Message-ID: <cffdf4e2-220d-cc2e-67e9-b83e848bdddf@fb.com>
Date:   Thu, 22 Apr 2021 16:57:59 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 11/17] libbpf: add linker extern resolution
 support for functions and global variables



On 4/22/21 3:12 PM, Andrii Nakryiko wrote:
> On Thu, Apr 22, 2021 at 2:27 PM Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
>>> Add BPF static linker logic to resolve extern variables and functions across
>>> multiple linked together BPF object files.
>>>
>>> For that, linker maintains a separate list of struct glob_sym structures,
>>> which keeps track of few pieces of metadata (is it extern or resolved global,
>>> is it a weak symbol, which ELF section it belongs to, etc) and ties together
>>> BTF type info and ELF symbol information and keeps them in sync.
>>>
>>> With adding support for extern variables/funcs, it's now possible for some
>>> sections to contain both extern and non-extern definitions. This means that
>>> some sections may start out as ephemeral (if only externs are present and thus
>>> there is not corresponding ELF section), but will be "upgraded" to actual ELF
>>> section as symbols are resolved or new non-extern definitions are appended.
>>>
>>> Additional care is taken to not duplicate extern entries in sections like
>>> .kconfig and .ksyms.
>>>
>>> Given libbpf requires BTF type to always be present for .kconfig/.ksym
>>> externs, linker extends this requirement to all the externs, even those that
>>> are supposed to be resolved during static linking and which won't be visible
>>> to libbpf. With BTF information always present, static linker will check not
>>> just ELF symbol matches, but entire BTF type signature match as well. That
>>> logic is stricter that BPF CO-RE checks. It probably should be re-used by
>>> .ksym resolution logic in libbpf as well, but that's left for follow up
>>> patches.
>>>
>>> To make it unnecessary to rewrite ELF symbols and minimize BTF type
>>> rewriting/removal, ELF symbols that correspond to externs initially will be
>>> updated in place once they are resolved. Similarly for BTF type info, VAR/FUNC
>>> and var_secinfo's (sec_vars in struct bpf_linker) are staying stable, but
>>> types they point to might get replaced when extern is resolved. This might
>>> leave some left-over types (even though we try to minimize this for common
>>> cases of having extern funcs with not argument names vs concrete function with
>>> names properly specified). That can be addresses later with a generic BTF
>>> garbage collection. That's left for a follow up as well.
>>>
>>> Given BTF type appending phase is separate from ELF symbol
>>> appending/resolution, special struct glob_sym->underlying_btf_id variable is
>>> used to communicate resolution and rewrite decisions. 0 means
>>> underlying_btf_id needs to be appended (it's not yet in final linker->btf), <0
>>> values are used for temporary storage of source BTF type ID (not yet
>>> rewritten), so -glob_sym->underlying_btf_id is BTF type id in obj-btf. But by
>>> the end of linker_append_btf() phase, that underlying_btf_id will be remapped
>>> and will always be > 0. This is the uglies part of the whole process, but
>>> keeps the other parts much simpler due to stability of sec_var and VAR/FUNC
>>> types, as well as ELF symbol, so please keep that in mind while reviewing.
>>
>> This is indeed complicated. I has some comments below. Please check
>> whether my understanding is correct or not.
>>
>>>
>>> BTF-defined maps require some extra custom logic and is addressed separate in
>>> the next patch, so that to keep this one smaller and easier to review.
