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Subject[PATCH 13/19] x86, mpx: Add temporary variable to reduce masking
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From: Dave Hansen <dave.hansen@linux.intel.com>

When we allocate a bounds table, we call mmap(), then add a
"valid" bit to the value before storing it in to the bounds
directory.

If we fail along the way, we go and mask that valid bit
_back_ out. That seems a little silly, and this makes it
much more clear when we have a plain address versus an
actual table _entry_.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---

b/arch/x86/mm/mpx.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff -puN arch/x86/mm/mpx.c~mpx-remove-unnecessary-masking arch/x86/mm/mpx.c
--- a/arch/x86/mm/mpx.c~mpx-remove-unnecessary-masking 2015-05-27 09:32:19.588673255 -0700
+++ b/arch/x86/mm/mpx.c 2015-05-27 09:32:19.592673436 -0700
@@ -429,6 +429,7 @@ static int allocate_bt(long __user *bd_e
unsigned long expected_old_val = 0;
unsigned long actual_old_val = 0;
unsigned long bt_addr;
+ unsigned long bd_new_entry;
int ret = 0;

/*
@@ -441,7 +442,7 @@ static int allocate_bt(long __user *bd_e
/*
* Set the valid flag (kinda like _PAGE_PRESENT in a pte)
*/
- bt_addr = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
+ bd_new_entry = bt_addr | MPX_BD_ENTRY_VALID_FLAG;

/*
* Go poke the address of the new bounds table in to the
@@ -455,7 +456,7 @@ static int allocate_bt(long __user *bd_e
* of the MPX code that have to pagefault_disable().
*/
ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
- expected_old_val, bt_addr);
+ expected_old_val, bd_new_entry);
if (ret)
goto out_unmap;

@@ -486,7 +487,7 @@ static int allocate_bt(long __user *bd_e
trace_mpx_new_bounds_table(bt_addr);
return 0;
out_unmap:
- vm_munmap(bt_addr & MPX_BT_ADDR_MASK, MPX_BT_SIZE_BYTES);
+ vm_munmap(bt_addr, MPX_BT_SIZE_BYTES);
return ret;
}

_

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