linux-kernel - Re: [PATCH] bpf: Avoid function casting when calculating immediate

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Date:   Mon, 17 Feb 2020 16:51:46 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Kees Cook <keescook@...omium.org>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Sami Tolvanen <samitolvanen@...gle.com>, bpf@...r.kernel.org
Subject: Re: [PATCH] bpf: Avoid function casting when calculating immediate

On 1/29/20 10:36 PM, Kees Cook wrote:
> In an effort to enable -Wcast-function-type in the top-level Makefile
> to support Control Flow Integrity builds, rework the BPF instruction
> immediate calculation macros to avoid mismatched function pointers. Since
> these calculations are only ever between function address (these are
> not function calls, just address calculations), they can be cast to u64
> instead, where the result will be assigned to the s32 insn->imm.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>   include/linux/filter.h |  6 +++---
>   kernel/bpf/hashtab.c   |  6 +++---
>   kernel/bpf/verifier.c  | 21 +++++++--------------
>   3 files changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index f349e2c0884c..b5beee7bf2ea 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -340,8 +340,8 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
>   
>   /* Function call */
>   
> -#define BPF_CAST_CALL(x)					\
> -		((u64 (*)(u64, u64, u64, u64, u64))(x))
> +#define BPF_FUNC_IMM(FUNC)					\
> +		((u64)(FUNC) - (u64)__bpf_call_base)

Looks good to me in general. My only concern is compilation on 32bit archs: I think
the cast needs to be of '(u64)(unsigned long)' to avoid introducing new warnings a la
'cast from pointer to integer of different size'.

Thanks,
Daniel