Re: [PATCH 05/10] bpf: Add cookie support to programs attached with kprobe multi link

Jiri Olsa
2022-03-06T17:29:19Z

On Fri, Mar 04, 2022 at 03:11:08PM -0800, Andrii Nakryiko wrote:
> On Tue, Feb 22, 2022 at 9:07 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Adding support to call bpf_get_attach_cookie helper from
> > kprobe programs attached with kprobe multi link.
> >
> > The cookie is provided by array of u64 values, where each
> > value is paired with provided function address or symbol
> > with the same array index.
> >
> > Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > include/linux/sort.h | 2 +
> > include/uapi/linux/bpf.h | 1 +
> > kernel/trace/bpf_trace.c | 103 ++++++++++++++++++++++++++++++++-
> > lib/sort.c | 2 +-
> > tools/include/uapi/linux/bpf.h | 1 +
> > 5 files changed, 107 insertions(+), 2 deletions(-)
> >
>
> [...]
>
> > BPF_CALL_1(bpf_get_attach_cookie_trace, void *, ctx)
> > {
> > struct bpf_trace_run_ctx *run_ctx;
> > @@ -1297,7 +1312,9 @@ kprobe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> > &bpf_get_func_ip_proto_kprobe_multi :
> > &bpf_get_func_ip_proto_kprobe;
> > case BPF_FUNC_get_attach_cookie:
> > - return &bpf_get_attach_cookie_proto_trace;
> > + return prog->expected_attach_type == BPF_TRACE_KPROBE_MULTI ?
> > + &bpf_get_attach_cookie_proto_kmulti :
> > + &bpf_get_attach_cookie_proto_trace;
> > default:
> > return bpf_tracing_func_proto(func_id, prog);
> > }
> > @@ -2203,6 +2220,9 @@ struct bpf_kprobe_multi_link {
> > struct bpf_link link;
> > struct fprobe fp;
> > unsigned long *addrs;
> > + struct bpf_run_ctx run_ctx;
>
> clever, I like it! Keep in mind, though, that this trick can only be
> used here because this run_ctx is read-only (I'd leave the comment
> here about this, I didn't realize immediately that this approach can't
> be used for run_ctx that needs to be modified).

hum, I don't see it at the moment.. I'll check on that and add the
comment or come up with more questions ;-)

>
> > + u64 *cookies;
> > + u32 cnt;
> > };
> >
> > static void bpf_kprobe_multi_link_release(struct bpf_link *link)
> > @@ -2219,6 +2239,7 @@ static void bpf_kprobe_multi_link_dealloc(struct bpf_link *link)
> >
> > kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
> > kvfree(kmulti_link->addrs);
> > + kvfree(kmulti_link->cookies);
> > kfree(kmulti_link);
> > }
> >
> > @@ -2227,10 +2248,57 @@ static const struct bpf_link_ops bpf_kprobe_multi_link_lops = {
> > .dealloc = bpf_kprobe_multi_link_dealloc,
> > };
> >
> > +static void bpf_kprobe_multi_cookie_swap(void *a, void *b, int size, const void *priv)
> > +{
> > + const struct bpf_kprobe_multi_link *link = priv;
> > + unsigned long *addr_a = a, *addr_b = b;
> > + u64 *cookie_a, *cookie_b;
> > +
> > + cookie_a = link->cookies + (addr_a - link->addrs);
> > + cookie_b = link->cookies + (addr_b - link->addrs);
> > +
> > + swap_words_64(addr_a, addr_b, size);
> > + swap_words_64(cookie_a, cookie_b, size);
>
> is it smart to call (now) non-inlined function just to swap two longs
> and u64s?..
>
> unsigned long tmp1;
> u64 tmp2;
>
> tmp1 = *addr_a; *addr_a = addr_b; *addr_b = tmp1;
> tmp2 = *cookie_a; *cookie_a = cookie_b; *cookie_b = tmp2;

the swap_words_64 has CONFIG_64BIT ifdef with some tweaks for 32bit,
so I wanted to use that.. but I agree with your other comment below
wrt performace, so will change

>
> ?
>
> > +}
> > +
> > +static int __bpf_kprobe_multi_cookie_cmp(const void *a, const void *b)
> > +{
> > + const unsigned long *addr_a = a, *addr_b = b;
> > +
> > + if (*addr_a == *addr_b)
> > + return 0;
> > + return *addr_a < *addr_b ? -1 : 1;
> > +}
> > +
>
> [...]
>
> > @@ -2238,12 +2306,16 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> > goto out;
> > }
> >
> > + old_run_ctx = bpf_set_run_ctx(&link->run_ctx);
> > +
> > rcu_read_lock();
>
> so looking at other code, I see that we first migrate_disable() and
> then rcu_read_lock(), so let's swap? We also normally set/reset
> run_ctx inside migrate+rcu_lock region. I'm not sure that's necessary,
> but also shouldn't hurt to stay consistent.

ok, will change

>
> > migrate_disable();
> > err = bpf_prog_run(link->link.prog, regs);
> > migrate_enable();
> > rcu_read_unlock();
> >
> > + bpf_reset_run_ctx(old_run_ctx);
> > +
> > out:
> > __this_cpu_dec(bpf_prog_active);
> > return err;
>
> [...]
>
> > diff --git a/lib/sort.c b/lib/sort.c
> > index b399bf10d675..91f7ce701cf4 100644
> > --- a/lib/sort.c
> > +++ b/lib/sort.c
> > @@ -80,7 +80,7 @@ static void swap_words_32(void *a, void *b, size_t n)
> > * but it's possible to have 64-bit loads without 64-bit pointers (e.g.
> > * x32 ABI). Are there any cases the kernel needs to worry about?
> > */
> > -static void swap_words_64(void *a, void *b, size_t n)
> > +void swap_words_64(void *a, void *b, size_t n)
>
> I'm worried that this might change performance unintentionally in
> other places (making the function global might pessimize inlining, I
> think). So let's not do that, just do a straightforward swap in cookie
> support code?

right, I did not realize this.. I'll add to cookie code directly

>
> > {
> > do {
> > #ifdef CONFIG_64BIT
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index 6c66138c1b9b..d18996502aac 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -1482,6 +1482,7 @@ union bpf_attr {
> > struct {
> > __aligned_u64 syms;
> > __aligned_u64 addrs;
> > + __aligned_u64 cookies;
>
> looks a bit weird to change layout of UAPI. That's not really a
> problem, because both patches will land at the same time. But if you
> move flags and cnt to the front of the struct it would a bit better.

I was following your previous comment:
https://lore.kernel.org/bpf/CAEf4BzbPeQbURZOD93TgPudOk3JD4odsZ9uwriNkrphes9V4dg@mail.gmail.com/

I like the idea that syms/addrs/cookies stay together,
because they are all related to cnt.. but yes, it's
'breaking' KABI in between these patches

jirka

>
>
> > __u32 cnt;
> > __u32 flags;
> > } kprobe_multi;
> > --
> > 2.35.1
> >