Re: BPF_PROG, BPF_KPROBE, BPF_KSYSCALL and enum conversions

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Thu, Apr 25, 2024 at 9:49 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> 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.

Ok, I will test a patch on that direction.

> Just please don't miss ___bpf_syscall_args* and ___bpf_kretprobe_args1
> as well.

k