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Message-ID: <CAEf4Bzab37Az_Aw4VJ8quenkec0YguSfALAH83mLKf3oh1f3aw@mail.gmail.com>
Date: Tue, 5 Jul 2022 21:48:55 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Alan Maguire <alan.maguire@...cle.com>,
Alexei Starovoitov <ast@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
john fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
David Vernet <void@...ifault.com>, swboyd@...omium.org,
Nick Desaulniers <ndesaulniers@...gle.com>,
Dmitrii Dolgov <9erthalion6@...il.com>,
Kenny Yu <kennyyu@...com>,
Geliang Tang <geliang.tang@...e.com>,
Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
bpf <bpf@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/2] bpf: add a ksym BPF iterator
On Tue, Jul 5, 2022 at 9:44 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Jul 1, 2022 at 10:58 PM Yonghong Song <yhs@...com> wrote:
> >
> >
> >
> > On 7/1/22 2:26 AM, Alan Maguire wrote:
> > > add a "ksym" iterator which provides access to a "struct kallsym_iter"
> > > for each symbol. Intent is to support more flexible symbol parsing
> > > as discussed in [1].
> > >
> > > [1] https://lore.kernel.org/all/YjRPZj6Z8vuLeEZo@krava/
> > >
> > > Suggested-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> > > Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> > > ---
> > > kernel/kallsyms.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 89 insertions(+)
> > >
> > > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > > index fbdf8d3..8b662da 100644
> > > --- a/kernel/kallsyms.c
> > > +++ b/kernel/kallsyms.c
> > > @@ -30,6 +30,7 @@
> > > #include <linux/module.h>
> > > #include <linux/kernel.h>
> > > #include <linux/bsearch.h>
> > > +#include <linux/btf_ids.h>
> > >
> > > /*
> > > * These will be re-linked against their real values
> > > @@ -799,6 +800,91 @@ static int s_show(struct seq_file *m, void *p)
> > > .show = s_show
> > > };
> > >
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > +
> > > +struct bpf_iter__ksym {
> > > + __bpf_md_ptr(struct bpf_iter_meta *, meta);
> > > + __bpf_md_ptr(struct kallsym_iter *, ksym);
> > > +};
> > > +
> > > +static int ksym_prog_seq_show(struct seq_file *m, bool in_stop)
> > > +{
> > > + struct bpf_iter__ksym ctx;
> > > + struct bpf_iter_meta meta;
> > > + struct bpf_prog *prog;
> > > +
> > > + meta.seq = m;
> > > + prog = bpf_iter_get_info(&meta, in_stop);
> > > + if (!prog)
> > > + return 0;
> > > +
> > > + ctx.meta = &meta;
> > > + ctx.ksym = m ? m->private : NULL;
> > > + return bpf_iter_run_prog(prog, &ctx);
> > > +}
> > > +
> > > +static int bpf_iter_ksym_seq_show(struct seq_file *m, void *p)
> > > +{
> > > + return ksym_prog_seq_show(m, false);
> > > +}
> > > +
> > > +static void bpf_iter_ksym_seq_stop(struct seq_file *m, void *p)
> > > +{
> > > + if (!p)
> > > + (void) ksym_prog_seq_show(m, true);
> > > + else
> > > + s_stop(m, p);
> > > +}
> > > +
> > > +static const struct seq_operations bpf_iter_ksym_ops = {
> > > + .start = s_start,
> > > + .next = s_next,
> > > + .stop = bpf_iter_ksym_seq_stop,
> > > + .show = bpf_iter_ksym_seq_show,
> > > +};
> > > +
> > > +static int bpf_iter_ksym_init(void *priv_data, struct bpf_iter_aux_info *aux)
> > > +{
> > > + struct kallsym_iter *iter = priv_data;
> > > +
> > > + reset_iter(iter, 0);
> > > +
> > > + iter->show_value = true;
> >
> > I think instead of always having show_value = true, we should have
> > iter->show_value = kallsyms_show_value(...);
> >
> > this is consistent with what `cat /proc/kallsyms` is doing, and
> > also consistent with bpf_dump_raw_ok() used when dumping various
> > kernel info in syscall.c.
> >
> > We don't have a file here, so credential can be from the current
> > process with current_cred().
>
> This seems wrong to use current_cred(). show_value is used to not
> "leak" pointer values to unprivileged user-space, right? In our case
> BPF iterator is privileged, so there is no need to hide (or mangle,
> didn't check) values.
>
> If it happens that a privileged process loads iter/ksym program and
> then passes prog FD to unprivileged one to read iterator output,
> iter/ksym should still get correct symbol values.
>
> I think the initial approach with show_value = true is the right one
> -- give all the information as it is to BPF iterator.
Ok, I should have looked at the selftest first. Seems like this just
passes indicator to iter/ksym program and program can choose to ignore
it, if necessary. In which case I retract my comment, sorry :)
>
>
> >
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +DEFINE_BPF_ITER_FUNC(ksym, struct bpf_iter_meta *meta, struct kallsym_iter *ksym)
> > > +
> > > +static const struct bpf_iter_seq_info ksym_iter_seq_info = {
> > > + .seq_ops = &bpf_iter_ksym_ops,
> > > + .init_seq_private = bpf_iter_ksym_init,
> > > + .fini_seq_private = NULL,
> > > + .seq_priv_size = sizeof(struct kallsym_iter),
> > > +};
> > > +
> > > +static struct bpf_iter_reg ksym_iter_reg_info = {
> > > + .target = "ksym",
> > > + .ctx_arg_info_size = 1,
> > > + .ctx_arg_info = {
> > > + { offsetof(struct bpf_iter__ksym, ksym),
> > > + PTR_TO_BTF_ID_OR_NULL },
> > > + },
> > > + .seq_info = &ksym_iter_seq_info,
> > > +};
> > > +
> > > +BTF_ID_LIST(btf_ksym_iter_id)
> > > +BTF_ID(struct, kallsym_iter)
> > > +
> > > +static void __init bpf_ksym_iter_register(void)
> > > +{
> > > + ksym_iter_reg_info.ctx_arg_info[0].btf_id = *btf_ksym_iter_id;
> > > + if (bpf_iter_reg_target(&ksym_iter_reg_info))
> > > + pr_warn("Warning: could not register bpf ksym iterator\n");
> > > +}
> > > +
> > > +#endif /* CONFIG_BPF_SYSCALL */
> > > +
> > > static inline int kallsyms_for_perf(void)
> > > {
> > > #ifdef CONFIG_PERF_EVENTS
> > > @@ -885,6 +971,9 @@ const char *kdb_walk_kallsyms(loff_t *pos)
> > > static int __init kallsyms_init(void)
> > > {
> > > proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> > > +#if defined(CONFIG_BPF_SYSCALL)
> > > + bpf_ksym_iter_register();
> >
> > You can inline this function here and if bpf_iter_reg_target(...)
> > failed, just return the error code.
> >
> > > +#endif
> > > return 0;
> > > }
> > > device_initcall(kallsyms_init);
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