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

[Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>]

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
>>> 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@redhat.com>
> 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@redhat.com>

Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>

> ---
>  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;
>