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

Dave Hansen
2015-03-30T16:57:16Z

On 03/28/2015 01:39 AM, Borislav Petkov wrote:
> - 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);
> - if (!ret)
> - *actual_old_val_ptr = actual_old_val_32;
> - }
> - return ret;
> + if (is_64bit_mm(mm))
> + return user_atomic_cmpxchg_inatomic(actual_old_val_ptr,
> + bd_entry_addr,
> + expected_old_val,
> + new_bd_entry);
> + else
> + return futex_atomic_cmpxchg_inatomic((u32 *)actual_old_val_ptr,
> + (u32 __user *)bd_entry_addr,
> + expected_old_val,
> + new_bd_entry);
> }

That does look tempting, and I appreciate the analysis.

But, I'd really rather not hide this behind another layer of abstraction
in order to save a few variable declarations. It's definitely _smaller_
code, but it's a little less obvious what is going on.