Date:   Fri, 2 Aug 2019 00:16:52 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Song Liu <songliubraving@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 02/12] libbpf: implement BPF CO-RE offset
 relocation algorithm

On Thu, Aug 1, 2019 at 4:50 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Jul 31, 2019 at 11:47:53PM -0700, Andrii Nakryiko wrote:
> > This patch implements the core logic for BPF CO-RE offsets relocations.
> > Every instruction that needs to be relocated has corresponding
> > bpf_offset_reloc as part of BTF.ext. Relocations are performed by trying
> > to match recorded "local" relocation spec against potentially many
> > compatible "target" types, creating corresponding spec. Details of the
> > algorithm are noted in corresponding comments in the code.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > Acked-by: Song Liu <songliubraving@...com>
> ...
> > +             if (btf_is_composite(t)) {
> > +                     const struct btf_member *m = (void *)(t + 1);
> > +                     __u32 offset;
> > +
> > +                     if (access_idx >= BTF_INFO_VLEN(t->info))
> > +                             return -EINVAL;
> > +
> > +                     m = &m[access_idx];
> > +
> > +                     if (BTF_INFO_KFLAG(t->info)) {
> > +                             if (BTF_MEMBER_BITFIELD_SIZE(m->offset))
> > +                                     return -EINVAL;
> > +                             offset = BTF_MEMBER_BIT_OFFSET(m->offset);
> > +                     } else {
> > +                             offset = m->offset;
> > +                     }
>
> very similar logic exists in btf_dump.c
> probably makes sense to make a common helper at some point.

Will add btf_member_bit_offset(type, member) and
btf_member_bit_size(type, member).

>
> > +static size_t bpf_core_essential_name_len(const char *name)
> > +{
> > +     size_t n = strlen(name);
> > +     int i = n - 3;
> > +
> > +     while (i > 0) {
> > +             if (name[i] == '_' && name[i + 1] == '_' && name[i + 2] == '_')
> > +                     return i;
> > +             i--;
> > +     }
> > +     return n;
> > +}
>
> that's a bit of an eye irritant. How about?
>         size_t n = strlen(name);
>         int i, cnt = 0;
>
>         for (i = n - 1; i >= 0; i--) {
>                 if (name[i] == '_') {
>                     cnt++;
>                 } else {
>                    if (cnt == 3)
>                       return i + 1;
>                    cnt = 0;
>                 }
>         }
>         return n;

I find this one much harder to read and understand. What's
eye-irritating about that loop?

Your loop will also handle `a____b` differently. My version will
return "a_" as essential name, yours "a____b". Was this intentional on
your part?

I'd rather use this instead, if you hate the first one:

size_t n = strlen(name);
int i;

for (i = n - 3; i > 0; i--) {
    if (strncmp(name + i, "___", 3) == 0)
        return i;
}

Is this better?

>
> > +     case BTF_KIND_ARRAY: {
> > +             const struct btf_array *loc_a, *targ_a;
> > +
> > +             loc_a = (void *)(local_type + 1);
> > +             targ_a = (void *)(targ_type + 1);
> > +             local_id = loc_a->type;
> > +             targ_id = targ_a->type;
>
> can we add a helper like:

Yes, we can. I was thinking about that, but decided to not expand
patch set. But we do need to extract all those small, but nice
helpers. I'll put them in libbpf_internal.h for now, but I think it
might be good idea to expose them as part of btf.h. Thoughts?

> const struct btf_array *btf_array(cosnt struct btf_type *t)
> {
>         return (const struct btf_array *)(t + 1);
> }
>
> then above will be:
>         case BTF_KIND_ARRAY: {
>                 local_id = btf_array(local_type)->type;
>                 targ_id = btf_array(targ_type)->type;
>
> and a bunch of code in btf.c and btf_dump.c would be cleaner as well?

Yep, some of those are already scattered around btf.c and btf_dump.c,
will clean up and add patch to this patch set.

