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

Andrii Nakryiko
2022-01-06T04:31:02Z

On Tue, Jan 4, 2022 at 12:10 AM Jiri Olsa <jolsa@redhat.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@kernel.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
>