arch/x86/include/asm/refcount.h

   1 #ifndef __ASM_X86_REFCOUNT_H
   2 #define __ASM_X86_REFCOUNT_H
   3 /*
   4  * x86-specific implementation of refcount_t. Based on PAX_REFCOUNT from
   5  * PaX/grsecurity.
   6  */
   7 #include <linux/refcount.h>
   8 #include <asm/bug.h>
   9
  10 /*
  11  * This is the first portion of the refcount error handling, which lives in
  12  * .text.unlikely, and is jumped to from the CPU flag check (in the
  13  * following macros). This saves the refcount value location into CX for
  14  * the exception handler to use (in mm/extable.c), and then triggers the
  15  * central refcount exception. The fixup address for the exception points
  16  * back to the regular execution flow in .text.
  17  */
  18 #define _REFCOUNT_EXCEPTION                             \
  19         ".pushsection .text..refcount\n"                \
  20         "111:\tlea %[counter], %%" _ASM_CX "\n"         \
  21         "112:\t" ASM_UD2 "\n"                           \
  22         ASM_UNREACHABLE                                 \
  23         ".popsection\n"                                 \
  24         "113:\n"                                        \
  25         _ASM_EXTABLE_REFCOUNT(112b, 113b)
  26
  27 /* Trigger refcount exception if refcount result is negative. */
  28 #define REFCOUNT_CHECK_LT_ZERO                          \
  29         "js 111f\n\t"                                   \
  30         _REFCOUNT_EXCEPTION
  31
  32 /* Trigger refcount exception if refcount result is zero or negative. */
  33 #define REFCOUNT_CHECK_LE_ZERO                          \
  34         "jz 111f\n\t"                                   \
  35         REFCOUNT_CHECK_LT_ZERO
  36
  37 /* Trigger refcount exception unconditionally. */
  38 #define REFCOUNT_ERROR                                  \
  39         "jmp 111f\n\t"                                  \
  40         _REFCOUNT_EXCEPTION
  41
  42 static __always_inline void refcount_add(unsigned int i, refcount_t *r)
  43 {
  44         asm volatile(LOCK_PREFIX "addl %1,%0\n\t"
  45                 REFCOUNT_CHECK_LT_ZERO
  46                 : [counter] "+m" (r->refs.counter)
  47                 : "ir" (i)
  48                 : "cc", "cx");
  49 }
  50
  51 static __always_inline void refcount_inc(refcount_t *r)
  52 {
  53         asm volatile(LOCK_PREFIX "incl %0\n\t"
  54                 REFCOUNT_CHECK_LT_ZERO
  55                 : [counter] "+m" (r->refs.counter)
  56                 : : "cc", "cx");
  57 }
  58
  59 static __always_inline void refcount_dec(refcount_t *r)
  60 {
  61         asm volatile(LOCK_PREFIX "decl %0\n\t"
  62                 REFCOUNT_CHECK_LE_ZERO
  63                 : [counter] "+m" (r->refs.counter)
  64                 : : "cc", "cx");
  65 }
  66
  67 static __always_inline __must_check
  68 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
  69 {
  70         GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl", REFCOUNT_CHECK_LT_ZERO,
  71                                   r->refs.counter, "er", i, "%0", e, "cx");
  72 }
  73
  74 static __always_inline __must_check bool refcount_dec_and_test(refcount_t *r)
  75 {
  76         GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl", REFCOUNT_CHECK_LT_ZERO,
  77                                  r->refs.counter, "%0", e, "cx");
  78 }
  79
  80 static __always_inline __must_check
  81 bool refcount_add_not_zero(unsigned int i, refcount_t *r)
  82 {
  83         int c, result;
  84
  85         c = atomic_read(&(r->refs));
  86         do {
  87                 if (unlikely(c == 0))
  88                         return false;
  89
  90                 result = c + i;
  91
  92                 /* Did we try to increment from/to an undesirable state? */
  93                 if (unlikely(c < 0 || c == INT_MAX || result < c)) {
  94                         asm volatile(REFCOUNT_ERROR
  95                                      : : [counter] "m" (r->refs.counter)
  96                                      : "cc", "cx");
  97                         break;
  98                 }
  99
 100         } while (!atomic_try_cmpxchg(&(r->refs), &c, result));
 101
 102         return c != 0;
 103 }
 104
 105 static __always_inline __must_check bool refcount_inc_not_zero(refcount_t *r)
 106 {
 107         return refcount_add_not_zero(1, r);
 108 }
 109
 110 #endif