Patch "powerpc/64e: Fix hang when debugging programs with relocated kernel" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-64e-fix-hang-when-debugging-programs-with-relocated-kernel.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: LiuHailong <[email protected]>
Date: Tue, 7 Feb 2017 10:35:52 +0800
Subject: powerpc/64e: Fix hang when debugging programs with relocated kernel

From: LiuHailong <[email protected]>

commit fd615f69a18a9d4aa5ef02a1dc83f319f75da8e7 upstream.

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

[scottwood: cleaned up commit message, and specified bad behavior
as a hang rather than an oops to correspond to mainline kernel behavior]

---
 arch/powerpc/kernel/exceptions-64e.S |   12 ++++++++++++
 1 file changed, 12 insertions(+)

@@ -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

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/powerpc-64e-fix-hang-when-debugging-programs-with-relocated-kernel.patch