Re: [PATCH RFC bpf-next 4/4] selftests/bpf: Fix kprobe get_func_ip tests for CONFIG_X86_KERNEL_IBT

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jul 18, 2022 at 02:09:54PM +0300, Martynas Pumputis wrote:
> 
> 
> On 7/18/22 00:43, Jiri Olsa wrote:
> > On Fri, Jul 08, 2022 at 12:16:35AM +0200, Jiri Olsa wrote:
> > > On Tue, Jul 05, 2022 at 10:29:17PM -0700, Andrii Nakryiko wrote:
> > > > On Tue, Jul 5, 2022 at 12:04 PM Jiri Olsa <jolsa@kernel.org> wrote:
> > > > > 
> > > > > The kprobe can be placed anywhere and user must be aware
> > > > > of the underlying instructions. Therefore fixing just
> > > > > the bpf program to 'fix' the address to match the actual
> > > > > function address when CONFIG_X86_KERNEL_IBT is enabled.
> > > > > 
> > > > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > > > ---
> > > > >   tools/testing/selftests/bpf/progs/get_func_ip_test.c | 7 +++++--
> > > > >   1 file changed, 5 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/tools/testing/selftests/bpf/progs/get_func_ip_test.c b/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > > > index a587aeca5ae0..220d56b7c1dc 100644
> > > > > --- a/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > > > +++ b/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > > > @@ -2,6 +2,7 @@
> > > > >   #include <linux/bpf.h>
> > > > >   #include <bpf/bpf_helpers.h>
> > > > >   #include <bpf/bpf_tracing.h>
> > > > > +#include <stdbool.h>
> > > > > 
> > > > >   char _license[] SEC("license") = "GPL";
> > > > > 
> > > > > @@ -13,6 +14,8 @@ extern const void bpf_modify_return_test __ksym;
> > > > >   extern const void bpf_fentry_test6 __ksym;
> > > > >   extern const void bpf_fentry_test7 __ksym;
> > > > > 
> > > > > +extern bool CONFIG_X86_KERNEL_IBT __kconfig __weak;
> > > > > +
> > > > >   __u64 test1_result = 0;
> > > > >   SEC("fentry/bpf_fentry_test1")
> > > > >   int BPF_PROG(test1, int a)
> > > > > @@ -37,7 +40,7 @@ __u64 test3_result = 0;
> > > > >   SEC("kprobe/bpf_fentry_test3")
> > > > >   int test3(struct pt_regs *ctx)
> > > > >   {
> > > > > -       __u64 addr = bpf_get_func_ip(ctx);
> > > > > +       __u64 addr = bpf_get_func_ip(ctx) - (CONFIG_X86_KERNEL_IBT ? 4 : 0);
> > > > 
> > > > so for kprobe bpf_get_func_ip() gets an address with 5 byte
> > > > compensation for `call __fentry__`, but not for endr? Why can't we
> > > > compensate for endbr inside the kernel code as well? I'd imagine we
> > > > either do no compensation (and thus we get &bpf_fentry_test3+5 or
> > > > &bpf_fentry_test3+9, depending on CONFIG_X86_KERNEL_IBT) or full
> > > > compensation (and thus always get &bpf_fentry_test3), but this
> > > > in-between solution seems to be the worst of both worlds?...
> > > 
> > > hm rigth, I guess we should be able to do that in bpf_get_func_ip,
> > > I'll check
> > 
> > sorry for late follow up..
> > 
> > so the problem is that you can place kprobe anywhere in the function
> > (on instruction boundary) but the IBT adjustment of kprobe address is
> > made only if it's at the function entry and there's endbr instruction
> 
> To add more fun to the issue, not all non-inlined functions get endbr64. For
> example "skb_release_head_state()" does, while "skb_free_head()" doesn't.

ah great.. thanks for info, will check

jirka

> 
> > 
> > and that kprobe address is what we return in helper:
> > 
> >    BPF_CALL_1(bpf_get_func_ip_kprobe, struct pt_regs *, regs)
> >    {
> >          struct kprobe *kp = kprobe_running();
> > 
> >          return kp ? (uintptr_t)kp->addr : 0;
> >    }
> > 
> > so the adjustment would work only for address at function entry, but
> > would be wrong for address within the function
> > 
> > perhaps we could add flag to kprobe to indicate the addr adjustment
> > was done and use it in helper
> > 
> > but that's why I thought I'd keep bpf_get_func_ip_kprobe as it and
> > leave it up to user
> > 
> > kprobe_multi and trampolines are different, because they can be
> > only at the function entry, so we can adjust the ip properly
> > 
> > jirka