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Message-ID: <202002171024.184D90B@keescook>
Date: Mon, 17 Feb 2020 10:25:22 -0800
From: Kees Cook <keescook@...omium.org>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Alexei Starovoitov <ast@...nel.org>, 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 Mon, Feb 17, 2020 at 04:51:46PM +0100, Daniel Borkmann wrote:
> 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'.
Oh, good point. I'll double-check the 32-bit builds. (I also have
another related change that I found several days later.) I'll get this
adjusted/tested and resend the patch.
Thanks!
-Kees
--
Kees Cook
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