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

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Date:   Fri, 2 Dec 2022 16:34:14 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     "Daniel T. Lee" <danieltimlee@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] samples: bpf: fix broken behavior of tracex2 write_size count

On Fri, Dec 2, 2022 at 8:29 AM Daniel T. Lee <danieltimlee@...il.com> wrote:
>
> 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);


use

SEC("ksyscall/write")
int BPF_KSYSCALL(bpf_prog3, unsigned int fd, const char *buf, size_t count)

instead?

And maybe let's update other samples to use SEC("ksyscall") and
BPF_KSYSCALL() macro as well?


> --
> 2.34.1
>