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

[Viktor Prutyanov]

> 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

Reviewed-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>