>>>
>>> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>>> ---
>>>    tools/lib/bpf/linker.c | 844 ++++++++++++++++++++++++++++++++++++++---
>>>    1 file changed, 785 insertions(+), 59 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
>>> index d5dc1d401f57..67d2d06e3cb6 100644
>>> --- a/tools/lib/bpf/linker.c
>>> +++ b/tools/lib/bpf/linker.c
>>> @@ -22,6 +22,8 @@
>>>    #include "libbpf_internal.h"
>>>    #include "strset.h"
>>>
>>> +#define BTF_EXTERN_SEC ".extern"
>>> +
>>>    struct src_sec {
>>>        const char *sec_name;
>>>        /* positional (not necessarily ELF) index in an array of sections */
>>> @@ -74,11 +76,36 @@ struct btf_ext_sec_data {
>>>        void *recs;
>>>    };
>>>
>>> +struct glob_sym {
>>> +     /* ELF symbol index */
>>> +     int sym_idx;
>>> +     /* associated section id for .ksyms, .kconfig, etc, but not .extern */
>>> +     int sec_id;
>>> +     /* extern name offset in STRTAB */
>>> +     int name_off;
>>> +     /* optional associated BTF type ID */
>>> +     int btf_id;
>>> +     /* BTF type ID to which VAR/FUNC type is pointing to; used for
>>> +      * rewriting types when extern VAR/FUNC is resolved to a concrete
>>> +      * definition
>>> +      */
>>> +     int underlying_btf_id;
>>> +     /* sec_var index in the corresponding dst_sec, if exists */
>>> +     int var_idx;
>>> +
>>> +     /* extern or resolved/global symbol */
>>> +     bool is_extern;
>>> +     /* weak or strong symbol, never goes back from strong to weak */
>>> +     bool is_weak;
>>> +};
>>> +
>>>    struct dst_sec {
>>>        char *sec_name;
>>>        /* positional (not necessarily ELF) index in an array of sections */
>>>        int id;
>>>
>>> +     bool ephemeral;
>>> +
>>>        /* ELF info */
>>>        size_t sec_idx;
>>>        Elf_Scn *scn;
>>> @@ -120,6 +147,10 @@ struct bpf_linker {
>>>
>>>        struct btf *btf;
>>>        struct btf_ext *btf_ext;
>>> +
>>> +     /* global (including extern) ELF symbols */
>>> +     int glob_sym_cnt;
>>> +     struct glob_sym *glob_syms;
>>>    };
>>>
>> [...]
>>> +
>>> +static bool glob_sym_btf_matches(const char *sym_name, bool exact,
>>> +                              const struct btf *btf1, __u32 id1,
>>> +                              const struct btf *btf2, __u32 id2)
>>> +{
>>> +     const struct btf_type *t1, *t2;
>>> +     bool is_static1, is_static2;
>>> +     const char *n1, *n2;
>>> +     int i, n;
>>> +
>>> +recur:
>>> +     n1 = n2 = NULL;
>>> +     t1 = skip_mods_and_typedefs(btf1, id1, &id1);
>>> +     t2 = skip_mods_and_typedefs(btf2, id2, &id2);
>>> +
[...]
>>> +
>>> +     case BTF_KIND_TYPEDEF:
>>> +     case BTF_KIND_VOLATILE:
>>> +     case BTF_KIND_CONST:
>>> +     case BTF_KIND_RESTRICT:
>>
>> We already did skip_mods_and_typedefs() before. Unless something serious
>> wrong, we should not hit the above four types. So I think we can skip
>> them here.
> 
> This is the way of documenting explicitly that I'm aware of those
> kinds and they shouldn't be encountered. Otherwise one might wonder if
> we just forgot to handle them.

maybe add a short comment like "shouldn't happen due to 
skip_mods_and_typedefs(), added here for completeness"?

> 
>>
>>> +     case BTF_KIND_DATASEC:
>>> +     default:
>>> +             pr_warn("global '%s': unsupported BTF kind %s\n",
>>> +                     sym_name, btf_kind_str(t1));
>>> +             return false;
>>> +     }
>>> +}
>>> +
>>> +static bool glob_syms_match(const char *sym_name,
>>> +                         struct bpf_linker *linker, struct glob_sym *glob_sym,
>>> +                         struct src_obj *obj, Elf64_Sym *sym, size_t sym_idx, int btf_id)
>>> +{
>>> +     const struct btf_type *src_t;
>>> +
>>> +     /* if we are dealing with externs, BTF types describing both global
>>> +      * and extern VARs/FUNCs should be completely present in all files
>>> +      */
>>> +     if (!glob_sym->btf_id || !btf_id) {
>>> +             pr_warn("BTF info is missing for global symbol '%s'\n", sym_name);
>>> +             return false;
>>> +     }
>>> +
>>> +     src_t = btf__type_by_id(obj->btf, btf_id);
>>> +     if (!btf_is_var(src_t) && !btf_is_func(src_t)) {
>>> +             pr_warn("only extern variables and functions are supported, but got '%s' for '%s'\n",
>>> +                     btf_kind_str(src_t), sym_name);
>>> +             return false;
>>> +     }
>>> +
>>> +     if (!glob_sym_btf_matches(sym_name, true /*exact*/,
>>> +                               linker->btf, glob_sym->btf_id, obj->btf, btf_id))
>>> +             return false;
>>> +
>>> +     return true;
>>> +}
>>> +
>> [...]
