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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
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