netdev - Re: pull-request: bpf-next 2022-03-21

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Date:   Mon, 21 Mar 2022 21:35:55 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: pull-request: bpf-next 2022-03-21

On Mon, Mar 21, 2022 at 7:36 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> Hi Linus and Alexei,
>
> At first, sorry about this issue. I missed to Cc'ed to arch maintainers.
>
> On Mon, 21 Mar 2022 17:31:28 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>
> > On Mon, Mar 21, 2022 at 4:59 PM Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > >
> > > On Mon, Mar 21, 2022 at 4:11 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > Did you look at the code?
> > > > In particular:
> > > > https://lore.kernel.org/bpf/164735286243.1084943.7477055110527046644.stgit@devnote2/
> > > >
> > > > it's a copy paste of arch/x86/kernel/kprobes/core.c
> > > >
> > > > How is it "bad architecture code" ?
> > >
> > > It's "bad architecture code" because the architecture maintainers have
> > > made changes to check ENDBR in the meantime.
> > >
> > > So it used to be perfectly fine. It's not any longer - and the
> > > architecture maintainers were clearly never actually cc'd on the
> > > changes, so they didn't find out until much too late.
>
> Let me retry porting fprobe on top of ENDBR things and confirm with
> arch maintainers.

Just look at linux-next.
objtool warning is the only issue.

> >
> > Not denying that missing cc was an issue.
> >
> > We can drop just arch patches:
> >       rethook: x86: Add rethook x86 implementation
> >       arm64: rethook: Add arm64 rethook implementation
> >       powerpc: Add rethook support
> >       ARM: rethook: Add rethook arm implementation
> >
> > or everything including Jiri's work on top of it.
> > Which would be a massive 27 patches.
> >
> > We'd prefer the former, of course.
> > Later during the merge window we can add a single
> > 'rethook: x86' patch that takes endbr into account,
> > so that multi-kprobe feature will work on x86.
> > For the next merge window we can add other archs.
> > Would that work?
>
> BTW, As far as I can see the ENDBR things, the major issue on fprobe
> is that the ftrace'ed ip address will be different from the symbol
> address (even) on x86. That must be ensured to work before merge.
> Let me check it on Linus's tree at first.

That's not an issue. Peter tweaked ftrace logic and fprobe plugs
into that.
The fprobe/multi-kprobe works fine in linux-next.

bpf selftest for multi kprobe needs this hack:
diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi.c
b/tools/testing/selftests/bpf/progs/kprobe_multi.c
index af27d2c6fce8..530a64e2996a 100644
--- a/tools/testing/selftests/bpf/progs/kprobe_multi.c
+++ b/tools/testing/selftests/bpf/progs/kprobe_multi.c
@@ -45,7 +45,7 @@ static void kprobe_multi_check(void *ctx, bool is_return)
        __u64 addr = bpf_get_func_ip(ctx);

 #define SET(__var, __addr, __cookie) ({                        \
-       if (((const void *) addr == __addr) &&          \
+       if (((const void *) addr == __addr + 4) &&              \
             (!test_cookie || (cookie == __cookie)))    \

to pass when both CONFIG_FPROBE=y and CONFIG_X86_KERNEL_IBT=y.
The test is too strict. It didn't account for the possibility of endbr.

So I'm inclined to drop only 4 arch patches instead of the whole thing.