>>> +
>>> +static void sym_update_visibility(Elf64_Sym *sym, int sym_vis)
>>> +{
>>> +     /* libelf doesn't provide setters for ST_VISIBILITY,
>>> +      * but it is stored in the lower 2 bits of st_other
>>> +      */
>>> +     sym->st_other &= 0x03;
>>> +     sym->st_other |= sym_vis;
>>> +}
>>> +
>>> +static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
>>> +                              Elf64_Sym *sym, const char *sym_name, int src_sym_idx)
>>> +{
>>> +     struct src_sec *src_sec = NULL;
>>> +     struct dst_sec *dst_sec = NULL;
>>> +     struct glob_sym *glob_sym = NULL;
>>> +     int name_off, sym_type, sym_bind, sym_vis, err;
>>> +     int btf_sec_id = 0, btf_id = 0;
>>> +     size_t dst_sym_idx;
>>> +     Elf64_Sym *dst_sym;
>>> +     bool sym_is_extern;
>>> +
>>> +     sym_type = ELF64_ST_TYPE(sym->st_info);
>>> +     sym_bind = ELF64_ST_BIND(sym->st_info);
>>> +     sym_vis = ELF64_ST_VISIBILITY(sym->st_other);
>>> +     sym_is_extern = sym->st_shndx == SHN_UNDEF;
>>> +
>>> +     if (sym_is_extern) {
>>> +             if (!obj->btf) {
>>> +                     pr_warn("externs without BTF info are not supported\n");
>>> +                     return -ENOTSUP;
>>> +             }
>>> +     } else if (sym->st_shndx < SHN_LORESERVE) {
>>
>> So what happens if sym->st_shndx >= SHN_LORESERVE. Maybe return failures
>> here? In general, bpf program shouldn't hit sym->st_shndx >= SHN_LORESERVE.
> 
> There is at least SHN_ABS (0xfff1), which is an informational STT_FILE
> symbol. libbpf doesn't error out on such special symbols, and linker
> will just pass-through them and append to the final object file.

Okay, I see. Never paid attention to it.

> 
>>
>>> +             src_sec = &obj->secs[sym->st_shndx];
>>> +             if (src_sec->skipped)
>>> +                     return 0;
>>> +             dst_sec = &linker->secs[src_sec->dst_id];
>>> +
>>> +             /* allow only one STT_SECTION symbol per section */
>>> +             if (sym_type == STT_SECTION && dst_sec->sec_sym_idx) {
>>> +                     obj->sym_map[src_sym_idx] = dst_sec->sec_sym_idx;
>>> +                     return 0;
>>> +             }
>>> +     }
>>> +
>>> +     if (sym_bind == STB_LOCAL)
>>> +             goto add_sym;
>>> +
>>> +     /* find matching BTF info */
>>> +     err = find_glob_sym_btf(obj, sym, sym_name, &btf_sec_id, &btf_id);
>>> +     if (err)
>>> +             return err;
>>> +
>>> +     if (sym_is_extern && btf_sec_id) {
>>> +             const char *sec_name = NULL;
>>> +             const struct btf_type *t;
>>> +
>>> +             t = btf__type_by_id(obj->btf, btf_sec_id);
>>> +             sec_name = btf__str_by_offset(obj->btf, t->name_off);
>>> +
>>> +             /* Clang puts unannotated extern vars into
>>> +              * '.extern' BTF DATASEC. Treat them the same
>>> +              * as unannotated extern funcs (which are
>>> +              * currently not put into any DATASECs).
>>> +              * Those don't have associated src_sec/dst_sec.
>>> +              */
>>> +             if (strcmp(sec_name, BTF_EXTERN_SEC) != 0) {
>>> +                     src_sec = find_src_sec_by_name(obj, sec_name);
>>> +                     if (!src_sec) {
>>> +                             pr_warn("failed to find matching ELF sec '%s'\n", sec_name);
>>> +                             return -ENOENT;
>>> +                     }
>>> +                     dst_sec = &linker->secs[src_sec->dst_id];
>>> +             }
>>> +     }
>>> +
>>> +     glob_sym = find_glob_sym(linker, sym_name);
>>> +     if (glob_sym) {
>>> +             /* Preventively resolve to existing symbol. This is
>>> +              * needed for further relocation symbol remapping in
>>> +              * the next step of linking.
