[Crash-utility] Re: backtrace - how do I get local and formal parametes - kernel is compiled -O0.

[Dave Anderson]

Pete/Piet Delaney wrote:
> |  > When I was using gdb on SunOS 4.1.4 I was able to set $sp and
> |  > $fp to switch processes. When I took the SP and FP and used
> |  > a gdb set $sp and %fp it didn't seen to allow me to do this:
> |  > - -----------------------------------------------------------
> |  > crash> gdb set $sp = 0xd76bbbb4
> |  > No registers.
> |  >
> |  > crash> gdb set $fp = 0xd76bbc24
> |  > No registers.
> |  > - -----------------------------------------------------------
> |  > Why is is saying 'No registers'?
> |
> | I'm not positive, but the gdb sources seem to display that message
> | whenever (!target_has_registers), and that's #define'd like so:
> |
> |   /* Does the target have registers?  (Exec files don't.)  */
> |
> |   #define target_has_registers    \
> |        (current_target.to_has_registers)
> |
> | And since it's being invoked as "gdb vmlinux", it only knows
> | about the exec file, and has no clue about registers.
> 
> Perhaps gdb changed since SunOS 4.1.4 in this area. We could
> consider patching gdb to think it has registers just to allow
> crash and/or the user from setting $sp and $fp to facilitate
> better back trace information. As I recall you make environment
> isn't building a virgin gdb.

That's exactly what I tried many years ago -- just forcibly setting
the stack and frame pointer values.  But it still required more
"user environment" information to work correctly.  I had some limited
success on the alpha platform, but never on x86.

And that's only the beginning.  To be of any use, it would have to
be able to deal with in-kernel exception frames, which from a user
debugger standpoint, it has no idea what they are.  And it would need
to be able to deal with the stack-switching done by the various
architectures to handle IRQ's, special-case exceptions, soft IRQ
handling, dealing with the assembly-code magic performed by the
entry.S entry points, and son on.  I noted that your crash example
was a "clean" panic() call -- as opposed to a the far-more-likely
crash scenario where the kernel is running in kernel mode, takes
an exception, lays down an exception frame, switches stacks, etc...

> 
> 
> 
> |  >
> |  > Also why don't you set up gdb with the stack info for the current
> |  > task and either allow the user to do a 'gdb bt' or do it with a
> |  > crash 'bt' option?
> |  >
> |
> | I don't even come close to understanding gdb internals well enough
> | to know what "set up gdb with the stack info" entails.  Send me
> | a patch when you've got it working -- for all arches, for all tasks,
> | and without requiring any vmcore file argument...  ;-)
> |
> |  > Will gdb work directly with the kdump core file? If not when did
> |  > it break and why? Docs with 2.6.16 suggest to use gdb of the core
> |  > file.
> |
> | I don't know if it *has* broke...
> |
> | As I said this morning, I don't know if the latest and greatest 32-bit
> | gdb can work with 64-bit ELF kdump vmcores, which kdump uses by default
> | for both 32-bit and 64-bit architectures.  The kdump.txt file says this:
> |
> |   * By default, the ELF headers are stored in ELF64 format to support
> |     systems with more than 4GB memory. The --elf32-core-headers option 
> can
> |     be used to force the generation of ELF32 headers. This is necessary
> |     because GDB currently cannot open vmcore files with ELF64 headers on
> |     32-bit systems. ELF32 headers can be used on non-PAE systems (that 
> is,
> |     less than 4GB of memory).
> |
> | But at least with a 32-bit ELF vmcore, gdb should be able to work.
> 
> ok, I'll trying pulling out a DIMM and running on 2GB.

But you'll still get a 64-bit vmcore unless you deal with the
kexec/kdump configuration.  And why not just put "mem=2048m" on
the boot command line?


> | Your memory's wrong, especially w/respect to local variables -- are
> | you kidding me?
> 
> No, I wasn't joking, I thought back traces with crash had the same
> format as with gdb. Last I worked on IA64 and gdb I found messing
> with it's accessing of registers rather convolved. I was modifying
> Andy Kleen's gdb interface to SGI's kdb debugger, it's called 'SKDB'.
> At the time SGI had kdb working on IA64 kernels and I tried modifying
> if from ia32 to IA64. The register storage in gdb was a bit odd.
> 
> Seems to be worthy of consideration to fool gdb into thing it has
> registers just so crash can get gdb to provide backtrace info.
> What happens of you simply change the #define to something
> like:
> 
>     #define target_has_registers    1
> 
> I take it that crash in doing the stack unwind it's self now
> instead of having gdb do it. Have you thought about having gdb
> do it and what's needed?

See above...

> 
> On Friday you mentioned an embedded  gdb module to hijack read
> attempts and back them to crash. I tried a Google for more info
> but didn't find anything. Could you tell me more about that?

That's *my* term for the stock version of gdb-6.1 that I hacked/married
with the crash sources.  If you look at the gdb sources you can see
that it already had hooks to "embed" it into another bimary,
typically into a GUI, or some such thing.  You basically hijack
the user interface from the gdb prompt, and pass it back to your
own user interface.

Dave