Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Tue, Nov 14, 2017 at 03:59:21PM +0530, Naveen N. Rao wrote:
> Kamalesh Babulal wrote:
> > From: Josh Poimboeuf <jpoimboe@redhat.com>
> > 
> > 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@redhat.com>
> > Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kernel/module_64.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> > index 39b01fd..9e5391f 100644
> > --- a/arch/powerpc/kernel/module_64.c
> > +++ b/arch/powerpc/kernel/module_64.c
> > @@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct module *me)
> >  	if (is_early_mcount_callsite(instruction - 1))
> >  		return 1;
> > 
> > +	/* Sibling calls don't return, so they don't need to restore r2 */
> > +	if (instruction[-1] == PPC_INST_BRANCH)
> > +		return 1;
> > +
> 
> This looks quite fragile, unless we know for sure that gcc will _always_
> emit this instruction form for sibling calls with relocations.
> 
> As an alternative, does it make sense to do the following check instead?
> 	if ((instr_is_branch_iform(insn) || instr_is_branch_bform(insn))
> 		&& !(insn & 0x1))

Yes, good point.  How about something like this?

(completely untested because I don't have access to a box at the moment)


diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index abef812de7f8..302e4368debc 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_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 9cb007bc7075..b5148a206b4d 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -487,11 +487,13 @@ 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;
 
 	/* Sibling calls don't return, so they don't need to restore r2 */
-	if (instruction[-1] == PPC_INST_BRANCH)
+	if (!instr_is_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..4727fafd37e4 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -304,6 +304,12 @@ int instr_is_relative_branch(unsigned int instr)
 	return instr_is_branch_iform(instr) || instr_is_branch_bform(instr);
 }
 
+int instr_is_link_branch(unsigned int instr)
+{
+	return (instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) &&
+	       (instr & BRANCH_SET_LINK);
+}
+
 static unsigned long branch_iform_target(const unsigned int *instr)
 {
 	signed long imm;