>>> +              */
>>> +             obj->sym_map[src_sym_idx] = glob_sym->sym_idx;
>>> +
>>> +             /* If both symbols are non-externs, at least one of
>>> +              * them has to be STB_WEAK, otherwise they are in
>>> +              * a conflict with each other.
>>> +              */
>>> +             if (!sym_is_extern && !glob_sym->is_extern
>>> +                 && !glob_sym->is_weak && sym_bind != STB_WEAK) {
>>> +                     pr_warn("conflicting non-weak symbol #%d (%s) definition in '%s'\n",
>>> +                             src_sym_idx, sym_name, obj->filename);
>>> +                     return -EINVAL;
>>>                }
>>>
>>> +             if (!glob_syms_match(sym_name, linker, glob_sym, obj, sym, src_sym_idx, btf_id))
>>> +                     return -EINVAL;
>>> +
>>> +             dst_sym = get_sym_by_idx(linker, glob_sym->sym_idx);
>>> +
>>> +             /* If new symbol is strong, then force dst_sym to be strong as
>>> +              * well; this way a mix of weak and non-weak extern
>>> +              * definitions will end up being strong.
>>> +              */
>>> +             if (sym_bind == STB_GLOBAL) {
>>> +                     /* We still need to preserve type (NOTYPE or
>>> +                      * OBJECT/FUNC, depending on whether the symbol is
>>> +                      * extern or not)
>>> +                      */
>>> +                     sym_update_bind(dst_sym, STB_GLOBAL);
>>> +                     glob_sym->is_weak = false;
>>> +             }
>>> +
>>> +             /* Non-default visibility is "contaminating", with stricter
>>> +              * visibility overwriting more permissive ones, even if more
>>> +              * permissive visibility comes from just an extern definition
>>> +              */
>>> +             if (sym_vis > ELF64_ST_VISIBILITY(dst_sym->st_other))
>>> +                     sym_update_visibility(dst_sym, sym_vis);
>>
>> For visibility, maybe we can just handle DEFAULT and HIDDEN, and others
>> are not supported? DEFAULT + DEFAULT/HIDDEN => DEFAULT, HIDDEN + HIDDEN
>> => HIDDEN?
>>
> 
> Sure, we can restrict this to STV_DEFAULT and STV_HIDDEN for now.
> 
>>> +
>>> +             /* If the new symbol is extern, then regardless if
>>> +              * existing symbol is extern or resolved global, just
>>> +              * keep the existing one untouched.
>>> +              */
>>> +             if (sym_is_extern)
>>> +                     return 0;
>>> +
>>> +             /* If existing symbol is a strong resolved symbol, bail out,
>>> +              * because we lost resolution battle have nothing to
>>> +              * contribute. We already checked abover that there is no
>>> +              * strong-strong conflict. We also already tightened binding
>>> +              * and visibility, so nothing else to contribute at that point.
>>> +              */
>>> +             if (!glob_sym->is_extern && sym_bind == STB_WEAK)
>>> +                     return 0;
>>> +
>>> +             /* At this point, new symbol is strong non-extern,
>>> +              * so overwrite glob_sym with new symbol information.
>>> +              * Preserve binding and visibility.
>>> +              */
>>> +             sym_update_type(dst_sym, sym_type);
>>> +             dst_sym->st_shndx = dst_sec->sec_idx;
>>> +             dst_sym->st_value = src_sec->dst_off + sym->st_value;
>>> +             dst_sym->st_size = sym->st_size;
>>> +
>>> +             /* see comment below about dst_sec->id vs dst_sec->sec_idx */
>>> +             glob_sym->sec_id = dst_sec->id;
>>> +             glob_sym->is_extern = false;
>>> +             /* never relax strong to weak binding */
>>> +             if (sym_bind == STB_GLOBAL)
>>> +                     glob_sym->is_weak = false;
>>
>> In the above, we already set glob_sym->is_weak to false if STB_GLOBAL.
