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From: Nick Desaulniers <ndesaulniers@google.com>
To: Kees Cook <keescook@chromium.org>, George Burgess IV <gbiv@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang
Date: Thu, 3 Feb 2022 12:18:24 -0800	[thread overview]
Message-ID: <CAKwvOdmzdQ+X2BsUbuVBWzFtaE2Vst4=ihNNR=LmOdozqQ5Ukg@mail.gmail.com> (raw)
In-Reply-To: <20220203173307.1033257-2-keescook@chromium.org>

On Thu, Feb 3, 2022 at 9:33 AM Kees Cook <keescook@chromium.org> wrote:
>
> In order to gain greater visibility to type information when using
> __builtin_object_size(), Clang has a function attribute "pass_object_size"
> that will make size information available for marked arguments in
> a function by way of implicit additional function arguments that are
> then wired up the __builtin_object_size().
>
> This is needed to implement FORTIFY_SOURCE in Clang, as a workaround
> to Clang's __builtin_object_size() having limited visibility[1] into types
> across function calls (even inlines).
>
> Since any usage must also be const, include it in the macro.

I really don't like hiding the qualifier in the argument-attribute
macro like that.  I'd rather we be more explicit about changing the
function interfaces in include/linux/fortify-string.h.

For instance, in patch 4/4, let's take a look at this hunk:

-__FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
...
+__FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
+char *strncpy(char * POS p, const char *q, __kernel_size_t size)

manually expanded, this has changed the qualifiers on the type of the
first parameter from `char *` to `char * const`.

I think it might be helpful to quote the docs
(https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size)

>> Additionally, any parameter that pass_object_size is applied to must be marked const at its function’s definition.

One thing that's concerning to me is though:

>> It is an error to take the address of a function with pass_object_size on any of its parameters.

Surely the kernel has indirect calls to some of these functions
somewhere? Is that just an issue for C++ name-mangling perhaps?

>
> This attribute has an additional benefit that it can be used even on
> non-inline functions to gain argument size information.
>
> [1] https://github.com/llvm/llvm-project/issues/53516
>
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: llvm@lists.linux.dev
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/compiler_attributes.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 37e260020221..4ce370094e3a 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -263,6 +263,20 @@
>   */
>  #define __packed                        __attribute__((__packed__))
>
> +/*
> + * Note: the "type" argument should match any __builtin_object_size(p, type) usage.
> + *
> + * Optional: not supported by gcc.
> + * Optional: not supported by icc.
> + *
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
> + */
> +#if __has_attribute(__pass_object_size__)
> +# define __pass_object_size(type)      const __attribute__((__pass_object_size__(type)))
> +#else
> +# define __pass_object_size(type)
> +#endif
> +
>  /*
>   *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
>   */
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers

  reply	other threads:[~2022-02-03 20:18 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-03 17:33 [PATCH v6 0/4] fortify: Add Clang support Kees Cook
2022-02-03 17:33 ` [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang Kees Cook
2022-02-03 20:18   ` Nick Desaulniers [this message]
2022-02-03 20:58     ` Kees Cook
2022-02-03 22:01       ` Nick Desaulniers
2022-02-04  0:29         ` Kees Cook
2022-02-03 17:33 ` [PATCH v6 2/4] Compiler Attributes: Add __overloadable " Kees Cook
2022-02-03 20:26   ` Nick Desaulniers
2022-02-03 21:04     ` Kees Cook
2022-02-03 22:11       ` Nick Desaulniers
2022-02-04  0:26         ` Kees Cook
2022-02-04  0:58           ` Nick Desaulniers
2022-02-04  1:07       ` Miguel Ojeda
2022-02-03 17:33 ` [PATCH v6 3/4] Compiler Attributes: Add __diagnose_as " Kees Cook
2022-02-03 20:28   ` Nick Desaulniers
2022-02-03 17:33 ` [PATCH v6 4/4] fortify: Add Clang support Kees Cook
2022-02-03 20:37   ` Nick Desaulniers
2022-02-03 21:26     ` Kees Cook
2022-02-03 17:47 ` [PATCH v6 0/4] " Miguel Ojeda
2022-02-03 19:57   ` Kees Cook
2022-02-03 21:12     ` Miguel Ojeda

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