linux-kernel - Re: [PATCH v1 1/1] arm64: remove unnecessary ifdefs around is_compat

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Message-ID: <ZZwsPsBZad-VqZ-U@LeoBras>
Date: Mon,  8 Jan 2024 14:09:18 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Leonardo Bras <leobras@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Oleg Nesterov <oleg@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Guo Hui <guohui@...ontech.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] arm64: remove unnecessary ifdefs around is_compat_task()

On Mon, Jan 08, 2024 at 05:26:26PM +0100, Arnd Bergmann wrote:
> On Mon, Jan 8, 2024, at 17:04, Leonardo Bras wrote:
> > On Mon, Jan 08, 2024 at 12:07:48PM -0300, Leonardo Bras wrote:
> >> On Fri, Jan 05, 2024 at 03:38:05PM +0100, Arnd Bergmann wrote:
> >> > 
> >> > I suspect it's enough to remove all of the other
> >> > "#ifdef CONFIG_COMPAT" checks in this file and rely on
> >> > dead code elimination to remove the rest, but there might
> >> > be additional problems if some extern declarations are
> >> > hidden in an #ifdef as well.
> >
> > I could remove all CONFIG_COMPAT ifdefs from this file, and for compiling 
> > it required a few extra defines (in other files) to be moved outside of
> > their #ifdef CONFIG_COMPAT. Those being:
> >
> >  #define VFP_STATE_SIZE         ((32 * 8) + 4)
> >  #define VFP_FPSCR_STAT_MASK    0xf800009f
> >  #define VFP_FPSCR_CTRL_MASK    0x07f79f00
> >
> >  #define COMPAT_ELF_NGREG               18
> >  typedef unsigned int                   compat_elf_greg_t;
> >  typedef compat_elf_greg_t              compat_elf_gregset_t[COMPAT_ELF_NGREG];
> > 
> >
> > OTOH, the size of the final arch/arm64/kernel/ptrace.o went from 44768 to 
> > 56328 bytes, which I understand to be undesired.
> 
> Right, unfortunately it seems that compat_arch_ptrace() is
> globally visible and consequently not dropped by the compiler
> in dead code elimination.
> 
> > A different (and simpler) solution is to have an empty struct in case of 
> > !CONFIG_COMPAT, that will be optimized out in compile-time:
> >
> > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> > index 9f8781f1fdfda..d2f275d8a3e6e 100644
> > --- a/arch/arm64/kernel/ptrace.c
> > +++ b/arch/arm64/kernel/ptrace.c
> > @@ -2107,6 +2107,9 @@ long compat_arch_ptrace(struct task_struct 
> > *child, compat_long_t request,
> > 
> >         return ret;
> >  }
> > +#else
> > +static const struct user_regset_view user_aarch32_view = {};
> > +static const struct user_regset_view user_aarch32_ptrace_view = {};
> >  #endif /* CONFIG_COMPAT */
> > 
> >  const struct user_regset_view *task_user_regset_view(struct task_struct *task)
> >
> > With this the patch will build successfully and arch/arm64/kernel/ptrace.o 
> > will be able to keep it's original size.
> >
> > Arnd, is that ok?
> 
> I don't see it being worth it if you add extra unused lines
> in order to remove one more #ifdef. I would either leave the
> task_user_regset_view() function unchanged here, or (if this
> works) move the #ifdef down a few lines so the existing
> user_regset_view structures can be shared:
> 
> @@ -1595,7 +1595,6 @@ static const struct user_regset_view user_aarch64_view = {
>         .regsets = aarch64_regsets, .n = ARRAY_SIZE(aarch64_regsets)
>  };
>  
> -#ifdef CONFIG_COMPAT
>  enum compat_regset {
>         REGSET_COMPAT_GPR,
>         REGSET_COMPAT_VFP,
> @@ -1852,6 +1851,7 @@ static const struct user_regset_view user_aarch32_ptrace_view = {
>         .regsets = aarch32_ptrace_regsets, .n = ARRAY_SIZE(aarch32_ptrace_regsets)
>  };
>  
> +#ifdef CONFIG_COMPAT
>  static int compat_ptrace_read_user(struct task_struct *tsk, compat_ulong_t off,
>                                    compat_ulong_t __user *ret)
>  {
> 

Ok, with the previous defines moved outside !CONFIG_COMPAT, this works as a 
charm and keeps arch/arm64/kernel/ptrace.o with the same size. 

I will send a v2 soon.

Thanks!
Leo

> 
>      Arnd
>