Filling in struct mips64_watch_regs from a 32 bit process

Filling in struct mips64_watch_regs from a 32 bit process

[Earl Chew]

I notice that a 32 bit process running on a 64 bit kernel is expected to
know that it should fill in mips64_watch_regs --- even though it is running
against a 32 bit ABI.

Is this an oversight, or am I missing something ?

[ That same 32 bit process must fill in mips32_watch_regs when running on
  a 32 bit kernel. ]


In arch/mips/include/asm/ptrace.h:

struct mips32_watch_regs {
	unsigned int watchlo[8];
	...
};

and

struct mips64_watch_regs {
	unsigned long long watchlo[8];
	...
};

These are used in a union, but sizeof(mips64_watch_regs.watchlo) will not
match sizeof(mips32_watch_regs.watchlo).



For a 64 bit kernel, the code in arch/mips/kernel/ptrace.c reads:


        /* Check the values. */
        for (i = 0; i < current_cpu_data.watch_reg_use_cnt; i++) {
                __get_user(lt[i], &addr->WATCH_STYLE.watchlo[i]);
#ifdef CONFIG_32BIT
                if (lt[i] & __UA_LIMIT)
                        return -EINVAL;
#else
                if (test_tsk_thread_flag(child, TIF_32BIT_ADDR)) {
                        if (lt[i] & 0xffffffff80000000UL)
                                return -EINVAL;
                } else {
                        if (lt[i] & __UA_LIMIT)
                                return -EINVAL;
                }
#endif


Thus for a 64 bit kernel, WATCH_STYLE is defined to be mips64, and the code
goes on to obtain:

	addr->mips64.watchlo[i]

and to verify it based on TIF_32BIT_ADDR.


In other words, the 32 bit process is expected to fill in mips64_watch_regs
when it is running on a 64 bit kernel, and mips32_watch_regs when it is running
on a 32 bit kernel.



Earl

Re: Filling in struct mips64_watch_regs from a 32 bit process

[David Daney]

On 01/31/2011 04:41 PM, Earl Chew wrote:
> I notice that a 32 bit process running on a 64 bit kernel is expected to
> know that it should fill in mips64_watch_regs --- even though it is running
> against a 32 bit ABI.
>
> Is this an oversight, or am I missing something ?

It is intentional.

>
> [ That same 32 bit process must fill in mips32_watch_regs when running on
>    a 32 bit kernel. ]
>
>
> In arch/mips/include/asm/ptrace.h:
>
> struct mips32_watch_regs {
> 	unsigned int watchlo[8];
> 	...
> };
>
> and
>
> struct mips64_watch_regs {
> 	unsigned long long watchlo[8];
> 	...
> };
>
> These are used in a union, but sizeof(mips64_watch_regs.watchlo) will not
> match sizeof(mips32_watch_regs.watchlo).
>

The sizes are different and thus are ... different.  struct 
pt_watch_regs however, has a well defined size.

The important thing is that the style element of struct pt_watch_regs is 
always in the same place.

>
>
> For a 64 bit kernel, the code in arch/mips/kernel/ptrace.c reads:
>
>
>          /* Check the values. */
>          for (i = 0; i<  current_cpu_data.watch_reg_use_cnt; i++) {
>                  __get_user(lt[i],&addr->WATCH_STYLE.watchlo[i]);
> #ifdef CONFIG_32BIT
>                  if (lt[i]&  __UA_LIMIT)
>                          return -EINVAL;
> #else
>                  if (test_tsk_thread_flag(child, TIF_32BIT_ADDR)) {
>                          if (lt[i]&  0xffffffff80000000UL)
>                                  return -EINVAL;
>                  } else {
>                          if (lt[i]&  __UA_LIMIT)
>                                  return -EINVAL;
>                  }
> #endif
>
>
> Thus for a 64 bit kernel, WATCH_STYLE is defined to be mips64, and the code
> goes on to obtain:
>
> 	addr->mips64.watchlo[i]
>
> and to verify it based on TIF_32BIT_ADDR.
>
>
> In other words, the 32 bit process is expected to fill in mips64_watch_regs
> when it is running on a 64 bit kernel, and mips32_watch_regs when it is running
> on a 32 bit kernel.
>

Yes.  The debugging agent must check struct pt_watch_regs style to 
determine which type of watch registers the hardware is using.

Take a look at how GDB handles this in mips-linux-nat.c.

David Daney

Re: Filling in struct mips64_watch_regs from a 32 bit process

[Earl Chew]

>> I notice that a 32 bit process running on a 64 bit kernel is expected to
>> know that it should fill in mips64_watch_regs --- even though it is running
>> against a 32 bit ABI.
>>
>> Is this an oversight, or am I missing something ?
> 
> It is intentional.

Oh I see ... I have to call PTRACE_SET_WATCH_REGS first in order to figure
out whether the kernel is expecting me to use pt_watch_style_mips32 or
pt_watch_style_mips64.

Thanks.

Earl

Re: Filling in struct mips64_watch_regs from a 32 bit process

[David Daney]

On 01/31/2011 05:12 PM, Earl Chew wrote:
>>> I notice that a 32 bit process running on a 64 bit kernel is expected to
>>> know that it should fill in mips64_watch_regs --- even though it is running
>>> against a 32 bit ABI.
>>>
>>> Is this an oversight, or am I missing something ?
>>
>> It is intentional.
>
> Oh I see ... I have to call PTRACE_SET_WATCH_REGS first in order to figure
> out whether the kernel is expecting me to use pt_watch_style_mips32 or
> pt_watch_style_mips64.
>

That would be PTRACE_GET_WATCH_REGS.  It will tell you the width of the 
registers as well as how many are available (in the num_valid field) and 
the properties of each (in the watch_masks[] fields).

Fill in watchlo[] and watchhi[], write them back with 
PTRACE_SET_WATCH_REGS, then sit back and wait for SIGTRAP.

David Daney

Re: Filling in struct mips64_watch_regs from a 32 bit process

[Earl Chew]

> That would be PTRACE_GET_WATCH_REGS.

Yes, that's what I thought I typed  :-(

Earl