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.
