linux-ext4 - Re: [PATCH v2] x86: make DR*

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Date:	Sat, 27 Apr 2013 09:20:28 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	"Theodore Ts'o" <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2] x86: make DR*_RESERVED unsigned long

I don't know why this is uapi... finest make a lot of sense to me.

Oleg Nesterov <oleg@...hat.com> wrote:

>On 04/26, Oleg Nesterov wrote:
>
>> On 04/26, H. Peter Anvin wrote:
>> >
>> > On 04/26/2013 09:38 AM, Oleg Nesterov wrote:
>> > >
>> > > 	- do_debug:
>> > >
>> > > 		dr6 &= ~DR6_RESERVED;
>> > >
>> > > 	  this also wrongly clears 32-63 bits. Fortunately these
>> > > 	  bits are reserved and must be zero.
>> >
>> > I don't think this is wrongly at all.
>>
>> OK, I meant that it also clears the bits that are not specified in
>> DR6_RESERVED mask.
>>
>> > The whole point is to mask out
>> > the bits that the handler doesn't want to deal with, so masking out
>the
>> > reserved bits [63:32] seems reasonable to me.
>>
>> Then we should do
>>
>> 	- #define DR6_RESERVED    0xFFFF0FF0
>> 	+ #define DR6_RESERVED    0xFFFFFFFFFFFF0FF0
>>
>> ?
>>
>> or what? (just in case, I will happily agree with "do nothing" ;)
>
>Or we can do the s/reserved/mask/ change and avoid any "unexpected"
>effect of "long &= ~int". This allso allows to kill ifdef(__i386__).
>
>But this is include/uapi, I do not know if I can simply remove the
>old define's.
>
>In short: whatever you prefer, including "leave it alone".
>
>Oleg.
>
>diff --git a/arch/x86/include/uapi/asm/debugreg.h
>b/arch/x86/include/uapi/asm/debugreg.h
>index 3c0874d..2678b23 100644
>--- a/arch/x86/include/uapi/asm/debugreg.h
>+++ b/arch/x86/include/uapi/asm/debugreg.h
>@@ -14,8 +14,7 @@
> which debugging register was responsible for the trap.  The other bits
>    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_MASK	(0xF00FU)	/* Everything else is reserved */
> 
> #define DR_TRAP0	(0x1)		/* db0 */
> #define DR_TRAP1	(0x2)		/* db1 */
>@@ -32,6 +31,8 @@
>  and indicates what types of access we trap on, and how large the data
>    field is that we are looking at */
> 
>+#define DR_CONTROL_MASK (0xFFFF03FFU) /* Everything else is reserved
>*/
>+
> #define DR_CONTROL_SHIFT 16 /* Skip this many bits in ctl register */
> #define DR_CONTROL_SIZE 4   /* 4 control bits per register */
> 
>@@ -64,12 +65,6 @@
>   We can slow the instruction pipeline for instructions coming via the
>gdt or the ldt if we want to.  I am not sure why this is an advantage
>*/
> 
>-#ifdef __i386__
>-#define DR_CONTROL_RESERVED (0xFC00) /* Reserved by Intel */
>-#else
>-#define DR_CONTROL_RESERVED (0xFFFFFFFF0000FC00UL) /* Reserved */
>-#endif
>-
> #define DR_LOCAL_SLOWDOWN (0x100)   /* Local slow the pipeline */
> #define DR_GLOBAL_SLOWDOWN (0x200)  /* Global slow the pipeline */
> 
>diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
>index 7461f50..bc5fb98 100644
>--- a/arch/x86/kernel/ptrace.c
>+++ b/arch/x86/kernel/ptrace.c
>@@ -657,7 +657,7 @@ static int ptrace_write_dr7(struct task_struct
>*tsk, unsigned long data)
> 	bool second_pass = false;
> 	int i, rc, ret = 0;
> 
>-	data &= ~DR_CONTROL_RESERVED;
>+	data &= DR_CONTROL_MASK;
> 	old_dr7 = ptrace_get_dr7(thread->ptrace_bps);
> 
> restore:
>diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>index 68bda7a..42a635f 100644
>--- a/arch/x86/kernel/traps.c
>+++ b/arch/x86/kernel/traps.c
>@@ -402,7 +402,7 @@ dotraplinkage void __kprobes do_debug(struct
>pt_regs *regs, long error_code)
> 	get_debugreg(dr6, 6);
> 
> 	/* Filter out all the reserved bits which are preset to 1 */
>-	dr6 &= ~DR6_RESERVED;
>+	dr6 &= DR6_MASK;
> 
> 	/*
> 	 * If dr6 has no reason to give us about the origin of this trap,

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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