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From: David Laight <David.Laight@ACULAB.COM>
To: 'Kees Cook' <keescook@chromium.org>, Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"Gustavo A . R . Silva" <gustavoars@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hardening@vger.kernel.org"
	<linux-hardening@vger.kernel.org>
Subject: RE: [PATCH bpf-next v2 2/2] bpf: Replace callers of BPF_CAST_CALL with proper function typedef
Date: Sat, 2 Oct 2021 16:09:17 +0000	[thread overview]
Message-ID: <29ca9f764f07426093b570515dc8e025@AcuMS.aculab.com> (raw)
In-Reply-To: <20210928230946.4062144-3-keescook@chromium.org>

From: Kees Cook
> Sent: 29 September 2021 00:10
...
> In order to keep ahead of cases in the kernel where Control Flow
> Integrity (CFI) may trip over function call casts, enabling
> -Wcast-function-type is helpful. To that end, BPF_CAST_CALL causes
> various warnings and is one of the last places in the kernel
> triggering this warning.
...
> -static int bpf_for_each_array_elem(struct bpf_map *map, void *callback_fn,
> +static int bpf_for_each_array_elem(struct bpf_map *map, bpf_callback_t callback_fn,
>  				   void *callback_ctx, u64 flags)
>  {
>  	u32 i, key, num_elems = 0;
> @@ -668,9 +668,8 @@ static int bpf_for_each_array_elem(struct bpf_map *map, void *callback_fn,
>  			val = array->value + array->elem_size * i;
>  		num_elems++;
>  		key = i;
> -		ret = BPF_CAST_CALL(callback_fn)((u64)(long)map,
> -					(u64)(long)&key, (u64)(long)val,
> -					(u64)(long)callback_ctx, 0);
> +		ret = callback_fn((u64)(long)map, (u64)(long)&key,
> +				  (u64)(long)val, (u64)(long)callback_ctx, 0);
>  		/* return value: 0 - continue, 1 - stop and return */
>  		if (ret)
>  			break;

This is still entirely horrid and potentially error prone.
While a callback function seems a nice idea the code is
almost always better and much easier to read if some
kind of iterator function is used so that the calling
code is just a simple loop.
This is true even if you need a #define for the loop end.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  parent reply	other threads:[~2021-10-02 16:09 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-28 23:09 [PATCH bpf-next v2 0/2] bpf: Build with -Wcast-function-type Kees Cook
2021-09-28 23:09 ` [PATCH bpf-next v2 1/2] bpf: Replace "want address" users of BPF_CAST_CALL with BPF_CALL_IMM Kees Cook
2021-09-28 23:25   ` Gustavo A. R. Silva
2021-09-28 23:09 ` [PATCH bpf-next v2 2/2] bpf: Replace callers of BPF_CAST_CALL with proper function typedef Kees Cook
2021-09-28 23:26   ` Gustavo A. R. Silva
2021-10-02 16:09   ` David Laight [this message]
2021-09-28 23:33 ` [PATCH bpf-next v2 0/2] bpf: Build with -Wcast-function-type Alexei Starovoitov

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