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

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Date:   Fri, 10 Dec 2021 11:19:18 +0000
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 tracex2 due to empty sys_write count argument

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