netdev - Re: [PATCH RFC bpf-next 4/4] selftests/bpf: Fix kprobe get_func_ip tests for CONFIG_X86_KERNEL

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Date:   Tue, 19 Jul 2022 10:24:12 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Martynas Pumputis <m@...bda.lt>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Yutaro Hayakawa <yutaro.hayakawa@...valent.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC bpf-next 4/4] selftests/bpf: Fix kprobe get_func_ip
 tests for CONFIG_X86_KERNEL_IBT

On Mon, Jul 18, 2022 at 02:48:46PM +0200, Jiri Olsa wrote:
> 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@...nel.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@...nel.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

I checked with Peter and yes the endbr does not need to be there

<peterz> IBT is 'Indirect Branch Tracking' ENDBR needs to be at the target for "JMP *%reg" and "CALL *%reg"
<peterz> direct call/jmp don't need anything specal

so we will need to hold the +4 info somewhere for each address
and use that in get_func_ip helper or perhaps we could read
previous instruction and check if the previous instruction is
endbr with check like:

	if (is_endbr(*(u32 *)(addr - 4)))
		addr -= 4

jirka