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Marchesi" To: Andrii Nakryiko Cc: bpf@vger.kernel.org, david.faust@oracle.com, cupertino.miranda@oracle.com Subject: Re: BPF_PROG, BPF_KPROBE, BPF_KSYSCALL and enum conversions In-Reply-To: (Andrii Nakryiko's message of "Thu, 25 Apr 2024 11:40:37 -0700") References: <87edat1j7f.fsf@oracle.com> Date: Thu, 25 Apr 2024 20:58:22 +0200 Message-ID: <87zfth5kxd.fsf@oracle.com> User-Agent: Gnus/5.13 (Gnus v5.13) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: LO2P265CA0424.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a0::28) To DM6PR10MB3113.namprd10.prod.outlook.com (2603:10b6:5:1a7::12) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB3113:EE_|IA0PR10MB7182:EE_ X-MS-Office365-Filtering-Correlation-Id: 292d47d7-1e37-4c27-8b3c-08dc6559b0a4 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230031|1800799015|376005|366007; 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Marchesi > wrote: >> >> >> The BPF_PROG macro defined in tools/lib/bpf/bpf_tracing.h uses a clever >> hack in order to provide a convenient way to define entry points for BPF >> programs, that get their argument as elements in a single "context" >> array argument. >> >> It allows to write something like: >> >> SEC("struct_ops/cwnd_event") >> void BPF_PROG(cwnd_event, struct sock *sk, enum tcp_ca_event event) >> { >> bbr_cwnd_event(sk, event); >> dctcp_cwnd_event(sk, event); >> cubictcp_cwnd_event(sk, event); >> } >> >> That expands into a pair of functions: >> >> void ____cwnd_event (unsigned long long *ctx, struct sock *sk, enum tc= p_ca_event event) >> { >> bbr_cwnd_event(sk, event); >> dctcp_cwnd_event(sk, event); >> cubictcp_cwnd_event(sk, event); >> } >> >> void cwnd_event (unsigned long long *ctx) >> { >> _Pragma("GCC diagnostic push") >> _Pragma("GCC diagnostic ignored \"-Wint-conversion\"") >> return ____cwnd_event(ctx, (void*)ctx[0], (void*)ctx[1]); >> _Pragma("GCC diagnostic pop") >> } >> >> Note how the 64-bit unsigned integers in the incoming CTX get casted to >> a void pointer, and then implicitly casted to whatever type of the >> actual argument in the wrapped function. In this case: >> >> Arg1: unsigned long long -> void * -> struct sock * >> Arg2: unsigned long long -> void * -> enum tcp_ca_event >> >> The behavior of GCC and clang when facing such conversions differ: >> >> pointer -> pointer >> >> Allowed by the C standard. >> GCC: no warning nor error. >> clang: no warning nor error. >> >> pointer -> integer type >> >> [C standard says the result of this conversion is implementation >> defined, and it may lead to unaligned pointer etc.] >> >> GCC: error: integer from pointer without a cast [-Wint-conversion] >> clang: error: incompatible pointer to integer conversion [-Wint-conv= ersion] >> >> pointer -> enumerated type >> >> GCC: error: incompatible types in assigment (*) >> clang: error: incompatible pointer to integer conversion [-Wint-conv= ersion] >> >> BPF_PROG works because the pointer to integer conversion leads to the >> same value in 64-bit mode, much like when casting a pointer to >> uintptr_t. It also silences compiler errors by mean of the compiler >> pragma that installs -Wno-int-conversion temporarily. >> >> However, the GCC error marked with (*) above when assigning a pointer to >> an enumerated value is not associated with the -Wint-conversion warning, >> and it is not possible to turn it off. >> >> This is preventing building the BPF kernel selftests with GCC. >> >> The