Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy
From: Dave Hansen
Date: Tue Dec 08 2015 - 11:34:47 EST
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
#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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