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

[Yonghong Song <yhs@fb.com>]

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.