netdev - RE: [PATCH] samples: bpf: fix tracex2 due to empty sys

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Date:   Fri, 10 Dec 2021 10:00:26 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     "Daniel T. Lee" <danieltimlee@...il.com>,
        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: RE: [PATCH] samples: bpf: fix tracex2 due to empty sys_write count
 argument

Daniel T. Lee wrote:
> Currently from syscall entry, argument can't be fetched correctly as a
> result of register cleanup.
> 
>     commit 6b8cf5cc9965 ("x86/entry/64/compat: Clear registers for compat syscalls, to reduce speculation attack surface")
> 
> For example in upper commit, registers are cleaned prior to syscall.
> To be more specific, sys_write syscall has count size as a third argument.
> But this can't be fetched from __x64_sys_enter/__s390x_sys_enter due to
> register cleanup. (e.g. [x86] xorl %r8d, %r8d / [s390x] xgr %r7, %r7)
> 
> This commit fix this problem by modifying the trace event to ksys_write
> instead of sys_write syscall entry.
> 
>     # Wrong example of 'write()' syscall argument fetching
>     # ./tracex2
>     ...
>     pid 50909 cmd dd uid 0
>            syscall write() stats
>      byte_size       : count     distribution
>        1 -> 1        : 4968837  |************************************* |
> 
>     # Successful example of 'write()' syscall argument fetching
>     # (dd's write bytes at a time defaults to 512)
>     # ./tracex2
>     ...
>     pid 3095 cmd dd uid 0
>            syscall write() stats
>      byte_size       : count     distribution
>     ...
>      256 -> 511      : 0        |                                      |
>      512 -> 1023     : 4968844  |************************************* |
> 
> 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 5bc696bac27d..96dff3bea227 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);
> -- 
> 2.32.0
> 

LGTM

Acked-by: John Fastabend <john.fastabend@...il.com>