netdev - Re: [PATCH 02/13] kprobe: Keep traced function address

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Date:   Wed, 5 Jan 2022 20:30:48 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        lkml <linux-kernel@...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>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 02/13] kprobe: Keep traced function address

On Tue, Jan 4, 2022 at 12:10 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> The bpf_get_func_ip_kprobe helper should return traced function
> address, but it's doing so only for kprobes that are placed on
> the function entry.
>
> If kprobe is placed within the function, bpf_get_func_ip_kprobe
> returns that address instead of function entry.
>
> Storing the function entry directly in kprobe object, so it could
> be used in bpf_get_func_ip_kprobe helper.
>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  include/linux/kprobes.h                              |  3 +++
>  kernel/kprobes.c                                     | 12 ++++++++++++
>  kernel/trace/bpf_trace.c                             |  2 +-
>  tools/testing/selftests/bpf/progs/get_func_ip_test.c |  4 ++--
>  4 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index 8c8f7a4d93af..a204df4fef96 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -74,6 +74,9 @@ struct kprobe {
>         /* Offset into the symbol */
>         unsigned int offset;
>
> +       /* traced function address */
> +       unsigned long func_addr;
> +

keep in mind that we'll also need (maybe in a follow up series) to
store bpf_cookie somewhere close to this func_addr as well. Just
mentioning to keep in mind as you decide with Masami where to put it.


>         /* Called before addr is executed. */
>         kprobe_pre_handler_t pre_handler;
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index d20ae8232835..c4060a8da050 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1310,6 +1310,7 @@ static void init_aggr_kprobe(struct kprobe *ap, struct kprobe *p)
>         copy_kprobe(p, ap);
>         flush_insn_slot(ap);
>         ap->addr = p->addr;
> +       ap->func_addr = p->func_addr;
>         ap->flags = p->flags & ~KPROBE_FLAG_OPTIMIZED;
>         ap->pre_handler = aggr_pre_handler;
>         /* We don't care the kprobe which has gone. */
> @@ -1588,6 +1589,16 @@ static int check_kprobe_address_safe(struct kprobe *p,
>         return ret;
>  }
>
> +static unsigned long resolve_func_addr(kprobe_opcode_t *addr)
> +{
> +       char str[KSYM_SYMBOL_LEN];
> +       unsigned long offset;
> +
> +       if (kallsyms_lookup((unsigned long) addr, NULL, &offset, NULL, str))
> +               return (unsigned long) addr - offset;
> +       return 0;
> +}
> +
>  int register_kprobe(struct kprobe *p)
>  {
>         int ret;
> @@ -1600,6 +1611,7 @@ int register_kprobe(struct kprobe *p)
>         if (IS_ERR(addr))
>                 return PTR_ERR(addr);
>         p->addr = addr;
> +       p->func_addr = resolve_func_addr(addr);
>
>         ret = warn_kprobe_rereg(p);
>         if (ret)
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 21aa30644219..25631253084a 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1026,7 +1026,7 @@ BPF_CALL_1(bpf_get_func_ip_kprobe, struct pt_regs *, regs)
>  {
>         struct kprobe *kp = kprobe_running();
>
> -       return kp ? (uintptr_t)kp->addr : 0;
> +       return kp ? (uintptr_t)kp->func_addr : 0;
>  }
>
>  static const struct bpf_func_proto bpf_get_func_ip_proto_kprobe = {
> 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..e988aefa567e 100644
> --- a/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> +++ b/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> @@ -69,7 +69,7 @@ int test6(struct pt_regs *ctx)
>  {
>         __u64 addr = bpf_get_func_ip(ctx);
>
> -       test6_result = (const void *) addr == &bpf_fentry_test6 + 5;
> +       test6_result = (const void *) addr == &bpf_fentry_test6;
>         return 0;
>  }
>
> @@ -79,6 +79,6 @@ int test7(struct pt_regs *ctx)
>  {
>         __u64 addr = bpf_get_func_ip(ctx);
>
> -       test7_result = (const void *) addr == &bpf_fentry_test7 + 5;
> +       test7_result = (const void *) addr == &bpf_fentry_test7;

we can treat this as a bug fix for bpf_get_func_ip() for kprobes,
right? I think "Fixes: " tag is in order then.


>         return 0;
>  }

> --
> 2.33.1
>