On Thu, Jul 7, 2022 at 1:28 AM Yaniv Agman <yanivagman@xxxxxxxxx> wrote:
>
> בתאריך יום ה׳, 7 ביולי 2022 ב-3:48 מאת Andrii Nakryiko
> <andrii@xxxxxxxxxx>:
> >
> > This RFC patch set is to gather feedback about new
> > SEC("ksyscall") and SEC("kretsyscall") section definitions meant to simplify
> > life of BPF users that want to trace Linux syscalls without having to know or
> > care about things like CONFIG_ARCH_HAS_SYSCALL_WRAPPER and related arch-specific
> > vs arch-agnostic __<arch>_sys_xxx vs __se_sys_xxx function names, calling
> > convention woes ("nested" pt_regs), etc. All this is quite annoying to
> > remember and care about as BPF user, especially if the goal is to write
> > achitecture- and kernel version-agnostic BPF code (e.g., things like
> > libbpf-tools, etc).
> >
> > By using SEC("ksyscall/xxx")/SEC("kretsyscall/xxx") user clearly communicates
> > the desire to kprobe/kretprobe kernel function that corresponds to the
> > specified syscall. Libbpf will take care of all the details of determining
> > correct function name and calling conventions.
> >
> > This patch set also improves BPF_KPROBE_SYSCALL (and renames it to
> > BPF_KSYSCALL to match SEC("ksyscall")) macro to take into account
> > CONFIG_ARCH_HAS_SYSCALL_WRAPPER instead of hard-coding whether host
> > architecture is expected to use syscall wrapper or not (which is less reliable
> > and can change over time).
> >
>
> Hi Andrii,
> I would very much liked if there was such a macro, which will make
> things easier for syscall tracing,
> but I think that you can't assume that libbpf will have access to
> kconfig files all the time.
> For example, if running from a container and not mounting /boot (on
> environments where the config file is in /boot), libbpf will fail to
> load CONFIG_ARCH_HAS_SYSCALL_WRAPPER value and assume it to be not
> defined.
> Then, on any environment with a "new" kernel where the program runs
> from a container, it will return the wrong argument values.
> For this very reason we fall-back in [1] to assume
> CONFIG_ARCH_HAS_SYSCALL_WRAPPER is defined, as in most environments it
> will be.
>
> [1] https://github.com/aquasecurity/tracee/blob/0f28a2cc14b851308ebaa380d503dea9eaa67271/pkg/ebpf/initialization/kconfig.go#L37
>
I see, unfortunately without relying on
CONFIG_ARCH_HAS_SYSCALL_WRAPPER on BPF side it's hard to make this
correct in all kernel versions. One way would be to keep
BPF_KPROBE_SYSCALL as is assuming syscall wrapper for x86, s390 and
arm64, and add BPF_KSYSCALL() macro as I did here, which would depend
on __kconfig, so in your situation it won't work. SEC("ksyscall") by
itself will still work, though, if you find it useful.
> > It would be great to get feedback about the overall feature, but also I'd
> > appreciate help with testing this, especially for non-x86_64 architectures.
> >
> > Cc: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> > Cc: Kenta Tada <kenta.tada@xxxxxxxx>
> > Cc: Hengqi Chen <hengqi.chen@xxxxxxxxx>
> >
> > Andrii Nakryiko (3):
> > libbpf: improve and rename BPF_KPROBE_SYSCALL
> > libbpf: add ksyscall/kretsyscall sections support for syscall kprobes
> > selftests/bpf: use BPF_KSYSCALL and SEC("ksyscall") in selftests
> >
> > tools/lib/bpf/bpf_tracing.h | 44 +++++--
> > tools/lib/bpf/libbpf.c | 109 ++++++++++++++++++
> > tools/lib/bpf/libbpf.h | 16 +++
> > tools/lib/bpf/libbpf.map | 1 +
> > tools/lib/bpf/libbpf_internal.h | 2 +
> > .../selftests/bpf/progs/bpf_syscall_macro.c | 6 +-
> > .../selftests/bpf/progs/test_attach_probe.c | 6 +-
> > .../selftests/bpf/progs/test_probe_user.c | 27 +----
> > 8 files changed, 172 insertions(+), 39 deletions(-)
> >
> > --
> > 2.30.2
> >
