Re: [PATCH 1/8] bpf: Add support to attach kprobe program with fprobe

Jiri Olsa
2022-02-08T08:56:10Z

On Mon, Feb 07, 2022 at 10:59:14AM -0800, Andrii Nakryiko wrote:
> On Wed, Feb 2, 2022 at 5:53 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > Adding new link type BPF_LINK_TYPE_FPROBE that attaches kprobe program
> > through fprobe API.
> >
> > The fprobe API allows to attach probe on multiple functions at once very
> > fast, because it works on top of ftrace. On the other hand this limits
> > the probe point to the function entry or return.
> >
> > The kprobe program gets the same pt_regs input ctx as when it's attached
> > through the perf API.
> >
> > Adding new attach type BPF_TRACE_FPROBE that enables such link for kprobe
> > program.
> >
> > User provides array of addresses or symbols with count to attach the kprobe
> > program to. The new link_create uapi interface looks like:
> >
> > struct {
> > __aligned_u64 syms;
> > __aligned_u64 addrs;
> > __u32 cnt;
> > __u32 flags;
> > } fprobe;
> >
> > The flags field allows single BPF_F_FPROBE_RETURN bit to create return fprobe.
> >
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > include/linux/bpf_types.h | 1 +
> > include/uapi/linux/bpf.h | 13 ++
> > kernel/bpf/syscall.c | 248 ++++++++++++++++++++++++++++++++-
> > tools/include/uapi/linux/bpf.h | 13 ++
> > 4 files changed, 270 insertions(+), 5 deletions(-)
> >
>
> [...]
>
> >
> > +#ifdef CONFIG_FPROBE
> > +
> > +struct bpf_fprobe_link {
> > + struct bpf_link link;
> > + struct fprobe fp;
> > + unsigned long *addrs;
> > +};
> > +
> > +static void bpf_fprobe_link_release(struct bpf_link *link)
> > +{
> > + struct bpf_fprobe_link *fprobe_link;
> > +
> > + fprobe_link = container_of(link, struct bpf_fprobe_link, link);
> > + unregister_fprobe(&fprobe_link->fp);
> > +}
> > +
> > +static void bpf_fprobe_link_dealloc(struct bpf_link *link)
> > +{
> > + struct bpf_fprobe_link *fprobe_link;
> > +
> > + fprobe_link = container_of(link, struct bpf_fprobe_link, link);
> > + kfree(fprobe_link->addrs);
> > + kfree(fprobe_link);
> > +}
> > +
> > +static const struct bpf_link_ops bpf_fprobe_link_lops = {
> > + .release = bpf_fprobe_link_release,
> > + .dealloc = bpf_fprobe_link_dealloc,
> > +};
> > +
>
> should this whole new link implementation (including
> fprobe_link_prog_run() below) maybe live in kernel/trace/bpf_trace.c?
> Seems a bit more fitting than kernel/bpf/syscall.c

right, it's trace related

>
> > +static int fprobe_link_prog_run(struct bpf_fprobe_link *fprobe_link,
> > + struct pt_regs *regs)
> > +{
> > + int err;
> > +
> > + if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> > + err = 0;
> > + goto out;
> > + }
> > +
> > + rcu_read_lock();
> > + migrate_disable();
> > + err = bpf_prog_run(fprobe_link->link.prog, regs);
> > + migrate_enable();
> > + rcu_read_unlock();
> > +
> > + out:
> > + __this_cpu_dec(bpf_prog_active);
> > + return err;
> > +}
> > +
> > +static void fprobe_link_entry_handler(struct fprobe *fp, unsigned long entry_ip,
> > + struct pt_regs *regs)
> > +{
> > + unsigned long saved_ip = instruction_pointer(regs);
> > + struct bpf_fprobe_link *fprobe_link;
> > +
> > + /*
> > + * Because fprobe's regs->ip is set to the next instruction of
> > + * dynamic-ftrace insturction, correct entry ip must be set, so
> > + * that the bpf program can access entry address via regs as same
> > + * as kprobes.
> > + */
> > + instruction_pointer_set(regs, entry_ip);
> > +
> > + fprobe_link = container_of(fp, struct bpf_fprobe_link, fp);
> > + fprobe_link_prog_run(fprobe_link, regs);
> > +
> > + instruction_pointer_set(regs, saved_ip);
> > +}
> > +
> > +static void fprobe_link_exit_handler(struct fprobe *fp, unsigned long entry_ip,
> > + struct pt_regs *regs)
>
> isn't it identical to fprobe_lnk_entry_handler? Maybe use one callback
> for both entry and exit?

heh, did not notice that :) yep, looks that way, will check

>
> > +{
> > + unsigned long saved_ip = instruction_pointer(regs);
> > + struct bpf_fprobe_link *fprobe_link;
> > +
> > + instruction_pointer_set(regs, entry_ip);
> > +
> > + fprobe_link = container_of(fp, struct bpf_fprobe_link, fp);
> > + fprobe_link_prog_run(fprobe_link, regs);
> > +
> > + instruction_pointer_set(regs, saved_ip);
> > +}
> > +
> > +static int fprobe_resolve_syms(const void *usyms, u32 cnt,
> > + unsigned long *addrs)
> > +{
> > + unsigned long addr, size;
> > + const char **syms;
> > + int err = -ENOMEM;
> > + unsigned int i;
> > + char *func;
> > +
> > + size = cnt * sizeof(*syms);
> > + syms = kzalloc(size, GFP_KERNEL);
>
> any reason not to use kvzalloc() here?

probably just my ignorance ;-) will check

>
> > + if (!syms)
> > + return -ENOMEM;
> > +
>
> [...]
>
> > +
> > +static int bpf_fprobe_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> > +{
> > + struct bpf_fprobe_link *link = NULL;
> > + struct bpf_link_primer link_primer;
> > + unsigned long *addrs;
> > + u32 flags, cnt, size;
> > + void __user *uaddrs;
> > + void __user *usyms;
> > + int err;
> > +
> > + /* no support for 32bit archs yet */
> > + if (sizeof(u64) != sizeof(void *))
> > + return -EINVAL;
>
> -EOPNOTSUPP?

ok

>
> > +
> > + if (prog->expected_attach_type != BPF_TRACE_FPROBE)
> > + return -EINVAL;
> > +
> > + flags = attr->link_create.fprobe.flags;
> > + if (flags & ~BPF_F_FPROBE_RETURN)
> > + return -EINVAL;
> > +
> > + uaddrs = u64_to_user_ptr(attr->link_create.fprobe.addrs);
> > + usyms = u64_to_user_ptr(attr->link_create.fprobe.syms);
> > + if ((!uaddrs && !usyms) || (uaddrs && usyms))
> > + return -EINVAL;
>
> !!uaddrs == !!usyms ?

ah right, will change

>
> > +
> > + cnt = attr->link_create.fprobe.cnt;
> > + if (!cnt)
> > + return -EINVAL;
> > +
> > + size = cnt * sizeof(*addrs);
> > + addrs = kzalloc(size, GFP_KERNEL);
>
> same, why not kvzalloc? Also, aren't you overwriting each addrs entry
> anyway, so "z" is not necessary, right?

true, no need for zeroing

thanks,
jirka