From: Kees Cook <keescook@chromium.org>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>,
linux-kernel@vger.kernel.org, Martin KaFai Lau <kafai@fb.com>,
Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
Andrii Nakryiko <andriin@fb.com>,
Sami Tolvanen <samitolvanen@google.com>,
bpf@vger.kernel.org
Subject: Re: [PATCH] bpf: Avoid function casting when calculating immediate
Date: Mon, 17 Feb 2020 10:25:22 -0800 [thread overview]
Message-ID: <202002171024.184D90B@keescook> (raw)
In-Reply-To: <92bcfdab-79df-c3f4-bae8-00116b39e015@iogearbox.net>
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@chromium.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
prev parent reply other threads:[~2020-02-17 18:25 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-29 21:36 [PATCH] bpf: Avoid function casting when calculating immediate Kees Cook
2020-02-17 15:51 ` Daniel Borkmann
2020-02-17 18:25 ` Kees Cook [this message]
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