Re: Filling in struct mips64_watch_regs from a 32 bit process

[David Daney <ddaney@caviumnetworks.com>]

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