linux-kernel - Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy

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Date:	Tue, 8 Dec 2015 08:34:41 -0800
From:	Dave Hansen <dave@...1.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy

On 12/08/2015 07:15 AM, Thomas Gleixner wrote:
> On Thu, 3 Dec 2015, Dave Hansen wrote:
>>  
>> +static inline int vma_pkey(struct vm_area_struct *vma)
> 
> Shouldn't this return something unsigned?

Ingo had asked that we use 'int' in the syscalls at some point.  We also
use a -1 to mean "no pkey set" (to differentiate it from pkey=0) at
least at the very top of the syscall level.

>> +{
>> +	u16 pkey = 0;
>> +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>> +	unsigned long vma_pkey_mask = VM_PKEY_BIT0 | VM_PKEY_BIT1 |
>> +				      VM_PKEY_BIT2 | VM_PKEY_BIT3;
>> +	/*
>> +	 * ffs is one-based, not zero-based, so bias back down by 1.
>> +	 */
>> +	int vm_pkey_shift = __builtin_ffsl(vma_pkey_mask) - 1;
> 
> Took me some time to figure out that this will resolve to a compile
> time constant (hopefully). Is there a reason why we don't have a
> VM_PKEY_SHIFT constant in the header file which makes that code just
> simple and intuitive?

All of the VM_* flags are #defined as bitmaps directly and don't define
shifts:

#define VM_MAYWRITE     0x00000020
#define VM_MAYEXEC      0x00000040
#define VM_MAYSHARE     0x00000080
...

So to get a shift we've either got to do a ffs somewhere, or we have to
define the VM_PKEY_BIT*'s differently from all of the other VM_* flags.
 Or, we do something along the lines of:

#define VM_PKEY_BIT0 0x100000000UL
#define __VM_PKEY_SHIFT (32)

and we run a small risk that somebody will desynchronize the shift and
the bit definition.

We only need this shift in this *one* place, so that's why I opted for
the local variable and ffs.

>> +	/*
>> +	 * gcc generates better code if we do this rather than:
>> +	 * pkey = (flags & mask) >> shift
>> +	 */
>> +	pkey = (vma->vm_flags >> vm_pkey_shift) &
>> +	       (vma_pkey_mask >> vm_pkey_shift);
> 
> My gcc (4.9) does it the other way round for whatever reason.

I'll go recheck.

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