Re: [PATCH] elf2dmp: Don't abandon when Prcb is set to 0

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2023/06/12 12:42, Viktor Prutyanov wrote:
>> Prcb may be set to 0 for some CPUs if the dump was taken before they
>> start. The dump may still contain valuable information for started CPUs
>> so don't abandon conversion in such a case.
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>> ---
>> contrib/elf2dmp/main.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/contrib/elf2dmp/main.c b/contrib/elf2dmp/main.c
>> index d77b8f98f7..91c58e4424 100644
>> --- a/contrib/elf2dmp/main.c
>> +++ b/contrib/elf2dmp/main.c
>> @@ -312,6 +312,11 @@ static int fill_context(KDDEBUGGER_DATA64 *kdbg,
>> return 1;
>> }
>>
>> + if (!Prcb) {
>> + eprintf("Context for CPU #%d is missing\n", i);
>> + continue;
>> + }
>> +
>> if (va_space_rw(vs, Prcb + kdbg->OffsetPrcbContext,
>> &Context, sizeof(Context), 0)) {
>> eprintf("Failed to read CPU #%d ContextFrame location\n", i);
>>
>> --
>> 2.40.1
> 
> Hi Akihiko,
> 
> How this fix can be tested?

It is a bit difficult to test it as you need to interrupt the very early 
stage of boot. I applied the following change to TCG so that it stops 
immediately after the first processor configures Prcb.

diff --git a/target/i386/tcg/sysemu/misc_helper.c 
b/target/i386/tcg/sysemu/misc_helper.c
index e1528b7f80..f68eba9cac 100644
--- a/target/i386/tcg/sysemu/misc_helper.c
+++ b/target/i386/tcg/sysemu/misc_helper.c
@@ -25,6 +25,9 @@
  #include "exec/address-spaces.h"
  #include "exec/exec-all.h"
  #include "tcg/helper-tcg.h"
+#include "exec/gdbstub.h"
+#include "hw/core/cpu.h"
+#include "sysemu/runstate.h"

  void helper_outb(CPUX86State *env, uint32_t port, uint32_t data)
  {
@@ -217,7 +220,10 @@ void helper_wrmsr(CPUX86State *env)
          env->segs[R_FS].base = val;
          break;
      case MSR_GSBASE:
+        printf("%s: %" PRIx64 "\n", __func__, val);
          env->segs[R_GS].base = val;
+        gdb_set_stop_cpu(current_cpu);
+        vm_stop(RUN_STATE_PAUSED);
          break;
      case MSR_KERNELGSBASE:
          env->kernelgsbase = val;

> 
> NumberProcessors field is still set to qemu_elf.state_nr, how does WinDbg react to this?

If Prcb for processor 1 is missing, WinDbg outputs: KiProcessorBlock[1] 
is null.
You can still debug the started processors with no issue.

Regards,
Akihiko Odaki

> 
> Viktor