>
> > +             goto recur;
> > +     }
> > +     default:
> > +             pr_warning("unexpected kind %d relocated, local [%d], target [%d]\n",
> > +                        kind, local_id, targ_id);
> > +             return 0;
> > +     }
> > +}
> > +
> > +/*
> > + * Given single high-level named field accessor in local type, find
> > + * corresponding high-level accessor for a target type. Along the way,
> > + * maintain low-level spec for target as well. Also keep updating target
> > + * offset.
> > + *
> > + * Searching is performed through recursive exhaustive enumeration of all
> > + * fields of a struct/union. If there are any anonymous (embedded)
> > + * structs/unions, they are recursively searched as well. If field with
> > + * desired name is found, check compatibility between local and target types,
> > + * before returning result.
> > + *
> > + * 1 is returned, if field is found.
> > + * 0 is returned if no compatible field is found.
> > + * <0 is returned on error.
> > + */
> > +static int bpf_core_match_member(const struct btf *local_btf,
> > +                              const struct bpf_core_accessor *local_acc,
> > +                              const struct btf *targ_btf,
> > +                              __u32 targ_id,
> > +                              struct bpf_core_spec *spec,
> > +                              __u32 *next_targ_id)
> > +{
> > +     const struct btf_type *local_type, *targ_type;
> > +     const struct btf_member *local_member, *m;
> > +     const char *local_name, *targ_name;
> > +     __u32 local_id;
> > +     int i, n, found;
> > +
> > +     targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
> > +     if (!targ_type)
> > +             return -EINVAL;
> > +     if (!btf_is_composite(targ_type))
> > +             return 0;
> > +
> > +     local_id = local_acc->type_id;
> > +     local_type = btf__type_by_id(local_btf, local_id);
> > +     local_member = (void *)(local_type + 1);
> > +     local_member += local_acc->idx;
> > +     local_name = btf__name_by_offset(local_btf, local_member->name_off);
> > +
> > +     n = BTF_INFO_VLEN(targ_type->info);
> > +     m = (void *)(targ_type + 1);
>
> new btf_member() helper?
>
> > +     for (i = 0; i < n; i++, m++) {
> > +             __u32 offset;
> > +
> > +             /* bitfield relocations not supported */
> > +             if (BTF_INFO_KFLAG(targ_type->info)) {
> > +                     if (BTF_MEMBER_BITFIELD_SIZE(m->offset))
> > +                             continue;
> > +                     offset = BTF_MEMBER_BIT_OFFSET(m->offset);
> > +             } else {
> > +                     offset = m->offset;
> > +             }
> > +             if (offset % 8)
> > +                     continue;
>
> same bit of code again?
> definitely could use a helper.

Different handling (continue here, return error above), but can use
those helpers I mentioned above.

>
> > +     for (i = 0; i < local_spec->len; i++, local_acc++, targ_acc++) {
> > +             targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id,
> > +                                                &targ_id);
> > +             if (!targ_type)
> > +                     return -EINVAL;
> > +
> > +             if (local_acc->name) {
> > +                     matched = bpf_core_match_member(local_spec->btf,
> > +                                                     local_acc,
> > +                                                     targ_btf, targ_id,
> > +                                                     targ_spec, &targ_id);
> > +                     if (matched <= 0)
> > +                             return matched;
> > +             } else {
> > +                     /* for i=0, targ_id is already treated as array element
> > +                      * type (because it's the original struct), for others
> > +                      * we should find array element type first
> > +                      */
> > +                     if (i > 0) {
> > +                             const struct btf_array *a;
> > +
> > +                             if (!btf_is_array(targ_type))
> > +                                     return 0;
> > +
> > +                             a = (void *)(targ_type + 1);
> > +                             if (local_acc->idx >= a->nelems)
> > +                                     return 0;
>
> am I reading it correctly that the local spec requested out-of-bounds
> index in the target array type?
> Why this is 'ignore' instead of -EINVAL?

