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

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Date: Mon,  8 Jan 2024 13:04:58 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Leonardo Bras <leobras@...hat.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	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 12:07:48PM -0300, Leonardo Bras wrote:
> On Fri, Jan 05, 2024 at 03:38:05PM +0100, Arnd Bergmann wrote:
> > On Fri, Jan 5, 2024, at 14:14, Mark Rutland wrote:
> > > On Fri, Jan 05, 2024 at 01:15:00AM -0300, Leonardo Bras wrote:
> > > arch/arm64/kernel/ptrace.c:2121:25: note: each undeclared identifier is 
> > > reported only once for each function it appears in
> > > arch/arm64/kernel/ptrace.c:2123:25: error: 'user_aarch32_ptrace_view' 
> > > undeclared (first use in this function)
> > >  2123 |                 return &user_aarch32_ptrace_view;
> > >       |                         ^~~~~~~~~~~~~~~~~~~~~~~~
> > > make[4]: *** [scripts/Makefile.build:243: arch/arm64/kernel/ptrace.o] 
> > > Error 1
> > > make[3]: *** [scripts/Makefile.build:480: arch/arm64/kernel] Error 2
> > > make[2]: *** [scripts/Makefile.build:480: arch/arm64] Error 2
> > > make[1]: *** [/home/mark/src/linux/Makefile:1911: .] Error 2
> > > make: *** [Makefile:234: __sub-make] Error 2
> > >
> > > ... and looking at the code, user_aarch32_view and user_aarch32_ptrace_view are
> > > both defined under ifdeffery for CONFIG_COMPAT, so that's obviously not going
> > > to work...
> 
> Thanks for noticing, Mark!
> 
> > 
> > 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.

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?

Thanks!
Leo



> > 
> >     Arnd
> 
> Sure, I sill send a v2 soon.
> 
> Thanks!
> Leo
> 
> >