Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

[Alexei Starovoitov]

On Mon, Jul 31, 2023 at 4:57 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Mon, 31 Jul 2023 14:59:47 -0700
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>
> > On Mon, Jul 31, 2023 at 12:30 AM Masami Hiramatsu (Google)
> > <mhiramat@xxxxxxxxxx> wrote:
> > >
> > > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > >
> > > Add btf_find_struct_member() API to search a member of a given data structure
> > > or union from the member's name.
> > >
> > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > > Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > > ---
> > >  Changes in v3:
> > >   - Remove simple input check.
> > >   - Fix unneeded IS_ERR_OR_NULL() check for btf_type_by_id().
> > >   - Move the code next to btf_get_func_param().
> > >   - Use for_each_member() macro instead of for-loop.
> > >   - Use btf_type_skip_modifiers() instead of btf_type_by_id().
> > >  Changes in v4:
> > >   - Use a stack for searching in anonymous members instead of nested call.
> > > ---
> > >  include/linux/btf.h |    3 +++
> > >  kernel/bpf/btf.c    |   40 ++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 43 insertions(+)
> > >
> > > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > > index 20e3a07eef8f..4b10d57ceee0 100644
> > > --- a/include/linux/btf.h
> > > +++ b/include/linux/btf.h
> > > @@ -226,6 +226,9 @@ const struct btf_type *btf_find_func_proto(const char *func_name,
> > >                                            struct btf **btf_p);
> > >  const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
> > >                                            s32 *nr);
> > > +const struct btf_member *btf_find_struct_member(struct btf *btf,
> > > +                                               const struct btf_type *type,
> > > +                                               const char *member_name);
> > >
> > >  #define for_each_member(i, struct_type, member)                        \
> > >         for (i = 0, member = btf_type_member(struct_type);      \
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index f7b25c615269..8d81a4ffa67b 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -958,6 +958,46 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
> > >                 return NULL;
> > >  }
> > >
> > > +#define BTF_ANON_STACK_MAX     16
> > > +
> > > +/*
> > > + * Find a member of data structure/union by name and return it.
> > > + * Return NULL if not found, or -EINVAL if parameter is invalid.
> > > + */
> > > +const struct btf_member *btf_find_struct_member(struct btf *btf,
> > > +                                               const struct btf_type *type,
> > > +                                               const char *member_name)
> > > +{
> > > +       const struct btf_type *anon_stack[BTF_ANON_STACK_MAX];
> > > +       const struct btf_member *member;
> > > +       const char *name;
> > > +       int i, top = 0;
> > > +
> > > +retry:
> > > +       if (!btf_type_is_struct(type))
> > > +               return ERR_PTR(-EINVAL);
> > > +
> > > +       for_each_member(i, type, member) {
> > > +               if (!member->name_off) {
> > > +                       /* Anonymous union/struct: push it for later use */
> > > +                       type = btf_type_skip_modifiers(btf, member->type, NULL);
> > > +                       if (type && top < BTF_ANON_STACK_MAX)
> > > +                               anon_stack[top++] = type;
> > > +               } else {
> > > +                       name = btf_name_by_offset(btf, member->name_off);
> > > +                       if (name && !strcmp(member_name, name))
> > > +                               return member;
> > > +               }
> > > +       }
> > > +       if (top > 0) {
> > > +               /* Pop from the anonymous stack and retry */
> > > +               type = anon_stack[--top];
> > > +               goto retry;
> > > +       }
> >
> > Looks good, but I don't see a test case for this.
> > The logic is a bit tricky. I'd like to have a selftest that covers it.
>
> Thanks, and I agree about selftest.
>
> >
> > You probably need to drop Alan's reviewed-by, since the patch is quite
> > different from the time he reviewed it.
>
> OK. BTW, I found a problem on this function. I guess the member->offset will
> be the offset from the intermediate anonymous union, it is usually 0, but
> I need the offset from the given structure. Thus the interface design must
> be changed. Passing a 'u32 *offset' and set the correct offset in it. If
> it has nested intermediate anonymous unions, that offset must also be pushed.

With all that piling up have you considering reusing btf_struct_walk() ?
It's doing the opposite off -> btf_id while you need name -> btf_id.
But it will give an idea of overall complexity if you want to solve it
for nested arrays and struct/union.

> >
> > Assuming that is addressed. How do we merge the series?
> > The first 3 patches have serious conflicts with bpf trees.
> >
> > Maybe send the first 3 with extra selftest for above recursion
> > targeting bpf-next then we can have a merge commit that Steven can pull
> > into tracing?
> >
> > Or if we can have acks for patches 4-9 we can pull the whole set into bpf-next.
>
> That's a good question. I don't like splitting the whole series in 2 -next
> branches. So I can send this to the bpf-next.

Works for me.

> I need to work on another series(*) on fprobes which will not have conflicts with
> this series. (*Replacing pt_regs with ftrace_regs on fprobe, which will take longer
> time, and need to adjust with eBPF).

ftrace_regs?
Ouch. For bpf we rely on pt_regs being an argument.
fprobe should be 100% compatible replacement of kprobe-at-the-func-start.
If it diverges from that it's a big issue for bpf.
We'd have to remove all of fprobe usage.
I could be missing something, of course.