samples/bpf: Fix tracex2 by using BPF_KSYSCALL macro

Currently, there is a problem with tracex2, as it doesn't print the
histogram properly and the results are misleading. (all results report
as 0)

The problem is caused by a change in arguments of the function to which
the kprobe connects. This tracex2 bpf program uses kprobe (attached
to __x64_sys_write) to figure out the size of the write system call. In
order to achieve this, the third argument 'count' must be intact.

The following is a prototype of the sys_write variant. (checked with
pfunct)

    ~/git/linux$ pfunct -P fs/read_write.o | grep sys_write
    ssize_t ksys_write(unsigned int fd, const char  * buf, size_t count);
    long int __x64_sys_write(const struct pt_regs  * regs);
    ... cross compile with s390x ...
    long int __s390_sys_write(struct pt_regs * regs);

Since the nature of SYSCALL_WRAPPER function wraps the argument once,
additional process of argument extraction is required to properly parse
the argument.

    #define BPF_KSYSCALL(name, args...)
    ... snip ...
    struct pt_regs *regs = LINUX_HAS_SYSCALL_WRAPPER                    \
			   ? (struct pt_regs *)PT_REGS_PARM1(ctx)       \
			   : ctx;                                       \

In order to fix this problem, the BPF_SYSCALL macro has been used. This
reduces the hassle of parsing arguments from pt_regs. Since the macro
uses the CORE version of argument extraction, additional portability
comes too.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221224071527.2292-5-danieltimlee@gmail.com

samples/bpf/tracex2.bpf.c

@@ -8,6 +8,7 @@
 #include <linux/version.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
 
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
@@ -76,14 +77,13 @@ struct {
 } my_hist_map SEC(".maps");
 
 SEC("ksyscall/write")
-int bpf_prog3(struct pt_regs *ctx)
+int BPF_KSYSCALL(bpf_prog3, unsigned int fd, const char *buf, size_t count)
 {
-	long write_size = PT_REGS_PARM3(ctx);
 	long init_val = 1;
 	long *value;
 	struct hist_key key;
 
-	key.index = log2l(write_size);
+	key.index = log2l(count);
 	key.pid_tgid = bpf_get_current_pid_tgid();
 	key.uid_gid = bpf_get_current_uid_gid();
 	bpf_get_current_comm(&key.comm, sizeof(key.comm));