magic in the BPF_PROG macro leads down to these macros: >> >> #define ___bpf_ctx_cast1(x) ___bpf_ctx_cast0(), (void *)ctx[= 0] >> #define ___bpf_ctx_cast2(x, args...) ___bpf_ctx_cast1(args), (void *)= ctx[1] >> #define ___bpf_ctx_cast3(x, args...) ___bpf_ctx_cast2(args), (void *)= ctx[2] >> etc >> >> An option would be to change all the usages of BPF_PROG that use >> enumerated arguments in order to use integers instead. But this is not >> very nice for obvious reasons. >> >> Another option would be to omit the casts to (void *) from the >> definitions above. This would lead to conversions from 'unsigned long >> long' to typed pointers, integer types and enumerated types. As far as >> I can tell this should imply no difference in the generated code in >> 64-bit mode (is there any particular reason for this cast?). Since the >> pointer->enum conversion would not happen, errors in both compilers >> would be successfully silenced with the -Wno-int-conversion pragma. >> >> This option would lead to: >> >> #define ___bpf_ctx_cast1(x) ___bpf_ctx_cast0(), ctx[0] >> #define ___bpf_ctx_cast2(x, args...) ___bpf_ctx_cast1(args), ctx[1] >> #define ___bpf_ctx_cast3(x, args...) ___bpf_ctx_cast2(args), ctx[2] >> #define ___bpf_ctx_cast4(x, args...) ___bpf_ctx_cast3(args), ctx[3] >> etc >> >> Then there is BPF_KPROBE, which is very much like BPF_PROG but the >> context is an array of pointers to ptregs instead of an array of >> unsigned long longs. >> >> The BPF_KPROBE arguments and handled by: >> >> #define ___bpf_kprobe_args0() ctx >> #define ___bpf_kprobe_args1(x) ___bpf_kprobe_args0(), (void *= )PT_REGS_PARM1(ctx) >> #define ___bpf_kprobe_args2(x, args...) ___bpf_kprobe_args1(args), (vo= id *)PT_REGS_PARM2(ctx) >> #define ___bpf_kprobe_args3(x, args...) ___bpf_kprobe_args2(args), (vo= id *)PT_REGS_PARM3(ctx) >> etc >> >> There is currently only one BPF_KPROBE usage that uses an enumerated >> value (handle__kprobe in progs/test_vmlinux.c) but a similar solution to >> the above could be used, by casting the ptregs pointers to unsigned long >> long: >> >> #define ___bpf_kprobe_args0() ctx >> #define ___bpf_kprobe_args1(x) ___bpf_kprobe_args0(),(unsigne= d long long )PT_REGS_PARM1(ctx) >> #define ___bpf_kprobe_args2(x, args...) ___bpf_kprobe_args1(args),(uns= igned long long)PT_REGS_PARM2(ctx) >> #define ___bpf_kprobe_args3(x, args...) ___bpf_kprobe_args2(args),(uns= igned long long)PT_REGS_PARM3(ctx) >> etc >> >> Similar situation with BPF_KSYSCALL: >> >> #define ___bpf_syswrap_args1(x) ___bpf_syswrap_args0(), (void= *)PT_REGS_PARM1_CORE_SYSCALL(regs) >> #define ___bpf_syswrap_args2(x, args...) ___bpf_syswrap_args1(args), (= void *)PT_REGS_PARM2_CORE_SYSCALL(regs) >> etc >> >> There is currently no usage of BPF_KSYSCALL with enumerated types, but >> the same change would lead to: >> >> #define ___bpf_syswrap_args1(x) ___bpf_syswrap_args0(),(unsig= ned long long)PT_REGS_PARM1_CORE_SYSCALL(regs) >> #define ___bpf_syswrap_args2(x, args...) ___bpf_syswrap_args1(args),(u= nsigned long long )PT_REGS_PARM2_CORE_SYSCALL(regs) >> etc >> >> Opinions? >> > > I don't remember why I did (void *), but I think I was just banging my > head against the compiler until I made it work, and once it worked, I > didn't try to improve it further :) If casting to (unsigned long long) > works just as well as (void *) and helps in GCC case, let's convert. Ok, I will test a patch on that direction. > Just please don't miss ___bpf_syscall_args* and ___bpf_kretprobe_args1 > as well. k