Re: [RFC PATCH bpf-next 0/7] bpf: Support struct value argument for trampoline base progs

[Kui-Feng Lee <kuifeng@fb.com>]

On Thu, 2022-07-28 at 10:46 -0700, Yonghong Song wrote:
> 
> 
> On 7/28/22 8:46 AM, Kui-Feng Lee wrote:
> > On Tue, 2022-07-26 at 10:11 -0700, Yonghong Song wrote:
> > > Currently struct arguments are not supported for trampoline based
> > > progs.
> > > One of major reason is that struct argument may pass by value
> > > which
> > > may
> > > use more than one registers. This breaks trampoline progs where
> > > each argument is assumed to take one register. bcc community
> > > reported
> > > the
> > > issue ([1]) where struct argument is not supported for fentry
> > > program.
> > >    typedef struct {
> > >          uid_t val;
> > >    } kuid_t;
> > >    typedef struct {
> > >          gid_t val;
> > >    } kgid_t;
> > >    int security_path_chown(struct path *path, kuid_t uid, kgid_t
> > > gid);
> > > Inside Meta, we also have a use case to attach to
> > > tcp_setsockopt()
> > >    typedef struct {
> > >          union {
> > >                  void            *kernel;
> > >                  void __user     *user;
> > >          };
> > >          bool            is_kernel : 1;
> > >    } sockptr_t;
> > >    int tcp_setsockopt(struct sock *sk, int level, int optname,
> > >                       sockptr_t optval, unsigned int optlen);
> > > 
> > > This patch added struct value support for bpf tracing programs
> > > which
> > > uses trampoline. struct argument size needs to be 16 or less so
> > > it can fit in one or two registers. Based on analysis on llvm and
> > > experiments, atruct argument size greater than 16 will be passed
> > > as pointer to the struct.
> > 
> > Is it possible to force llvm to always pass a pointer to a struct
> > over
> > 8 bytes (the size of single register) for the BPF traget?
> 
> This is already the case for bpf target. Any struct parameter (1
> byte, 2 
> bytes, ..., 8 types, ..., 16 bytes, ...) will be passed as a
> reference.
> 
> But this is not the case for most other architectures. For example,
> for
> x86_64, in most cases, struct size <= 16 will be passed with two
> registers instead of as a reference.

I ask this question because you modify the signature of a bpf program
to a pointer to a struct in patch #4.  Is that necessary if the
compiler passes a struct paramter as a reference?