linux-kernel - Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc

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Date:   Sun, 16 May 2021 06:09:50 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     vbabka@...e.cz, akpm@...ux-foundation.org, iamjoonsoo.kim@....com,
        rientjes@...gle.com, penberg@...nel.org, cl@...ux.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        nathan@...nel.org, naresh.kamboju@...aro.org,
        clang-built-linux@...glegroups.com, linux-next@...r.kernel.org,
        ndesaulniers@...gle.com, lkft-triage@...ts.linaro.org,
        sfr@...b.auug.org.au, arnd@...db.de, akpm@...ux-foundation.org
Subject: Re: [PATCH v3] mm, slub: change run-time assertion in
 kmalloc_index() to compile-time

Hello Vlastimil, recently kbuild-all test bot reported compile error on
clang 10.0.1, with defconfig.

Nathan Chancellor wrote:
> I think this happens because arch_prepare_optimized_kprobe() calls kzalloc()
> with a size of MAX_OPTINSN_SIZE, which is
> 
> #define MAX_OPTINSN_SIZE                                \
>       (((unsigned long)optprobe_template_end -        \
>          (unsigned long)optprobe_template_entry) +     \
>         MAX_OPTIMIZED_LENGTH + JMP32_INSN_SIZE)

> and the optprobe_template_{end,entry} are not evaluated as constants.
>
> I am not sure what the solution is. There seem to be a growing list of issues
> with LLVM 10 that were fixed in LLVM 11, which might necessitate requiring
> LLVM 11 and newer to build the kernel, given this affects a defconfig.
> Cheers,
> Nathan


I think it's because kmalloc compiles successfully when size is constant,
and kmalloc_index isn't. so I think compiler seems to be confused.

currently if size is non-constant, kmalloc calls dummy function __kmalloc,
which always returns NULL.

so what about changing kmalloc to do compile-time assertion too, and track
all callers that are calling kmalloc with non-constant argument.

How do you think? If you think it is the solution, I'll do that work.