Similar to any other mismatch (e.g., int in local type vs int64 in
target type). It just makes candidate not matching. Why would that be
error that will stop the whole relocation and subsequently object
loading process?

>
> > +/*
> > + * Probe few well-known locations for vmlinux kernel image and try to load BTF
> > + * data out of it to use for target BTF.
> > + */
> > +static struct btf *bpf_core_find_kernel_btf(void)
> > +{
> > +     const char *locations[] = {
> > +             "/lib/modules/%1$s/vmlinux-%1$s",
> > +             "/usr/lib/modules/%1$s/kernel/vmlinux",
> > +     };
> > +     char path[PATH_MAX + 1];
> > +     struct utsname buf;
> > +     struct btf *btf;
> > +     int i, err;
> > +
> > +     err = uname(&buf);
> > +     if (err) {
> > +             pr_warning("failed to uname(): %d\n", err);
>
> defensive programming ?
> I think uname() can fail only if &buf points to non-existing page like null.

I haven't checked source for this syscall, but man page specified that
it might return -1 on error.

>
> > +             return ERR_PTR(err);
> > +     }
> > +
> > +     for (i = 0; i < ARRAY_SIZE(locations); i++) {
> > +             snprintf(path, PATH_MAX, locations[i], buf.release);
> > +             pr_debug("attempting to load kernel BTF from '%s'\n", path);
>
> I think this debug message would have been more useful after access().

Sure, will move.

>
> > +
> > +             if (access(path, R_OK))
> > +                     continue;
> > +
> > +             btf = btf__parse_elf(path, NULL);
> > +             if (IS_ERR(btf))
> > +                     continue;
> > +
> > +             pr_debug("successfully loaded kernel BTF from '%s'\n", path);
> > +             return btf;
> > +     }
> > +
> > +     pr_warning("failed to find valid kernel BTF\n");
> > +     return ERR_PTR(-ESRCH);
> > +}
> > +
> > +/* Output spec definition in the format:
> > + * [<type-id>] (<type-name>) + <raw-spec> => <offset>@<spec>,
> > + * where <spec> is a C-syntax view of recorded field access, e.g.: x.a[3].b
> > + */
> > +static void bpf_core_dump_spec(int level, const struct bpf_core_spec *spec)
> > +{
> > +     const struct btf_type *t;
> > +     const char *s;
> > +     __u32 type_id;
> > +     int i;
> > +
> > +     type_id = spec->spec[0].type_id;
> > +     t = btf__type_by_id(spec->btf, type_id);
> > +     s = btf__name_by_offset(spec->btf, t->name_off);
> > +     libbpf_print(level, "[%u] (%s) + ", type_id, s);
>
> imo extra []() don't improve readability of the dump.

[<num>] is the general convention I've been using throughout libbpf to
specify type ID, so I'd rather keep it for consistency. I can drop
parens, though, no problem.

>
> > +
> > +     for (i = 0; i < spec->raw_len; i++)
> > +             libbpf_print(level, "%d%s", spec->raw_spec[i],
> > +                          i == spec->raw_len - 1 ? " => " : ":");
> > +
> > +     libbpf_print(level, "%u @ &x", spec->offset);
> > +
> > +     for (i = 0; i < spec->len; i++) {
> > +             if (spec->spec[i].name)
> > +                     libbpf_print(level, ".%s", spec->spec[i].name);
> > +             else
> > +                     libbpf_print(level, "[%u]", spec->spec[i].idx);
> > +     }
> > +
> > +}
> > +
> > +static size_t bpf_core_hash_fn(const void *key, void *ctx)
> > +{
> > +     return (size_t)key;
> > +}
> > +
> > +static bool bpf_core_equal_fn(const void *k1, const void *k2, void *ctx)
> > +{
> > +     return k1 == k2;
> > +}
> > +
> > +static void *u32_to_ptr(__u32 x)
> > +{
> > +     return (void *)(uintptr_t)x;
> > +}
>
> u32 to pointer on 64-bit arch?!
> That surely needs a comment.

