Re: [PATCH v4] libbpf: kprobe.multi: Filter with available_filter_functions

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, May 25, 2023 at 6:38 PM Jackie Liu <liu.yun@linux.dev> wrote:
>
> Hi Andrii.
>
> 在 2023/5/26 04:43, Andrii Nakryiko 写道:
> > On Thu, May 25, 2023 at 3:28 AM Jackie Liu <liu.yun@linux.dev> wrote:
> >>
> >> From: Jackie Liu <liuyun01@kylinos.cn>
> >>
> >> When using regular expression matching with "kprobe multi", it scans all
> >> the functions under "/proc/kallsyms" that can be matched. However, not all
> >> of them can be traced by kprobe.multi. If any one of the functions fails
> >> to be traced, it will result in the failure of all functions. The best
> >> approach is to filter out the functions that cannot be traced to ensure
> >> proper tracking of the functions.
> >>
> >> Use available_filter_functions check first, if failed, fallback to
> >> kallsyms.
> >>
> >> Here is the test eBPF program [1].
> >> [1] https://github.com/JackieLiu1/ketones/commit/a9e76d1ba57390e533b8b3eadde97f7a4535e867
> >>
> >> Suggested-by: Jiri Olsa <olsajiri@gmail.com>
> >> Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
> >> ---
> >>   tools/lib/bpf/libbpf.c | 92 +++++++++++++++++++++++++++++++++++++-----
> >>   1 file changed, 83 insertions(+), 9 deletions(-)
> >>
> >
> > Question to you and Jiri: what happens when multi-kprobe's syms has
> > duplicates? Will the program be attached multiple times? If yes, then
> > it sounds like a problem? Both available_filters and kallsyms can have
> > duplicate function names in them, right?
>
> If I understand correctly, there should be no problem with repeated
> function registration, because the bottom layer is done through fprobe
> registration addrs, kprobe.multi itself does not do this work, but
> fprobe is based on ftrace, it will register addr by makes a hash,
> that is, if it is the same address, it should be filtered out.
>

Looking at kernel code, it seems kernel will actually return error if
user specifies multiple duplicated names. Because kernel will
bsearch() to the first instance, and never resolve the second
duplicated instance. And then will assume that not all symbols are
resolved.

So, it worries me that we'll switch from kallsyms to available_filters
by default, because that introduces new failure modes.

Either way, let's add a selftest that uses a duplicate function name
and see what happens?

> The main problem here is not the problem of repeated registration of
> functions, but some functions are not allowed to hook. For example, when
> I track vfs_*, vfs_set_acl_prepare_kgid and vfs_set_acl_prepare_kuid are
> not allowed to hook. These exist under kallsyms, but
> available_filter_functions does not, I have observed for a while,
> matching through available_filter_functions can effectively prevent this
> from happening.

Yeah, I understand that. My point above is that a)
available_filter_functions contains duplicates and b) doesn't contain
addresses. So we are forced to rely on kernel string -> addr
resolution, which doesn't seem to handle duplicate entries well (let's
test).

So it's a regression to switch to that without taking any other precautions.

>
> >
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index ad1ec893b41b..3dd72d69cdf7 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -10417,13 +10417,14 @@ static bool glob_match(const char *str, const char *pat)
> >>   struct kprobe_multi_resolve {
> >>          const char *pattern;
> >>          unsigned long *addrs;
> >> +       const char **syms;
> >>          size_t cap;
> >>          size_t cnt;
> >>   };
> >>

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