tools: Prevent invalid mem access when reading PAGE_OFFSET on s390

[jeromemarchand]

Since kernel 6.10, PAGE_OFFSET on s390 is not a constant anymore but points to vm.identity_base and needs to be dereferenced properly with bpf_probe_read_kernel().

Fixes the following error:
bpf: Failed to load program: Permission denied
0: R1=ctx() R10=fp0
; int trace_0(struct pt_regs *ctx) { @ main.c:172
0: (bf) r6 = r1 ; R1=ctx() R6_w=ctx()
; u64 tgid_pid = bpf_get_current_pid_tgid(); @ main.c:68
1: (85) call bpf_get_current_pid_tgid#14 ; R0_w=scalar()
2: (7b) *(u64 *)(r10 -8) = r0 ; R0_w=scalar(id=1) R10=fp0 fp-8_w=scalar(id=1)
3: (b7) r7 = 0 ; R7_w=0
; struct entry_t entry = {}; @ main.c:75
4: (7b) *(u64 *)(r10 -16) = r7 ; R7_w=0 R10=fp0 fp-16_w=0
5: (7b) *(u64 *)(r10 -24) = r7 ; R7_w=0 R10=fp0 fp-24_w=0
; entry.start_ns = bpf_ktime_get_ns(); @ main.c:76
6: (85) call bpf_ktime_get_ns#5 ; R0=scalar()
7: (7b) *(u64 *)(r10 -32) = r0 ; R0=scalar(id=2) R10=fp0 fp-32_w=scalar(id=2)
; entry.id = id; @ main.c:77
8: (7b) *(u64 *)(r10 -40) = r7 ; R7=0 R10=fp0 fp-40_w=0
; entry.kernel_stack_id = bcc_get_stackid(bpf_pseudo_fd(1, -3), ctx, 0); @ main.c:94
9: (18) r2 = 0xfd038000 ; R2_w=map_ptr(map=stacks,ks=4,vs=1016)
; return bcc_get_stackid_(ctx, (void *)map, flags); @ helpers.h:660
11: (bf) r1 = r6 ; R1_w=ctx() R6=ctx()
12: (b7) r3 = 0 ; R3_w=0
13: (85) call bpf_get_stackid#27 ; R0_w=scalar()
; entry.kernel_stack_id = bcc_get_stackid(bpf_pseudo_fd(1, -3), ctx, 0); @ main.c:94
14: (63) *(u32 *)(r10 -24) = r0 ; R0_w=scalar() R10=fp0 fp-24=mmmmscalar()
; if (entry.kernel_stack_id >= 0) { @ main.c:96
15: (6d) if r7 s> r0 goto pc+6 ; R0_w=scalar(smin=0,umax=0x7fffffffffffffff,var_off=(0x0; 0x7fffffffffffffff)) R7=0
; u64 ip = PT_REGS_IP(ctx); @ main.c:97
16: (79) r1 = *(u64 *)(r6 +16) ; R1_w=scalar() R6=ctx()
; page_offset = PAGE_OFFSET; @ main.c:114
17: (18) r2 = 0x0 ; R2_w=0
19: (79) r2 = *(u64 *)(r2 +16)
R2 invalid mem access 'scalar'
processed 18 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 1

Traceback (most recent call last):
File "/usr/share/bcc/tools/funcslower", line 266, in
b.attach_kprobe(event=function, fn_name="trace_%d" % i)
File "/usr/lib/python3.12/site-packages/bcc/init.py", line 877, in attach_kprobe
fn = self.load_func(fn_name, BPF.KPROBE)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/site-packages/bcc/init.py", line 552, in load_func
raise Exception("Failed to load BPF program %s: %s" %
Exception: Failed to load BPF program b'trace_0': Permission denied