linux - 如何使用 ftrace 仅跟踪系统调用事件而不显示 Linux 内核中的任何其他函数？

Question

例如，要监控所有 mkdir 调用，我能想到的最好办法是:

#!/bin/sh
set -eux

d=debug/tracing

mkdir -p debug
if ! mountpoint -q debug; then
  mount -t debugfs nodev debug
fi

# Stop tracing.
echo 0 > "${d}/tracing_on"

# Clear previous traces.
echo > "${d}/trace"

# Enable tracing mkdir
echo sys_enter_mkdir > "${d}/set_event"

# Set tracer type.
echo function > "${d}/current_tracer"

# Filter only sys_mkdir as a workaround.
echo SyS_mkdir > "${d}/set_ftrace_filter"

# Start tracing.
echo 1 > "${d}/tracing_on"

# Generate two mkdir calls.
rm -rf /tmp/a
rm -rf /tmp/b
mkdir /tmp/a
mkdir /tmp/b

# View the trace.
cat "${d}/trace"

# Stop tracing.
echo 0 > "${d}/tracing_on"

umount debug

然后在使用 sudo 运行后，它给出:

# tracer: function
#
# entries-in-buffer/entries-written: 4/4   #P:16
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
           mkdir-31254 [015] .... 2010985.576760: sys_mkdir(pathname: 7ffc54b32c77, mode: 1ff)
           mkdir-31254 [015] .... 2010985.576763: SyS_mkdir <-tracesys_phase2
           mkdir-31255 [007] .... 2010985.578363: sys_mkdir(pathname: 7fff02d90c77, mode: 1ff)
           mkdir-31255 [007] .... 2010985.578365: SyS_mkdir <-tracesys_phase2

我的问题是它为每个系统调用输出两行:

• sys_mkdir 这是我想要的事件
• SyS_mkdir 这是过滤后的函数解决方法，我不想看到它

如果我尝试这样做:

echo > "${d}/set_ftrace_filter"

或者根本不碰那个文件，那么它会显示大量函数，并且很难找到系统调用。

是否有更好的方法来禁用常规功能并只保留系统调用事件？

我可以只使用 SyS_mkdir 并禁用我猜的系统调用事件，但如果我可以使用更具体的事件，感觉会更干净吗？还有:

• 事件显示参数，这样更好。
• syscall 函数名称在内核版本之间发生变化。例如，在 Linux v4.18 上它已经是 __x64_sys_mkdir 而不是 SyS_mkdir。

相关:

• How to trace the write system call in the Linux kernel?

在 Ubuntu 18.04、Linux 内核 4.15 上测试。

Answer 1

使用nop 跟踪器

作为proposed by sruffell ，我们所要做的就是使用 nop 跟踪器而不是 function，这将禁用函数跟踪，但不会禁用事件。

使用 sudo 运行:

#!/bin/sh
set -eux

d=debug/tracing

mkdir -p debug
if ! mountpoint -q debug; then
  mount -t debugfs nodev debug
fi

# Stop tracing.
echo 0 > "${d}/tracing_on"

# Clear previous traces.
echo > "${d}/trace"

# Find the tracer name.
cat "${d}/available_tracers"

# Disable tracing functions, show only system call events.
echo nop > "${d}/current_tracer"

# Find the event name with.
grep mkdir "${d}/available_events"

# Enable tracing mkdir.
# Both statements below seem to do the exact same thing,
# just with different interfaces.
# https://www.kernel.org/doc/html/v4.18/trace/events.html
echo sys_enter_mkdir > "${d}/set_event"
# echo 1 > "${d}/events/syscalls/sys_enter_mkdir/enable"

# Start tracing.
echo 1 > "${d}/tracing_on"

# Generate two mkdir calls by two different processes.
rm -rf /tmp/a /tmp/b
mkdir /tmp/a
mkdir /tmp/b

# View the trace.
cat "${d}/trace"

# Stop tracing.
echo 0 > "${d}/tracing_on"

umount debug

这给出了所需的输出:

   mkdir-26064 [007] .... 2014370.909743: sys_mkdir(pathname: 7fffbd461c77, mode: 1ff)
   mkdir-26065 [014] .... 2014370.911615: sys_mkdir(pathname: 7ffea53bac77, mode: 1ff)

替代方案不是最佳方案

这也行，但绝对不那么好，替换:

echo SyS_mkdir > "${d}/set_ftrace_filter"

与:

echo '*' > "${d}/set_ftrace_notrace"

这将关闭文档中提到的所有功能:https://www.kernel.org/doc/html/v4.18/trace/ftrace.html#the-file-system

set_ftrace_notrace:

This has an effect opposite to that of set_ftrace_filter. Any function that is added here will not be traced. If a function exists in both set_ftrace_filter and set_ftrace_notrace, the function will not be traced.