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Re: Filling in struct mips64_watch_regs from a 32 bit process

To: Earl Chew <echew@ixiacom.com>
Subject: Re: Filling in struct mips64_watch_regs from a 32 bit process
From: David Daney <ddaney@caviumnetworks.com>
Date: Mon, 31 Jan 2011 17:03:07 -0800
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
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References: <4D4756B5.4010100@ixiacom.com>
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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

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