I should probably call it u32_to_hash_key() to make it obvious it's
conversion to hashmap's generic `void *` key type.

>
> > +
> > +/*
> > + * CO-RE relocate single instruction.
> > + *
> > + * The outline and important points of the algorithm:
> > + * 1. For given local type, find corresponding candidate target types.
> > + *    Candidate type is a type with the same "essential" name, ignoring
> > + *    everything after last triple underscore (___). E.g., `sample`,
> > + *    `sample___flavor_one`, `sample___flavor_another_one`, are all candidates
> > + *    for each other. Names with triple underscore are referred to as
> > + *    "flavors" and are useful, among other things, to allow to
> > + *    specify/support incompatible variations of the same kernel struct, which
> > + *    might differ between different kernel versions and/or build
> > + *    configurations.
> > + *
> > + *    N.B. Struct "flavors" could be generated by bpftool's BTF-to-C
> > + *    converter, when deduplicated BTF of a kernel still contains more than
> > + *    one different types with the same name. In that case, ___2, ___3, etc
> > + *    are appended starting from second name conflict. But start flavors are
> > + *    also useful to be defined "locally", in BPF program, to extract same
> > + *    data from incompatible changes between different kernel
> > + *    versions/configurations. For instance, to handle field renames between
> > + *    kernel versions, one can use two flavors of the struct name with the
> > + *    same common name and use conditional relocations to extract that field,
> > + *    depending on target kernel version.
>
> there are actual kernel types that have ___ in the name.
> Ex: struct lmc___media
> We probably need to revisit this 'flavor' convention.

There are only these:
- lmc___softc
- lmc___media
- lmc___ctl (all three in drivers/net/wan/lmc/lmc_var.h)
- ____ftrace_##name set of structs

I couldn't come up with anything cleaner-looking. I think we can still
keep ___ convention, but:

1. Match only exactly 3 underscores, delimited by non-underscore from
both sides (so similar to your proposed loop above);
2. We can also try matching candidates assuming full name without
___xxx part removed, in addition to current logic. This seems like an
overkill at this point and unlikely to be useful in practice, so I'd
postpone implementing this until we really need it.

What do you think? Which other convention did you have in mind?

>
> > +     for (i = 0, j = 0; i < cand_ids->len; i++) {
> > +             cand_id = cand_ids->data[i];
> > +             cand_type = btf__type_by_id(targ_btf, cand_id);
> > +             cand_name = btf__name_by_offset(targ_btf, cand_type->name_off);
> > +
> > +             err = bpf_core_spec_match(&local_spec, targ_btf,
> > +                                       cand_id, &cand_spec);
> > +             if (err < 0) {
> > +                     pr_warning("prog '%s': relo #%d: failed to match spec ",
> > +                                prog_name, relo_idx);
> > +                     bpf_core_dump_spec(LIBBPF_WARN, &local_spec);
> > +                     libbpf_print(LIBBPF_WARN,
> > +                                  " to candidate #%d [%d] (%s): %d\n",
> > +                                  i, cand_id, cand_name, err);
> > +                     return err;
> > +             }
> > +             if (err == 0) {
> > +                     pr_debug("prog '%s': relo #%d: candidate #%d [%d] (%s) doesn't match spec ",
> > +                              prog_name, relo_idx, i, cand_id, cand_name);
> > +                     bpf_core_dump_spec(LIBBPF_DEBUG, &local_spec);
> > +                     libbpf_print(LIBBPF_DEBUG, "\n");
> > +                     continue;
> > +             }
> > +
> > +             pr_debug("prog '%s': relo #%d: candidate #%d matched as spec ",
> > +                      prog_name, relo_idx, i);
>
> did you mention that you're going to make a helper for this debug dumps?

yeah, I added bpf_core_dump_spec(), but I don't know how to shorten
this further... This output is extremely useful to understand what's
happening and will be invaluable when users will inevitably report
confusing behavior in some cases, so I still want to keep it.

>

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