Re: [lkp@intel.com: [norov:fns 9/23] include/asm-generic/percpu.h:21:44: warning: array subscript 4 is above array bounds of 'long unsigned int[4]']

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Yury Norov <yury.norov@gmail.com>
Date: Fri, 1 Jul 2022 19:33:24 -0700

> Hi Alexander,

Hi!

> 
> I'm working on top of your recent cons bitmap series, and again
> received a bug report from LKP regarding one of you patches. Can
> you please look at this report?
> 
> I tried reproduce it myself, but the build is failed for me:
> arch/arc/kernel/entry.S: Assembler messages:
> arch/arc/kernel/entry.S:280: Error: operand out of range (260 is not between -256 and 255)
> make[2]: *** [scripts/Makefile.build:322: arch/arc/kernel/entry-compact.o] Error 1
> make[2]: *** Waiting for unfinished jobs....
> 
> If you think it's mis-attributed, we should let LKP people know that
> something is wrong on their side.
> 
> Thanks,
> Yury
> 
> ----- Forwarded message from kernel test robot <lkp@intel.com> -----
> 
> Date: Sat, 2 Jul 2022 08:42:59 +0800
> From: kernel test robot <lkp@intel.com>
> To: Yury Norov <yury.norov@gmail.com>
> Cc: kbuild-all@lists.01.org
> Subject: [norov:fns 9/23] include/asm-generic/percpu.h:21:44: warning: array subscript 4 is above array bounds of
> 	'long unsigned int[4]'
> 
> tree:   https://github.com/norov/linux fns
> head:   1df9509fd92a58cb37e9a419235823bbb13367de
> commit: 3e7e5baaaba78075a7f3a57432609e363bf2a486 [9/23] bitmap: don't assume compiler evaluates small mem*() builtins calls
> config: arc-randconfig-r035-20220629 (https://download.01.org/0day-ci/archive/20220702/202207020812.oTFxIVue-lkp@intel.com/config)
> compiler: arc-elf-gcc (GCC) 11.3.0
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # https://github.com/norov/linux/commit/3e7e5baaaba78075a7f3a57432609e363bf2a486
>         git remote add norov https://github.com/norov/linux
>         git fetch --no-tags norov fns
>         git checkout 3e7e5baaaba78075a7f3a57432609e363bf2a486
>         # save the config file
>         mkdir build_dir && cp config build_dir/.config
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.3.0 make.cross W=1 O=build_dir ARCH=arc SHELL=/bin/bash kernel/
> 
> If you fix the issue, kindly add following tag where applicable
> Reported-by: kernel test robot <lkp@intel.com>
> 
> All warnings (new ones prefixed by >>):
> 
>    In file included from include/linux/compiler_types.h:93,
>                     from <command-line>:
>    kernel/sched/topology.c: In function 'build_sched_domains':
> >> include/asm-generic/percpu.h:21:44: warning: array subscript 4 is above array bounds of 'long unsigned int[4]' [-Warray-bounds]
>       21 | #define per_cpu_offset(x) (__per_cpu_offset[x])
>    include/linux/compiler-gcc.h:35:33: note: in definition of macro 'RELOC_HIDE'
>       35 |         (typeof(ptr)) (__ptr + (off));                                  \
>          |                                 ^~~
>    include/linux/percpu-defs.h:236:9: note: in expansion of macro 'SHIFT_PERCPU_PTR'
>      236 |         SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu)));                 \
>          |         ^~~~~~~~~~~~~~~~
>    include/linux/percpu-defs.h:236:33: note: in expansion of macro 'per_cpu_offset'
>      236 |         SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu)));                 \
>          |                                 ^~~~~~~~~~~~~~
>    kernel/sched/topology.c:969:20: note: in expansion of macro 'per_cpu_ptr'
>      969 |         sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
>          |                    ^~~~~~~~~~~
>    In file included from ./arch/arc/include/generated/asm/percpu.h:1,
>                     from include/linux/irqflags.h:17,
>                     from include/linux/rcupdate.h:26,
>                     from include/linux/rculist.h:11,
>                     from include/linux/sched/signal.h:5,
>                     from include/linux/sched/cputime.h:5,
>                     from kernel/sched/build_utility.c:13:
>    include/asm-generic/percpu.h:19:22: note: while referencing '__per_cpu_offset'
>       19 | extern unsigned long __per_cpu_offset[NR_CPUS];
>          |                      ^~~~~~~~~~~~~~~~

Ah, I've seen that one and tried to reproduce it hardly, but didn't
manage to do that. Then I took a look at the code mentioned here,
but again couldn't find any code pieces which would go past the
array (with CPU ID higher than 3 on a 4-CPU kernel).
I've heard that `-Warray-bounds` is still somewhat noisy and
produces a fair amount of false-positives, so that it's been even
disabled for 5.19-rc1 after being enabled during the merge window.

I'm super fine with letting lkp guys know that it's an unrelated
stuff, whom should I write to?
BTW, I didn't receive any reports from the linux-next bots and
haven't found a single over the LKML, so looks like it should be
okay in general :)

> 
> 
> vim +21 include/asm-generic/percpu.h
> 
> ^1da177e4c3f415 Linus Torvalds 2005-04-16  20  
> a875a69f8b00a38 Ingo Molnar    2006-07-03 @21  #define per_cpu_offset(x) (__per_cpu_offset[x])
> acdac87202a4081 travis@sgi.com 2008-01-30  22  #endif
> acdac87202a4081 travis@sgi.com 2008-01-30  23  
> 
> :::::: The code at line 21 was first introduced by commit
> :::::: a875a69f8b00a38b4f40d9632a4fc71a159f0e0d [PATCH] lockdep: add per_cpu_offset()
> 
> :::::: TO: Ingo Molnar <mingo@elte.hu>
> :::::: CC: Linus Torvalds <torvalds@g5.osdl.org>
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://01.org/lkp
> 
> ----- End forwarded message -----

Thanks,
Olek