Re: [PATCH 15/17] x86, mpx: do 32-bit-only cmpxchg for 32-bit apps

[Dave Hansen <dave@sr71.net>]

On 03/27/2015 10:29 AM, Borislav Petkov wrote:
>> > +static int mpx_cmpxchg_bd_entry(struct mm_struct *mm,
>> > +		unsigned long *actual_old_val_ptr, long __user *bd_entry_addr,
>> > +		unsigned long expected_old_val, unsigned long new_bd_entry)
>> > +{
>> > +	int ret;
>> > +	/*
>> > +	 * user_atomic_cmpxchg_inatomic() actually uses sizeof()
>> > +	 * the pointer thatt we pass to it to figure out how much
>> > +	 * data to cmpxchg.  We have to be careful here not to
>> > +	 * pass a pointer to a 64-bit data type when we only want
>> > +	 * a 32-bit copy.
>> > +	 */
>> > +	if (is_64bit_mm(mm)) {
>> > +		ret = user_atomic_cmpxchg_inatomic(actual_old_val_ptr,
>> > +				bd_entry_addr, expected_old_val, new_bd_entry);
>> > +	} else {
>> > +		u32 uninitialized_var(actual_old_val_32);
>> > +		u32 expected_old_val_32 = expected_old_val;
>> > +		u32 new_bd_entry_32 = new_bd_entry;
>> > +		u32 __user *bd_entry_32 = (u32 __user *)bd_entry_addr;
>> > +		ret = user_atomic_cmpxchg_inatomic(&actual_old_val_32,
>> > +				bd_entry_32, expected_old_val_32,
>> > +				new_bd_entry_32);
> Hmm, I would've added a user_atomic_cmpxchg_inatomic_size() macro which
> calls __user_atomic_cmpxchg_inatomic().

That would have saved creating 'u32 __user *bd_entry_32' so that we
could implicitly do sizeof(*bd_entry_32).  But, what else does it buy us?