powerpc/64e: Fix hang when debugging programs with relocated kernel

Debug interrupts can be taken during interrupt entry, since interrupt
entry does not automatically turn them off.  The kernel will check
whether the faulting instruction is between [interrupt_base_book3e,
__end_interrupts], and if so clear MSR[DE] and return.

However, when the kernel is built with CONFIG_RELOCATABLE, it can't use
LOAD_REG_IMMEDIATE(r14,interrupt_base_book3e) and
LOAD_REG_IMMEDIATE(r15,__end_interrupts), as they ignore relocation.
Thus, if the kernel is actually running at a different address than it
was built at, the address comparison will fail, and the exception entry
code will hang at kernel_dbg_exc.

r2(toc) is also not usable here, as r2 still holds data from the
interrupted context, so LOAD_REG_ADDR() doesn't work either.  So we use
the *name@got* to get the EV of two labels directly.

Test programs test.c shows as follows:
int main(int argc, char *argv[])
{
	if (access("/proc/sys/kernel/perf_event_paranoid", F_OK) == -1)
		printf("Kernel doesn't have perf_event support\n");
}

Steps to reproduce the bug, for example:
 1) ./gdb ./test
 2) (gdb) b access
 3) (gdb) r
 4) (gdb) s

Signed-off-by: Liu Hailong <liu.hailong6@zte.com.cn>
Signed-off-by: Jiang Xuexin <jiang.xuexin@zte.com.cn>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>
Reviewed-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Huang Jian <huang.jian@zte.com.cn>
[scottwood: cleaned up commit message, and specified bad behavior
 as a hang rather than an oops to correspond to mainline kernel behavior]
Fixes: 1cb6e0649248 ("powerpc/book3e: support CONFIG_RELOCATABLE")
Cc: <stable@vger.kernel.org> # 4.4.x-
Signed-off-by: Scott Wood <oss@buserror.net>

arch/powerpc/kernel/exceptions-64e.S

@@ -735,8 +735,14 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
 	andis.	r15,r14,(DBSR_IC|DBSR_BT)@h
 	beq+	1f
 
+#ifdef CONFIG_RELOCATABLE
+	ld	r15,PACATOC(r13)
+	ld	r14,interrupt_base_book3e@got(r15)
+	ld	r15,__end_interrupts@got(r15)
+#else
 	LOAD_REG_IMMEDIATE(r14,interrupt_base_book3e)
 	LOAD_REG_IMMEDIATE(r15,__end_interrupts)
+#endif
 	cmpld	cr0,r10,r14
 	cmpld	cr1,r10,r15
 	blt+	cr0,1f
@@ -799,8 +805,14 @@ kernel_dbg_exc:
 	andis.	r15,r14,(DBSR_IC|DBSR_BT)@h
 	beq+	1f
 
+#ifdef CONFIG_RELOCATABLE
+	ld	r15,PACATOC(r13)
+	ld	r14,interrupt_base_book3e@got(r15)
+	ld	r15,__end_interrupts@got(r15)
+#else
 	LOAD_REG_IMMEDIATE(r14,interrupt_base_book3e)
 	LOAD_REG_IMMEDIATE(r15,__end_interrupts)
+#endif
 	cmpld	cr0,r10,r14
 	cmpld	cr1,r10,r15
 	blt+	cr0,1f