netdev - Re: [PATCH bpf-next 1/4] samples: bpf: fix bpf programs with kprobe/sys

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Date:   Thu, 2 Jul 2020 20:13:47 +0900
From:   "Daniel T. Lee" <danieltimlee@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Andrii Nakryiko <andriin@...com>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] samples: bpf: fix bpf programs with
 kprobe/sys_connect event

On Thu, Jul 2, 2020 at 2:13 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 7/1/20 7:16 PM, Daniel T. Lee wrote:
> > Currently, BPF programs with kprobe/sys_connect does not work properly.
> >
> > Commit 34745aed515c ("samples/bpf: fix kprobe attachment issue on x64")
> > This commit modifies the bpf_load behavior of kprobe events in the x64
> > architecture. If the current kprobe event target starts with "sys_*",
> > add the prefix "__x64_" to the front of the event.
> >
> > Appending "__x64_" prefix with kprobe/sys_* event was appropriate as a
> > solution to most of the problems caused by the commit below.
> >
> >      commit d5a00528b58c ("syscalls/core, syscalls/x86: Rename struct
> >      pt_regs-based sys_*() to __x64_sys_*()")
> >
> > However, there is a problem with the sys_connect kprobe event that does
> > not work properly. For __sys_connect event, parameters can be fetched
> > normally, but for __x64_sys_connect, parameters cannot be fetched.
> >
> > Because of this problem, this commit fixes the sys_connect event by
> > specifying the __sys_connect directly and this will bypass the
> > "__x64_" appending rule of bpf_load.
>
> In the kernel code, we have
>
> SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr,
>                  int, addrlen)
> {
>          return __sys_connect(fd, uservaddr, addrlen);
> }
>
> Depending on compiler, there is no guarantee that __sys_connect will
> not be inlined. I would prefer to still use the entry point
> __x64_sys_* e.g.,
>     SEC("kprobe/" SYSCALL(sys_write))
>

As you mentioned, there is clearly a possibility that problems may arise
because the symbol does not exist according to the compiler.

However, in x64, when using Kprobe for __x64_sys_connect event, the
tests are not working properly because the parameters cannot be fetched,
and the test under selftests/bpf is using "kprobe/_sys_connect" directly.

I'm not sure how to deal with this problem. Any advice and suggestions
will be greatly appreciated.

Thanks for your time and effort for the review.
Daniel

> >
> > Fixes: 34745aed515c ("samples/bpf: fix kprobe attachment issue on x64")
> > Signed-off-by: Daniel T. Lee <danieltimlee@...il.com>
> > ---
> >   samples/bpf/map_perf_test_kern.c         | 2 +-
> >   samples/bpf/test_map_in_map_kern.c       | 2 +-
> >   samples/bpf/test_probe_write_user_kern.c | 2 +-
> >   3 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/samples/bpf/map_perf_test_kern.c b/samples/bpf/map_perf_test_kern.c
> > index 12e91ae64d4d..cebe2098bb24 100644
> > --- a/samples/bpf/map_perf_test_kern.c
> > +++ b/samples/bpf/map_perf_test_kern.c
> > @@ -154,7 +154,7 @@ int stress_percpu_hmap_alloc(struct pt_regs *ctx)
> >       return 0;
> >   }
> >
> > -SEC("kprobe/sys_connect")
> > +SEC("kprobe/__sys_connect")
> >   int stress_lru_hmap_alloc(struct pt_regs *ctx)
> >   {
> >       char fmt[] = "Failed at stress_lru_hmap_alloc. ret:%dn";
> > diff --git a/samples/bpf/test_map_in_map_kern.c b/samples/bpf/test_map_in_map_kern.c
> > index 6cee61e8ce9b..b1562ba2f025 100644
> > --- a/samples/bpf/test_map_in_map_kern.c
> > +++ b/samples/bpf/test_map_in_map_kern.c
> > @@ -102,7 +102,7 @@ static __always_inline int do_inline_hash_lookup(void *inner_map, u32 port)
> >       return result ? *result : -ENOENT;
> >   }
> >
> > -SEC("kprobe/sys_connect")
> > +SEC("kprobe/__sys_connect")
> >   int trace_sys_connect(struct pt_regs *ctx)
> >   {
> >       struct sockaddr_in6 *in6;
> > diff --git a/samples/bpf/test_probe_write_user_kern.c b/samples/bpf/test_probe_write_user_kern.c
> > index 6579639a83b2..9b3c3918c37d 100644
> > --- a/samples/bpf/test_probe_write_user_kern.c
> > +++ b/samples/bpf/test_probe_write_user_kern.c
> > @@ -26,7 +26,7 @@ struct {
> >    * This example sits on a syscall, and the syscall ABI is relatively stable
> >    * of course, across platforms, and over time, the ABI may change.
> >    */
> > -SEC("kprobe/sys_connect")
> > +SEC("kprobe/__sys_connect")
> >   int bpf_prog1(struct pt_regs *ctx)
> >   {
> >       struct sockaddr_in new_addr, orig_addr = {};
> >