perf augmented_syscalls: Avoid optimization to pass older BPF validators

perf augmented_syscalls: Avoid optimization to pass older BPF validators

See https://www.spinics.net/lists/netdev/msg480099.html for the whole
discussio, but to make the augmented_syscalls.c BPF program to get built
and loaded successfully in a greater range of kernels, add an extra
check.

Related patch:

  a60dd35d2e39 ("bpf: change bpf_perf_event_output arg5 type to ARG_CONST_SIZE_OR_ZERO")

That is in the kernel since v4.15, I couldn't figure why this is hitting
me with 4.17.17, but adding the workaround discussed there makes this
work with this fedora kernel and with 4.18.recent.

Before:

  # uname -a
  Linux seventh 4.17.17-100.fc27.x86_64 #1 SMP Mon Aug 20 15:53:11 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
  # perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
  libbpf: load bpf program failed: Permission denied
  libbpf: -- BEGIN DUMP LOG ---
  libbpf:
  0: (bf) r6 = r1
  1: (b7) r1 = 0
  2: (7b) *(u64 *)(r10 -8) = r1
  3: (7b) *(u64 *)(r10 -16) = r1
  4: (7b) *(u64 *)(r10 -24) = r1
  5: (7b) *(u64 *)(r10 -32) = r1
  6: (7b) *(u64 *)(r10 -40) = r1
  7: (7b) *(u64 *)(r10 -48) = r1
  8: (7b) *(u64 *)(r10 -56) = r1
  9: (7b) *(u64 *)(r10 -64) = r1
  10: (7b) *(u64 *)(r10 -72) = r1
  11: (7b) *(u64 *)(r10 -80) = r1
  12: (7b) *(u64 *)(r10 -88) = r1
  13: (7b) *(u64 *)(r10 -96) = r1
  14: (7b) *(u64 *)(r10 -104) = r1
  15: (7b) *(u64 *)(r10 -112) = r1
  16: (7b) *(u64 *)(r10 -120) = r1
  17: (7b) *(u64 *)(r10 -128) = r1
  18: (7b) *(u64 *)(r10 -136) = r1
  19: (7b) *(u64 *)(r10 -144) = r1
  20: (7b) *(u64 *)(r10 -152) = r1
  21: (7b) *(u64 *)(r10 -160) = r1
  22: (7b) *(u64 *)(r10 -168) = r1
  23: (7b) *(u64 *)(r10 -176) = r1
  24: (7b) *(u64 *)(r10 -184) = r1
  25: (7b) *(u64 *)(r10 -192) = r1
  26: (7b) *(u64 *)(r10 -200) = r1
  27: (7b) *(u64 *)(r10 -208) = r1
  28: (7b) *(u64 *)(r10 -216) = r1
  29: (7b) *(u64 *)(r10 -224) = r1
  30: (7b) *(u64 *)(r10 -232) = r1
  31: (7b) *(u64 *)(r10 -240) = r1
  32: (7b) *(u64 *)(r10 -248) = r1
  33: (7b) *(u64 *)(r10 -256) = r1
  34: (7b) *(u64 *)(r10 -264) = r1
  35: (7b) *(u64 *)(r10 -272) = r1
  36: (7b) *(u64 *)(r10 -280) = r1
  37: (7b) *(u64 *)(r10 -288) = r1
  38: (7b) *(u64 *)(r10 -296) = r1
  39: (7b) *(u64 *)(r10 -304) = r1
  40: (7b) *(u64 *)(r10 -312) = r1
  41: (bf) r7 = r10
  42: (07) r7 += -312
  43: (bf) r1 = r7
  44: (b7) r2 = 48
  45: (bf) r3 = r6
  46: (85) call bpf_probe_read#4
  47: (79) r3 = *(u64 *)(r6 +24)
  48: (bf) r1 = r10
  49: (07) r1 += -256
  50: (b7) r8 = 256
  51: (b7) r2 = 256
  52: (85) call bpf_probe_read_str#45
  53: (bf) r1 = r0
  54: (67) r1 <<= 32
  55: (77) r1 >>= 32
  56: (bf) r5 = r0
  57: (07) r5 += 56
  58: (2d) if r8 > r1 goto pc+1
   R0=inv(id=0) R1=inv(id=0,umin_value=256,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R5=inv(id=0) R6=ctx(id=0,off=0,imm=0) R7=fp-312,call_-1 R8=inv256 R10=fp0,call_-1 fp-264=0
  59: (b7) r5 = 312
  60: (63) *(u32 *)(r10 -264) = r0
  61: (67) r5 <<= 32
  62: (77) r5 >>= 32
  63: (bf) r1 = r6
  64: (18) r2 = 0xffff8b9120cc8500
  66: (18) r3 = 0xffffffff
  68: (bf) r4 = r7
  69: (85) call bpf_perf_event_output#25
  70: (b7) r0 = 0
  71: (95) exit

  from 58 to 60: R0=inv(id=0) R1=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R5=inv(id=0) R6=ctx(id=0,off=0,imm=0) R7=fp-312,call_-1 R8=inv256 R10=fp0,call_-1 fp-264=0
  60: (63) *(u32 *)(r10 -264) = r0
  61: (67) r5 <<= 32
  62: (77) r5 >>= 32
  63: (bf) r1 = r6
  64: (18) r2 = 0xffff8b9120cc8500
  66: (18) r3 = 0xffffffff
  68: (bf) r4 = r7
  69: (85) call bpf_perf_event_output#25
  R5 unbounded memory access, use 'var &= const' or 'if (var < const)'

  libbpf: -- END LOG --
  libbpf: failed to load program 'syscalls:sys_enter_openat'
  libbpf: failed to load object 'tools/perf/examples/bpf/augmented_syscalls.c'
  bpf: load objects failed: err=-4007: (Kernel verifier blocks program loading)
  event syntax error: 'tools/perf/examples/bpf/augmented_syscalls.c'
                       \___ Kernel verifier blocks program loading

After:

  # perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
     0.000 cat/29249 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC)
     0.008 cat/29249 syscalls:sys_exit_openat:0x3
     0.021 cat/29249 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC)
     0.025 cat/29249 syscalls:sys_exit_openat:0x3
     0.180 cat/29249 open(filename: /usr/lib/locale/locale-archive, flags: CLOEXEC)
     0.185 cat/29249 syscalls:sys_exit_open:0x3
     0.242 cat/29249 openat(dfd: CWD, filename: /etc/passwd)
     0.245 cat/29249 syscalls:sys_exit_openat:0x3
  #

It also works with a more recent kernel:

  # uname -a
  Linux jouet 4.18.0-00014-g4e67b2a5df5d #6 SMP Thu Aug 30 17:34:17 -03 2018 x86_64 x86_64 x86_64 GNU/Linux
  # perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
     0.000 cat/26451 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC)
     0.020 cat/26451 syscalls:sys_exit_openat:0x3
     0.039 cat/26451 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC)
     0.044 cat/26451 syscalls:sys_exit_openat:0x3
     0.231 cat/26451 open(filename: /usr/lib/locale/locale-archive, flags: CLOEXEC)
     0.238 cat/26451 syscalls:sys_exit_open:0x3
     0.278 cat/26451 openat(dfd: CWD, filename: /etc/passwd)
     0.282 cat/26451 syscalls:sys_exit_openat:0x3
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: Gianluca Borello <g.borello@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-wkpsivs1a9afwldbul46btbv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>