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Date:   Fri, 10 Sep 2021 12:05:03 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ardb@...nel.org>, llvm@...ts.linux.dev,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Compiler Attributes: Check GCC version for __alloc_size attribute

On Fri, Sep 10, 2021 at 9:58 AM Kees Cook <keescook@...omium.org> wrote:
>
> Unfortunately, just version checking the use of -Wno-alloc-size-larger-than
> is not sufficient to make the __alloc_size attribute behave correctly
> under older GCC versions. The attribute itself must be disabled in those
> situations too, as there appears to be no way to reliably silence the
> SIZE_MAX constant expression cases for GCC versions less than 9.1:
>
> In file included from ./include/linux/resource_ext.h:11,
>                  from ./include/linux/pci.h:40,
>                  from drivers/net/ethernet/intel/ixgbe/ixgbe.h:9,
>                  from drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c:4:
> In function 'kmalloc_node',
>     inlined from 'ixgbe_alloc_q_vector' at ./include/linux/slab.h:743:9:
> ./include/linux/slab.h:618:9: error: argument 1 value '18446744073709551615' exceeds maximum object size 9223372036854775807 [-Werror=alloc-size-larger-than=]
>   return __kmalloc_node(size, flags, node);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/slab.h: In function 'ixgbe_alloc_q_vector':
> ./include/linux/slab.h:455:7: note: in a call to allocation function '__kmalloc_node' declared here
>  void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_slab_alignment __malloc;
>        ^~~~~~~~~~~~~~
>
> Specifically:
> -Wno-alloc-size-larger-than is not correctly handled by GCC < 9.1
>   https://godbolt.org/z/hqsfG7q84 (doesn't disable)

(heh, clang has had similar bugs with command line flags with `=` seperators)

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Though some of the below examples don't make sense to me (the one
above is fine).

>   https://godbolt.org/z/P9jdrPTYh (doesn't admit to not knowing about option)

^ technically your first link demonstrates that. This link doesn't add
anything new and makes it look like there are more issues that there
are.

>   https://godbolt.org/z/465TPMWKb (only warns when other warnings appear)

^ this example doesn't make sense to me.  I couldn't reproduce what
you're describing.

>
> -Walloc-size-larger-than=18446744073709551615 is not handled by GCC < 8
>   https://godbolt.org/z/73hh1EPxz (ignores numeric value)

Should this be GCC < 8.2?


Some other feedback on the general use of godbolt.  Under Output,
please always disable Intel Asm Syntax; it causes me physical pain.
Also under output, usually disabling Execute the Output is what you
want, too.

>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Miguel Ojeda <ojeda@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Arvind Sankar <nivedita@...m.mit.edu>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Sami Tolvanen <samitolvanen@...gle.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: llvm@...ts.linux.dev
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> I've been digging through older GCC build logs and found this needed to
> be fixed even harder than prior attempts from what Randy had been seeing.
> ---
>  include/linux/compiler-gcc.h        | 9 +++++++++
>  include/linux/compiler_attributes.h | 6 ------
>  include/linux/compiler_types.h      | 5 +++++
>  3 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index 01985821944b..ffe0a7570d66 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -151,3 +151,12 @@
>  #else
>  #define __diag_GCC_8(s)
>  #endif
> +
> +/*
> + * https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
> + * However, prior to 9.1, -Wno-alloc-size-larger-than does not work,
> + * making this attribute unusable.
> + */
> +#if GCC_VERSION < 90100
> +#define __alloc_size           /**/
> +#endif
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 19f178e20e61..2487be0e7199 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -53,12 +53,6 @@
>  #define __aligned(x)                    __attribute__((__aligned__(x)))
>  #define __aligned_largest               __attribute__((__aligned__))
>
> -/*
> - *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
> - * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
> - */
> -#define __alloc_size(x, ...)           __attribute__((__alloc_size__(x, ## __VA_ARGS__)))
> -
>  /*
>   * Note: users of __always_inline currently do not write "inline" themselves,
>   * which seems to be required by gcc to apply the attribute according
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index c43308b0a9a9..91de9bead40d 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -254,6 +254,11 @@ struct ftrace_likely_data {
>  #define asm_volatile_goto(x...) asm goto(x)
>  #endif
>
> +/* If not specifically disabled, allow the use of __alloc_size attribute. */
> +#ifndef __alloc_size
> +# define __alloc_size(x, ...)          __attribute__((__alloc_size__(x, ## __VA_ARGS__)))
> +#endif
> +
>  #ifdef CONFIG_CC_HAS_ASM_INLINE
>  #define asm_inline asm __inline
>  #else
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers

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