netdev - [PATCH] samples: bpf: fix broken behavior of tracex2 write

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Date:   Sat,  3 Dec 2022 01:29:07 +0900
From:   "Daniel T. Lee" <danieltimlee@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH] samples: bpf: fix broken behavior of tracex2 write_size count

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 __x64_sys_write (or s390x also) doesn't have the proper
argument, changing the kprobe event to ksys_write will fix the problem.

Signed-off-by: Daniel T. Lee <danieltimlee@...il.com>
---
 samples/bpf/tracex2_kern.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/samples/bpf/tracex2_kern.c b/samples/bpf/tracex2_kern.c
index 93e0b7680b4f..fc65c589e87f 100644
--- a/samples/bpf/tracex2_kern.c
+++ b/samples/bpf/tracex2_kern.c
@@ -78,7 +78,7 @@ struct {
 	__uint(max_entries, 1024);
 } my_hist_map SEC(".maps");

-SEC("kprobe/" SYSCALL(sys_write))
+SEC("kprobe/ksys_write")
 int bpf_prog3(struct pt_regs *ctx)
 {
 	long write_size = PT_REGS_PARM3(ctx);
-- 
2.34.1