Re: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a sibling call

From: Kamalesh Babulal
Date: Fri Nov 17 2017 - 03:17:17 EST

On Thursday 16 November 2017 11:15 PM, Josh Poimboeuf wrote:
On Thu, Nov 16, 2017 at 06:39:03PM +0530, Naveen N. Rao wrote:
Josh Poimboeuf wrote:
On Wed, Nov 15, 2017 at 02:58:33PM +0530, Naveen N. Rao wrote:
+int instr_is_link_branch(unsigned int instr)
+ return (instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) &&
+ (instr & BRANCH_SET_LINK);

Nitpicking here, but since we're not considering the other branch forms,
perhaps this can be renamed to instr_is_link_relative_branch() (or maybe
instr_is_relative_branch_link()), just so we're clear :)

My understanding is that the absolute/relative bit isn't a "form", but
rather a bit that can be set for either the b-form (conditional) or the
i-form (unconditional). And the above function isn't checking the
absolute bit, so it isn't necessarily a relative branch. Or did I miss

Ah, good point. I was coming from the fact that we are only considering the
i-form and b-form branches and not the lr/ctr/tar based branches, which are
always absolute branches, but can also set the link register.

Hm, RISC is more complicated than I realized ;-)

Thinking about this more, aren't we only interested in relative branches
here (for relocations), so can we actually filter out the absolute branches?
Something like this?

int instr_is_relative_branch_link(unsigned int instr)
return ((instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) &&
!(instr & BRANCH_ABSOLUTE) && (instr & BRANCH_SET_LINK));

Yeah, makes sense to me. Here's another try (also untested). If this
looks ok, Kamalesh would you mind testing again?


From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Subject: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a sibling call

When attempting to load a livepatch module, I got the following error:

module_64: patch_module: Expect noop after relocate, got 3c820000

The error was triggered by the following code in

14c: 00 00 00 48 b 14c <unregister_netdevice_queue+0x14c>
14c: R_PPC64_REL24 net_set_todo
150: 00 00 82 3c addis r4,r2,0

GCC didn't insert a nop after the branch to net_set_todo() because it's
a sibling call, so it never returns. The nop isn't needed after the
branch in that case.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>

arch/powerpc/include/asm/code-patching.h | 1 +
arch/powerpc/kernel/module_64.c | 12 +++++++++++-
arch/powerpc/lib/code-patching.c | 5 +++++
3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index abef812de7f8..2c895e8d07f7 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -33,6 +33,7 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags);
int patch_instruction(unsigned int *addr, unsigned int instr);

int instr_is_relative_branch(unsigned int instr);
+int instr_is_relative_link_branch(unsigned int instr);
int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr);
unsigned long branch_target(const unsigned int *instr);
unsigned int translate_branch(const unsigned int *dest,
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 759104b99f9f..180c16f04063 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -487,7 +487,17 @@ static bool is_early_mcount_callsite(u32 *instruction)
restore r2. */
static int restore_r2(u32 *instruction, struct module *me)
- if (is_early_mcount_callsite(instruction - 1))
+ u32 *prev_insn = instruction - 1;
+ if (is_early_mcount_callsite(prev_insn))
+ return 1;
+ /*
+ * Make sure the branch isn't a sibling call. Sibling calls aren't
+ * "link" branches and they don't return, so they don't need the r2
+ * restore afterwards.
+ */
+ if (!instr_is_relative_link_branch(*prev_insn))
return 1;

if (*instruction != PPC_INST_NOP) {
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index c9de03e0c1f1..d81aab7441f7 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -304,6 +304,11 @@ int instr_is_relative_branch(unsigned int instr)
return instr_is_branch_iform(instr) || instr_is_branch_bform(instr);

+int instr_is_relative_link_branch(unsigned int instr)
+ return instr_is_relative_branch(instr) && (instr & BRANCH_SET_LINK);
static unsigned long branch_iform_target(const unsigned int *instr)
signed long imm;