Josh Poimboeuf wrote:
On Tue, Nov 14, 2017 at 03:59:21PM +0530, Naveen N. Rao wrote:
Kamalesh Babulal wrote:
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
> 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>
> Signed-off-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>
> ---
> 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?
That looks good to me (with a very minor nit below).
Acked-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
(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);
+}
+
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 :)
- Naveen
static unsigned long branch_iform_target(const unsigned int *instr)
{
signed long imm;
--
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