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?
