dump the data at the
relevant address with "rd".

> 
> System info:
>     KERNEL: vmlinux.sles10sp2.smp.x86_64
>     DUMPFILE: 2008-08-08-18:57/vmcore
>     CPUS: 4
> 
>      RELEASE: 2.6.16.60-0.21-smp
>      VERSION: #1 SMP Tue May 6 12:41:02 UTC 2008
>      MACHINE: x86_64  (1995 Mhz)
>      MEMORY: 3.9 GB
> 
> 
> 
> [2]:
> I could get into live kernel debugging using crash on RHEL 5 after 
> installing relevant kernel-debug packages. How do I do the same on SLES 10?
> As of now, I have been compiling the kernel with CONFIG_DEBUG_INFO set. 
> I can use the resulting vmlinux for post crash analysis, but not for 
> live analysis.  Any light on how to accomplish this would be appreciated.

For SLES I'm guessing that you could use the post-built (CONFIG_DEBUG_INFO) kernel,
but also add the System.map file of the live kernel to the command line, as in:

   $ crash /boot/System.map vmlinux-post-built-with-debuginfo

Crash will use the debuginfo data from the vmlinux file, but then go in
and back-patch all the vmlinux file's minimal_symbols with the correct
symbol values from the System.map file.

But Bernhard Walle (bwalle at suse.de) can answer that part of your question.
He's active on this list, but I've added him to the cc: list to make
sure he sees this.

Dave Anderson


> 
> Thanks
> Durga Prasad
>