Re: Fwd: struct pt_regs missing in /usr/include/ tree for eBPF program compile

[Daniel Borkmann]

From: Thomas-Mich Richter <tmricht@xxxxxxxxxxxxxxxxxx>
Date: Wed, Aug 2, 2017 at 1:22 AM
[...]
> I work on the perf tool and its bpf support for IBM s390 and came across a
> strange issue compiling tools/testing/selftests/bpf/test_verifier.c on s390x.
>
> This is the compile error:
> gcc -Wall -O2 -I../../../include/uapi -I../../../lib
> -I../../../../include/generated
>    -DHAVE_GENHDR -I../../../include    test_verifier.c
>    /root/linux-devel/tools/testing/selftests/bpf/libbpf.a -lcap -lelf -o
>    /root/linux-devel/tools/testing/selftests/bpf/test_verifier
> In file included from test_verifier.c:63:0:
> ../../../include/uapi/linux/bpf_perf_event.h:14:17: error: field âregsâ has
>    incomplete type struct pt_regs regs;

I actually came across the same issue today on s390
while testing for something else.

> This shows up in test case "unpriv: spill/fill of different pointers ldx"
> at line 1811.
> This issue is located in file /usr/include/linux/bpf_perf_event.h which is a
> copy of the linux kernels include/uapi/linux/bpf_perf_event.h.
>
> It contains:
> struct bpf_perf_event_data {
>          struct pt_regs regs;
>          __u64 sample_period;
> };

Yeah, correct.

> On s390 struct pt_regs is not exported to user space and does not appear
> anywhere in /usr/include.
> How about other architectures beside Intel?
> As far as I know
> 1. the struct pt_regs contains only kernel registers, no user space registers?
> 2. Is part of the kernel API and should not be exported at all?

Looking into the tree, it appears that the following archs
export a definition of struct pt_regs as uapi typically in
arch/*/include/uapi/asm/ptrace.h: x86, sparc, power, mips,
microblaze, alpha, unicore32, parisc, score, sh, mn10300,
tile, m68k, m32r, ia64, hexagon, h8300, frv, cris, c6x.
And for these I couldn't find it: s390, arc, arm64, nios2.

Anyone knows if there's any guidance on whether they must
be exported or not? It appears at least the majority of
archs is exporting them in one way or another.

Looking at 2dbb4c05d048 ("bpf/samples: Fix PT_REGS_IP on
s390x and use it") and d912557b3460 ("samples: bpf: enable
trace samples for s390x"), this was added by Michael for
the programs themselves, which were using kernel headers
for walking structs in BPF tracing programs, so a bit
unrelated to the uapi issue actually, but given the
test_verifier has couple of test cases containing pt_regs,
they should probably do the same thing and be using kernel
headers given tracing programs inspect kernel-internal
data structures typically (see BPF tracing samples).

Now, I would like to avoid going down that road to pull
in kernel internal headers into test_verifier.c, could
we instead add a bpf_ptregs.h helper in tools/testing/selftests/bpf/,
where s390 and arm64 would put a definition to fallback when
otherwise not available? Admittedly, it's a bit of a hack
if exporting them is not an option, but 'normal' tracing
progs would consult kernel headers anyway. Thoughts?

> When I investigated the kernel side of the bpf() system call, the test case ends
> up in functions pe_prog_is_valid_access() and pe_prog_convert_ctx_access()
> via syscall(bpf)
>      +--> bpf_prog_load()
>           +--> find_prog_type() to load eBPF type specific verifiers
>           |       pe_prog_is_valid_access() and pe_prog_convert_ctx_access()
>           +--> bpf_check() to verify (and modify) the eBPF
>                +--> check_vfg()
>                     +--> do_check()
>                          +--> check_xadd()
>                               +--> check_mem_access()
>                                    +--> check_ctx_access()
>                                         +--> env->prog->aux->ops->is_valid_access
>                                              which is set to
>                                              pe_prog_is_valid_access()
>
> Now this last function expects and verifies struct pt_regs via struct member
> offsets which needs a correct struct pt_regs previously setup by user space
> eBPF program.
>
> This also requires a correct struct pt_regs in
> /usr/include/linux/bpf_perf_event.h
> (which includes /usr/include/{linux,asm,sym}/ptrace.h
>
> How to achieve this on a platform which does not export struct pt_regs to the
> user?
>
> Thanks a lot for your help.
>
> --
> Thomas Richter, Dept 3303, IBM LTC Boeblingen Germany
> --
> Vorsitzende des Aufsichtsrats: Martina Koederitz
> GeschÃftsfÃhrung: Dirk Wittkopp
> Sitz der Gesellschaft: BÃblingen / Registergericht: Amtsgericht
> Stuttgart, HRB 243294