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
> > something?
>
> 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?
----8<----
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
unregister_netdevice_queue():
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>
---
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;
--
2.13.6
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