Re: [PATCH 1/2] arm64: jump_label: mark arguments as const to satisfy asm constraints

[Ard Biesheuvel]

On Thu, 6 Oct 2022 at 10:55, Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
>
> On Thu, Oct 06, 2022 at 10:17:44AM +0200, Ard Biesheuvel wrote:
> > On Thu, 6 Oct 2022 at 10:05, Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
> > >
> > > Inspired by x86 commit 864b435514b2("x86/jump_label: Mark arguments as
> > > const to satisfy asm constraints"), mark arch_static_branch()'s and
> > > arch_static_branch_jump()'s arguments as const to satisfy asm
> > > constraints. And Steven in [1] also pointed out that "The "i"
> > > constraint needs to be a constant."
> > >
> > > Tested with building a simple external kernel module with "O0".
> > >
> > > [1]https://lore.kernel.org/all/20210212094059.5f8d05e8@xxxxxxxxxxxxxxxxxx/
> > >
> > > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/include/asm/jump_label.h | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> > > index cea441b6aa5d..48ddc0f45d22 100644
> > > --- a/arch/arm64/include/asm/jump_label.h
> > > +++ b/arch/arm64/include/asm/jump_label.h
> > > @@ -15,8 +15,8 @@
> > >
> > >  #define JUMP_LABEL_NOP_SIZE            AARCH64_INSN_SIZE
> > >
> > > -static __always_inline bool arch_static_branch(struct static_key *key,
> > > -                                              bool branch)
> > > +static __always_inline bool arch_static_branch(struct static_key * const key,
> > > +                                              const bool branch)
> > >  {
> > >         asm_volatile_goto(
> > >                 "1:     nop                                     \n\t"
> >
> > Is this still necessary if we specify the constraints in a more
> > reasonable manner:
> >
> >  "      .quad           %c0 - . + %1            \n\t"
> >  :  :  "i"(key), "i"(branch) :  : l_yes);
>
> Just tried this locally with the simple external kernel module, the
> asm operand 0 probably does not match constraints can still be
> reproduced with "O0".
>

OK so the key pointer needs to be pointer-to-const, which makes sense.
How about when using "Si" as the constraint for operand 0?


>
> >
> > instead of the horrid hack with the char* cast and using a bool as an
> > array index?
> >
> >
> >
> >
> > > @@ -32,8 +32,8 @@ static __always_inline bool arch_static_branch(struct static_key *key,
> > >         return true;
> > >  }
> > >
> > > -static __always_inline bool arch_static_branch_jump(struct static_key *key,
> > > -                                                   bool branch)
> > > +static __always_inline bool arch_static_branch_jump(struct static_key * const key,
> > > +                                                   const bool branch)
> > >  {
> > >         asm_volatile_goto(
> > >                 "1:     b               %l[l_yes]               \n\t"
> > > --
> > > 2.37.2
> > >