[PATCH] powerpc: Fix HMIs on big-endian with CONFIG_RELOCATABLE=y

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

This hack fixes it for me, but it's not great. Nick, any better idea ?

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index ea04dfb..752709cc8 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S

@@ -1119,7 +1119,11 @@ TRAMP_REAL_BEGIN(hmi_exception_early)
 	EXCEPTION_PROLOG_COMMON_2(PACA_EXGEN)
 	EXCEPTION_PROLOG_COMMON_3(0xe60)
 	addi	r3,r1,STACK_FRAME_OVERHEAD
+#ifdef PPC64_ELF_ABI_v1
+	BRANCH_LINK_TO_FAR(.hmi_exception_realmode) /* Function call ABI */
+#else
 	BRANCH_LINK_TO_FAR(hmi_exception_realmode) /* Function call ABI */
+#endif
 	cmpdi	cr0,r3,0
 
 	/* Windup the stack. */

On Mon, 08 Oct 2018 15:08:31 +1100
Benjamin Herrenschmidt [off-list ref] wrote:

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

This hack fixes it for me, but it's not great. Nick, any better idea ?

Is it a hack because the ifdef gunk, or because there's something
deeper wrong with using the .sym?

I guess all those handlers that load label address by hand could have
the bug silently creep in. Can we have them use the DOTSYM() macro?

Thanks,
Nick

On Mon, 2018-10-08 at 17:04 +1000, Nicholas Piggin wrote:

On Mon, 08 Oct 2018 15:08:31 +1100
Benjamin Herrenschmidt [off-list ref] wrote:

quoted

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

This hack fixes it for me, but it's not great. Nick, any better idea ?

Is it a hack because the ifdef gunk, or because there's something
deeper wrong with using the .sym?

I'd say ifdef gunk, also the KVM use doesn't need it bcs the kvm entry
isn't an OPD.

I guess all those handlers that load label address by hand could have
the bug silently creep in. Can we have them use the DOTSYM() macro?

The KVM one doesnt have a dotsym does it ?

Also should we load the TOC from the OPD ?

Thanks,
Nick

quoted

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index ea04dfb..752709cc8 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S

@@ -1119,7 +1119,11 @@ TRAMP_REAL_BEGIN(hmi_exception_early)
 	EXCEPTION_PROLOG_COMMON_2(PACA_EXGEN)
 	EXCEPTION_PROLOG_COMMON_3(0xe60)
 	addi	r3,r1,STACK_FRAME_OVERHEAD
+#ifdef PPC64_ELF_ABI_v1
+	BRANCH_LINK_TO_FAR(.hmi_exception_realmode) /* Function call ABI */
+#else
 	BRANCH_LINK_TO_FAR(hmi_exception_realmode) /* Function call ABI */
+#endif
 	cmpdi	cr0,r3,0
 
 	/* Windup the stack. */

Benjamin Herrenschmidt [off-list ref] writes:

On Mon, 2018-10-08 at 17:04 +1000, Nicholas Piggin wrote:

quoted

On Mon, 08 Oct 2018 15:08:31 +1100
Benjamin Herrenschmidt [off-list ref] wrote:

quoted

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

This hack fixes it for me, but it's not great. Nick, any better idea ?

Is it a hack because the ifdef gunk, or because there's something
deeper wrong with using the .sym?

I'd say ifdef gunk, also the KVM use doesn't need it bcs the kvm entry
isn't an OPD.

quoted

I guess all those handlers that load label address by hand could have
the bug silently creep in. Can we have them use the DOTSYM() macro?

The KVM one doesnt have a dotsym does it ?

Also should we load the TOC from the OPD ?

Technically yes. But I thought because we build with mcmodel=medium
we'll never actually get multiple TOCs in the kernel itself, so it
doesn't actually matter.

So this seems to work for now:

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 301a6a86a20f..e5566e248a02 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S

@@ -1149,7 +1149,7 @@ TRAMP_REAL_BEGIN(hmi_exception_early)
 	EXCEPTION_PROLOG_COMMON_2(PACA_EXGEN)
 	EXCEPTION_PROLOG_COMMON_3(0xe60)
 	addi	r3,r1,STACK_FRAME_OVERHEAD
-	BRANCH_LINK_TO_FAR(hmi_exception_realmode) /* Function call ABI */
+	BRANCH_LINK_TO_FAR(DOTSYM(hmi_exception_realmode)) /* Function call ABI */
 	cmpdi	cr0,r3,0
 
 	/* Windup the stack. */

cheers

On Tue, 2018-10-09 at 21:37 +1100, Michael Ellerman wrote:

Technically yes. But I thought because we build with mcmodel=medium
we'll never actually get multiple TOCs in the kernel itself, so it
doesn't actually matter.

So this seems to work for now:

Ok, fine. I forgot about DOTSYM. Will do for now.

Cheers,
Ben.

On Mon, 2018-10-08 at 04:08:31 UTC, Benjamin Herrenschmidt wrote:

HMIs will crash the kernel due to

	BRANCH_LINK_TO_FAR(hmi_exception_realmode)

Calling into the OPD instead of the actual code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/505a314fb28ce122091691c51426fa

cheers