On Thu, Apr 25, 2024 at 9:49 AM Jose E. Marchesi
<jose.marchesi@xxxxxxxxxx> 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 tcp_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-conversion]
>
> pointer -> enumerated type
>
> GCC: error: incompatible types in assigment (*)
> clang: error: incompatible pointer to integer conversion [-Wint-conversion]
>
> 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), (void *)PT_REGS_PARM2(ctx)
> #define ___bpf_kprobe_args3(x, args...) ___bpf_kprobe_args2(args), (void *)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(),(unsigned long long )PT_REGS_PARM1(ctx)
> #define ___bpf_kprobe_args2(x, args...) ___bpf_kprobe_args1(args),(unsigned long long)PT_REGS_PARM2(ctx)
> #define ___bpf_kprobe_args3(x, args...) ___bpf_kprobe_args2(args),(unsigned 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(),(unsigned long long)PT_REGS_PARM1_CORE_SYSCALL(regs)
> #define ___bpf_syswrap_args2(x, args...) ___bpf_syswrap_args1(args),(unsigned 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.
Just please don't miss ___bpf_syscall_args* and ___bpf_kretprobe_args1 as well.
