perf trace augmented_raw_syscalls: Add extra array index bounds check…

…ing to satisfy some BPF verifiers

In a RHEL8 kernel (4.18.0-513.11.1.el8_9.x86_64), that, as enterprise
kernels go, have backports from modern kernels, the verifier complains
about lack of bounds check for the index into the array of syscall
arguments, on a BPF bytecode generated by clang 17, with:

  ; } else if (size < 0 && size >= -6) { /* buffer */
  116: (b7) r1 = -6
  117: (2d) if r1 > r6 goto pc-30
   R0=map_value(id=0,off=0,ks=4,vs=24688,imm=0) R1_w=inv-6 R2=map_value(id=0,off=16,ks=4,vs=8272,imm=0) R3=inv(id=0) R5=inv40 R6=inv(id=0,umin_value=18446744073709551610,var_off=(0xffffffff00000000; 0xffffffff)) R7=map_value(id=0,off=56,ks=4,vs=8272,imm=0) R8=invP6 R9=map_value(id=0,off=20,ks=4,vs=24,imm=0) R10=fp0 fp-8=mmmmmmmm fp-16=map_value fp-24=map_value fp-32=inv40 fp-40=ctx fp-48=map_value fp-56=inv1 fp-64=map_value fp-72=map_value fp-80=map_value
  ; index = -(size + 1);
  118: (a7) r6 ^= -1
  119: (67) r6 <<= 32
  120: (77) r6 >>= 32
  ; aug_size = args->args[index];
  121: (67) r6 <<= 3
  122: (79) r1 = *(u64 *)(r10 -24)
  123: (0f) r1 += r6
  last_idx 123 first_idx 116
  regs=40 stack=0 before 122: (79) r1 = *(u64 *)(r10 -24)
  regs=40 stack=0 before 121: (67) r6 <<= 3
  regs=40 stack=0 before 120: (77) r6 >>= 32
  regs=40 stack=0 before 119: (67) r6 <<= 32
  regs=40 stack=0 before 118: (a7) r6 ^= -1
  regs=40 stack=0 before 117: (2d) if r1 > r6 goto pc-30
  regs=42 stack=0 before 116: (b7) r1 = -6
   R0_w=map_value(id=0,off=0,ks=4,vs=24688,imm=0) R1_w=inv1 R2_w=map_value(id=0,off=16,ks=4,vs=8272,imm=0) R3_w=inv(id=0) R5_w=inv40 R6_rw=invP(id=0,smin_value=-2147483648,smax_value=0) R7_w=map_value(id=0,off=56,ks=4,vs=8272,imm=0) R8_w=invP6 R9_w=map_value(id=0,off=20,ks=4,vs=24,imm=0) R10=fp0 fp-8=mmmmmmmm fp-16_w=map_value fp-24_r=map_value fp-32_w=inv40 fp-40=ctx fp-48=map_value fp-56_w=inv1 fp-64_w=map_value fp-72=map_value fp-80=map_value
  parent didn't have regs=40 stack=0 marks
  last_idx 110 first_idx 98
  regs=40 stack=0 before 110: (6d) if r1 s> r6 goto pc+5
  regs=42 stack=0 before 109: (b7) r1 = 1
  regs=40 stack=0 before 108: (65) if r6 s> 0x1000 goto pc+7
  regs=40 stack=0 before 98: (55) if r6 != 0x1 goto pc+9
   R0_w=map_value(id=0,off=0,ks=4,vs=24688,imm=0) R1_w=invP12 R2_w=map_value(id=0,off=16,ks=4,vs=8272,imm=0) R3_rw=inv(id=0) R5_w=inv24 R6_rw=invP(id=0,smin_value=-2147483648,smax_value=2147483647) R7_w=map_value(id=0,off=40,ks=4,vs=8272,imm=0) R8_rw=invP4 R9_w=map_value(id=0,off=12,ks=4,vs=24,imm=0) R10=fp0 fp-8=mmmmmmmm fp-16_rw=map_value fp-24_r=map_value fp-32_rw=invP24 fp-40_r=ctx fp-48_r=map_value fp-56_w=invP1 fp-64_rw=map_value fp-72_r=map_value fp-80_r=map_value
  parent already had regs=40 stack=0 marks
  124: (79) r6 = *(u64 *)(r1 +16)
   R0=map_value(id=0,off=0,ks=4,vs=24688,imm=0) R1_w=map_value(id=0,off=0,ks=4,vs=8272,umax_value=34359738360,var_off=(0x0; 0x7fffffff8),s32_max_value=2147483640,u32_max_value=-8) R2=map_value(id=0,off=16,ks=4,vs=8272,imm=0) R3=inv(id=0) R5=inv40 R6_w=invP(id=0,umax_value=34359738360,var_off=(0x0; 0x7fffffff8),s32_max_value=2147483640,u32_max_value=-8) R7=map_value(id=0,off=56,ks=4,vs=8272,imm=0) R8=invP6 R9=map_value(id=0,off=20,ks=4,vs=24,imm=0) R10=fp0 fp-8=mmmmmmmm fp-16=map_value fp-24=map_value fp-32=inv40 fp-40=ctx fp-48=map_value fp-56=inv1 fp-64=map_value fp-72=map_value fp-80=map_value
  R1 unbounded memory access, make sure to bounds check any such access
  processed 466 insns (limit 1000000) max_states_per_insn 2 total_states 20 peak_states 20 mark_read 3

