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

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Josh Poimboeuf wrote:
> 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?

That looks good to me (with a very minor nit below).
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

> 
> (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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