Subject: Re: [PATCH] x86: make DR*_RESERVED unsigned long

From: "H. Peter Anvin" <[email-masked]>
Date: Wed, 24 Apr 2013 16:06:38 -0700

On 04/24/2013 03:48 PM, Frederic Weisbecker wrote:
> 2013/4/24 Oleg Nesterov <[email-masked]>:
>> DR6_RESERVED and DR_CONTROL_RESERVED are used to clear the set
>> bits in the "unsigned long" data, make them long to ensure that
>> "&~" doesn't clear the upper bits.
>>
>> do_debug() and ptrace_write_dr7() which use DR*_RESERVED look
>> safe, but probably it makes sense to cleanup anyway.
> 
> Agreed. The code looks safe, but the pattern is error prone. I'm all
> for that cleanup.
> Just something below:
> 
>>
>> Reported-by: Linus Torvalds <[email-masked]>
>> Signed-off-by: Oleg Nesterov <[email-masked]>
>> ---
>>  arch/x86/include/uapi/asm/debugreg.h |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/include/uapi/asm/debugreg.h b/arch/x86/include/uapi/asm/debugreg.h
>> index 3c0874d..75d07dd 100644
>> --- a/arch/x86/include/uapi/asm/debugreg.h
>> +++ b/arch/x86/include/uapi/asm/debugreg.h
>> @@ -15,7 +15,7 @@
>>     are either reserved or not of interest to us. */
>>
>>  /* Define reserved bits in DR6 which are always set to 1 */
>> -#define DR6_RESERVED   (0xFFFF0FF0)
>> +#define DR6_RESERVED   (0xFFFF0FF0ul)
> 
> You told in an earlier email that intel manual says upper 32 bits of
> dr6 are reserved.
> In this case don't we need to expand the mask in 64 bits like is done
> for DR_CONTROL_RESERVED?
> 

Arguably this would be a *good* use for ~ ...

Instead of defining separate bitmasks for 32 and 64 bits have the
reciprocal (non-reserved bits):

#define DR6_RESERVED (~0x0000F00FUL)

That does have the right value on both 32 and 64 bits.  The leading
zeroes aren't even really needed.

Now, DR6 is a bit special in that a bunch of the reserved bits are
hardwired to 1, not 0; I don't know offhand if that is true for bits
[63:32].

	-hpa