If we add this line, as used in other BPF programs, to cap that index:

   index &= 7;

The generated BPF program is considered safe by that version of the BPF
verifier, allowing perf to collect the syscall args in one more kernel
using the BPF based pointer contents collector.

With the above one-liner it works with that kernel:

  [root@dell-per740-01 ~]# uname -a
  Linux dell-per740-01.khw.eng.rdu2.dc.redhat.com 4.18.0-513.11.1.el8_9.x86_64 #1 SMP Thu Dec 7 03:06:13 EST 2023 x86_64 x86_64 x86_64 GNU/Linux
  [root@dell-per740-01 ~]# ~acme/bin/perf trace -e *sleep* sleep 1.234567890
       0.000 (1234.704 ms): sleep/3863610 nanosleep(rqtp: { .tv_sec: 1, .tv_nsec: 234567890 })                  = 0
  [root@dell-per740-01 ~]#

As well as with the one in Fedora 40:

  root@number:~# uname -a
  Linux number 6.11.3-200.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Oct 10 22:31:19 UTC 2024 x86_64 GNU/Linux
  root@number:~# perf trace -e *sleep* sleep 1.234567890
       0.000 (1234.722 ms): sleep/14873 clock_nanosleep(rqtp: { .tv_sec: 1, .tv_nsec: 234567890 }, rmtp: 0x7ffe87311a40) = 0
  root@number:~#

Song Liu reported that this one-liner was being optimized out by clang
18, so I suggested and he tested that adding a compiler barrier before
it made clang v18 to keep it and the verifier in the kernel in Song's
case (Meta's 5.12 based kernel) also was happy with the resulting
bytecode.

I'll investigate using virtme-ng[1] to have all the perf BPF based
functionality thoroughly tested over multiple kernels and clang
versions.

[1] https://kernel-recipes.org/en/2024/virtme-ng/

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/lkml/Zw7JgJc0LOwSpuvx@x1
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c

@@ -477,6 +477,8 @@ static int augment_sys_enter(void *ctx, struct syscall_enter_args *args)
 				augmented = true;
 		} else if (size < 0 && size >= -6) { /* buffer */
 			index = -(size + 1);
+			barrier_var(index); // Prevent clang (noticed with v18) from removing the &= 7 trick.
+			index &= 7;	    // Satisfy the bounds checking with the verifier in some kernels.
 			aug_size = args->args[index];
 
 			if (aug_size > TRACE_AUG_MAX_BUF)