This patch introduces a new BPF script to test the BPF prologue adding
routines. The new script probes at null_lseek, which is the function pointer
used when we try to lseek on '/dev/null'.

The null_lseek function is chosen because it is used by function pointers, so
we don't need to consider inlining and LTO.

By extracting file->f_mode, bpf-script-test-prologue.c should know whether the
file is writable or readonly. According to llseek_loop() and
bpf-script-test-prologue.c, one fourth of total lseeks should be collected.

Committer note:

Testing it:

  # perf test -v BPF
  <SNIP>
  Kernel build dir is set to /lib/modules/4.3.0+/build
  set env: KBUILD_DIR=/lib/modules/4.3.0+/build
  unset env: KBUILD_OPTS
  include option is set to  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/4.9.2/include -I/home/git/linux/arch/x86/include -Iarch/x86/include/generated/uapi -Iarch/x86/include/generated  -I/home/git/linux/include -Iinclude -I/home/git/linux/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/git/linux/include/uapi -Iinclude/generated/uapi -include /home/git/linux/include/linux/kconfig.h
  set env: NR_CPUS=4
  set env: LINUX_VERSION_CODE=0x40300
  set env: CLANG_EXEC=/usr/libexec/icecc/bin/clang
  set env: CLANG_OPTIONS=-xc
  set env: KERNEL_INC_OPTIONS= -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/4.9.2/include -I/home/git/linux/arch/x86/include -Iarch/x86/include/generated/uapi -Iarch/x86/include/generated  -I/home/git/linux/include -Iinclude -I/home/git/linux/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/git/linux/include/uapi -Iinclude/generated/uapi -include /home/git/linux/include/linux/kconfig.h
  set env: WORKING_DIR=/lib/modules/4.3.0+/build
  set env: CLANG_SOURCE=-
  llvm compiling command template: echo '/*
   * bpf-script-test-prologue.c
   * Test BPF prologue
   */
  #ifndef LINUX_VERSION_CODE
  # error Need LINUX_VERSION_CODE
  # error Example: for 4.2 kernel, put 'clang-opt="-DLINUX_VERSION_CODE=0x40200" into llvm section of ~/.perfconfig'
  #endif
  #define SEC(NAME) __attribute__((section(NAME), used))

  #include <uapi/linux/fs.h>

  #define FMODE_READ		0x1
  #define FMODE_WRITE		0x2

  static void (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
	  (void *) 6;

  SEC("func=null_lseek file->f_mode offset orig")
  int bpf_func__null_lseek(void *ctx, int err, unsigned long f_mode,
			   unsigned long offset, unsigned long orig)
  {
	  if (err)
		  return 0;
	  if (f_mode & FMODE_WRITE)
		  return 0;
	  if (offset & 1)
		  return 0;
	  if (orig == SEEK_CUR)
		  return 0;
	  return 1;
  }

  char _license[] SEC("license") = "GPL";
  int _version SEC("version") = LINUX_VERSION_CODE;
  ' | $CLANG_EXEC -D__KERNEL__ -D__NR_CPUS__=$NR_CPUS -DLINUX_VERSION_CODE=$LINUX_VERSION_CODE $CLANG_OPTIONS $KERNEL_INC_OPTIONS -Wno-unused-value -Wno-pointer-sign -working-directory $WORKING_DIR -c "$CLANG_SOURCE" -target bpf -O2 -o -
  libbpf: loading object '[bpf_prologue_test]' from buffer
  libbpf: section .strtab, size 135, link 0, flags 0, type=3
  libbpf: section .text, size 0, link 0, flags 6, type=1
  libbpf: section .data, size 0, link 0, flags 3, type=1
  libbpf: section .bss, size 0, link 0, flags 3, type=8
  libbpf: section func=null_lseek file->f_mode offset orig, size 112, link 0, flags 6, type=1
  libbpf: found program func=null_lseek file->f_mode offset orig
  libbpf: section license, size 4, link 0, flags 3, type=1
  libbpf: license of [bpf_prologue_test] is GPL
  libbpf: section version, size 4, link 0, flags 3, type=1
  libbpf: kernel version of [bpf_prologue_test] is 40300
  libbpf: section .symtab, size 168, link 1, flags 0, type=2
  bpf: config program 'func=null_lseek file->f_mode offset orig'
  symbol:null_lseek file:(null) line:0 offset:0 return:0 lazy:(null)
  parsing arg: file->f_mode into file, f_mode(1)
  parsing arg: offset into offset
  parsing arg: orig into orig
  bpf: config 'func=null_lseek file->f_mode offset orig' is ok
  Looking at the vmlinux_path (7 entries long)
  Using /lib/modules/4.3.0+/build/vmlinux for symbols
  Open Debuginfo file: /lib/modules/4.3.0+/build/vmlinux
  Try to find probe point from debuginfo.
  Matched function: null_lseek
  Probe point found: null_lseek+0
  Searching 'file' variable in context.
  Converting variable file into trace event.
  converting f_mode in file
  f_mode type is unsigned int.
  Searching 'offset' variable in context.
  Converting variable offset into trace event.
  offset type is long long int.
  Searching 'orig' variable in context.
  Converting variable orig into trace event.
  orig type is int.
  Found 1 probe_trace_events.
  Opening /sys/kernel/debug/tracing//kprobe_events write=1
  Writing event: p:perf_bpf_probe/func _text+4840528 f_mode=+68(%di):u32 offset=%si:s64 orig=%dx:s32
  libbpf: don't need create maps for [bpf_prologue_test]
  prologue: pass validation
  prologue: slow path
  prologue: fetch arg 0, base reg is %di
  prologue: arg 0: offset 68
  prologue: fetch arg 1, base reg is %si
  prologue: fetch arg 2, base reg is %dx
  add bpf event perf_bpf_probe:func and attach bpf program 3
  adding perf_bpf_probe:func
  adding perf_bpf_probe:func to 0x51672c0
  mmap size 1052672B
  Opening /sys/kernel/debug/tracing//kprobe_events write=1
  Opening /sys/kernel/debug/tracing//uprobe_events write=1
  Parsing probe_events: p:perf_bpf_probe/func _text+4840528 f_mode=+68(%di):u32 offset=%si:s64 orig=%dx:s32
  Group:perf_bpf_probe Event:func probe:p
  Writing event: -:perf_bpf_probe/func
  test child finished with 0
  ---- end ----
  Test BPF filter: Ok
  #
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1447675815-166222-13-git-send-email-wangnan0@huawei.com
[ Added tools/perf/tests/llvm-src-prologue.c to .gitignore ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>