x86: Rename __glibc_reserved2 to ssp_base in tcbhead_t

This will be used to record the current shadow stack base for shadow
stack switching by getcontext, makecontext, setcontext and swapcontext.
If the target shadow stack base is the same as the current shadow stack
base, we unwind the shadow stack.  Otherwise it is a stack switch and
we look for a restore token to restore the target shadow stack.

	* sysdeps/i386/nptl/tcb-offsets.sym (SSP_BASE_OFFSET): New.
	* sysdeps/i386/nptl/tls.h (tcbhead_t): Replace __glibc_reserved2
	with ssp_base.
	* sysdeps/x86_64/nptl/tcb-offsets.sym (SSP_BASE_OFFSET): New.
	* sysdeps/x86_64/nptl/tls.h (tcbhead_t): Replace __glibc_reserved2
	with ssp_base.

ChangeLog

@@ -1,3 +1,12 @@
+2018-07-25  H.J. Lu  <[email protected]>
+
+	* sysdeps/i386/nptl/tcb-offsets.sym (SSP_BASE_OFFSET): New.
+	* sysdeps/i386/nptl/tls.h (tcbhead_t): Replace __glibc_reserved2
+	with ssp_base.
+	* sysdeps/x86_64/nptl/tcb-offsets.sym (SSP_BASE_OFFSET): New.
+	* sysdeps/x86_64/nptl/tls.h (tcbhead_t): Replace __glibc_reserved2
+	with ssp_base.
+
 2018-07-25  Andreas Schwab  <[email protected]>
 
 	[BZ #23442]

sysdeps/i386/nptl/tcb-offsets.sym

@@ -13,3 +13,4 @@ CLEANUP_PREV		offsetof (struct _pthread_cleanup_buffer, __prev)
 MUTEX_FUTEX		offsetof (pthread_mutex_t, __data.__lock)
 POINTER_GUARD		offsetof (tcbhead_t, pointer_guard)
 FEATURE_1_OFFSET	offsetof (tcbhead_t, feature_1)
+SSP_BASE_OFFSET		offsetof (tcbhead_t, ssp_base)

sysdeps/i386/nptl/tls.h

@@ -49,7 +49,8 @@ typedef struct
   void *__private_tm[3];
   /* GCC split stack support.  */
   void *__private_ss;
-  void *__glibc_reserved2;
+  /* The lowest address of shadow stack,  */
+  unsigned long ssp_base;
 } tcbhead_t;
 
 /* morestack.S in libgcc uses offset 0x30 to access __private_ss,   */

sysdeps/x86_64/nptl/tcb-offsets.sym

@@ -13,6 +13,7 @@ MULTIPLE_THREADS_OFFSET	offsetof (tcbhead_t, multiple_threads)
 POINTER_GUARD		offsetof (tcbhead_t, pointer_guard)
 VGETCPU_CACHE_OFFSET	offsetof (tcbhead_t, vgetcpu_cache)
 FEATURE_1_OFFSET	offsetof (tcbhead_t, feature_1)
+SSP_BASE_OFFSET		offsetof (tcbhead_t, ssp_base)
 
 -- Not strictly offsets, but these values are also used in the TCB.
 TCB_CANCELSTATE_BITMASK	 CANCELSTATE_BITMASK

sysdeps/x86_64/nptl/tls.h

@@ -60,7 +60,8 @@ typedef struct
   void *__private_tm[4];
   /* GCC split stack support.  */
   void *__private_ss;
-  long int __glibc_reserved2;
+  /* The lowest address of shadow stack,  */
+  unsigned long long int ssp_base;
   /* Must be kept even if it is no longer used by glibc since programs,
      like AddressSanitizer, depend on the size of tcbhead_t.  */
   __128bits __glibc_unused2[8][4] __attribute__ ((aligned (32)));
@@ -72,10 +73,17 @@ typedef struct
 /* morestack.S in libgcc uses offset 0x40 to access __private_ss,   */
 _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x40,
 		"offset of __private_ss != 0x40");
+/* NB: ssp_base used to be "long int __glibc_reserved2", which was
+   changed from 32 bits to 64 bits.  Make sure that the offset of the
+   next field, __glibc_unused2, is unchanged.  */
+_Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x60,
+		"offset of __glibc_unused2 != 0x60");
 # else
 /* morestack.S in libgcc uses offset 0x70 to access __private_ss,   */
 _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x70,
 		"offset of __private_ss != 0x70");
+_Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
+		"offset of __glibc_unused2 != 0x80");
 # endif
 
 #else /* __ASSEMBLER__ */