> 
> yep, you are right, this is unnecessary, I'll remove
> 
>>
>>> +
>>> +             if (complete_extern_btf_info(linker->btf, glob_sym->btf_id,
>>> +                                          obj->btf, btf_id))
>>> +                     return -EINVAL;
>>> +
>>> +             /* request updating VAR's/FUNC's underlying BTF type when appending BTF type */
>>> +             glob_sym->underlying_btf_id = 0;
>>> +
>>> +             obj->sym_map[src_sym_idx] = glob_sym->sym_idx;
>>> +             return 0;
>>> +     }
>>> +
>>> +add_sym:
>>> +     name_off = strset__add_str(linker->strtab_strs, sym_name);
>>> +     if (name_off < 0)
>>> +             return name_off;
>>> +
>>> +     dst_sym = add_new_sym(linker, &dst_sym_idx);
>>> +     if (!dst_sym)
>>> +             return -ENOMEM;
>>> +
>>> +     dst_sym->st_name = name_off;
>>> +     dst_sym->st_info = sym->st_info;
>>> +     dst_sym->st_other = sym->st_other;
>>> +     dst_sym->st_shndx = dst_sec ? dst_sec->sec_idx : sym->st_shndx;
>>> +     dst_sym->st_value = (src_sec ? src_sec->dst_off : 0) + sym->st_value;
>>> +     dst_sym->st_size = sym->st_size;
>>> +
>>> +     obj->sym_map[src_sym_idx] = dst_sym_idx;
>>> +
>>> +     if (sym_type == STT_SECTION && dst_sym) {
>>> +             dst_sec->sec_sym_idx = dst_sym_idx;
>>> +             dst_sym->st_value = 0;
>>> +     }
>>> +
>>> +     if (sym_bind != STB_LOCAL) {
>>> +             glob_sym = add_glob_sym(linker);
>>> +             if (!glob_sym)
>>> +                     return -ENOMEM;
>>> +
>>> +             glob_sym->sym_idx = dst_sym_idx;
>>> +             /* we use dst_sec->id (and not dst_sec->sec_idx), because
>>> +              * ephemeral sections (.kconfig, .ksyms, etc) don't have
>>> +              * sec_idx (as they don't have corresponding ELF section), but
>>> +              * still have id. .extern doesn't have even ephemeral section
>>> +              * associated with it, so dst_sec->id == dst_sec->sec_idx == 0.
>>> +              */
>>> +             glob_sym->sec_id = dst_sec ? dst_sec->id : 0;
>>> +             glob_sym->name_off = name_off;
>>> +             /* we will fill btf_id in during BTF merging step */
>>> +             glob_sym->btf_id = 0;
>>> +             glob_sym->is_extern = sym_is_extern;
>>> +             glob_sym->is_weak = sym_bind == STB_WEAK;
>>>        }
>>>
>>>        return 0;
>>> @@ -1256,7 +1887,7 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
>>>                dst_sec->shdr->sh_info = dst_linked_sec->sec_idx;
>>>
>>>                src_sec->dst_id = dst_sec->id;
>>> -             err = extend_sec(dst_sec, src_sec);
>>> +             err = extend_sec(linker, dst_sec, src_sec);
>>>                if (err)
>>>                        return err;
>>>
>>> @@ -1309,21 +1940,6 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
>>>        return 0;
>>>    }
>>>
>> [...]
>>> @@ -1442,6 +2078,7 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>>>    {
>>>        const struct btf_type *t;
>>>        int i, j, n, start_id, id;
>>> +     const char *name;
>>>
>>>        if (!obj->btf)
>>>                return 0;
>>> @@ -1454,12 +2091,40 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>>>                return -ENOMEM;
>>>
>>>        for (i = 1; i <= n; i++) {
>>> +             struct glob_sym *glob_sym = NULL;
>>> +
>>>                t = btf__type_by_id(obj->btf, i);
>>>
>>>                /* DATASECs are handled specially below */
>>>                if (btf_kind(t) == BTF_KIND_DATASEC)
>>>                        continue;
>>>
>>> +             if (btf_is_non_static(t)) {
>>> +                     /* there should be glob_sym already */
>>> +                     name = btf__str_by_offset(obj->btf, t->name_off);
>>> +                     glob_sym = find_glob_sym(linker, name);
>>> +
>>> +                     /* VARs without corresponding glob_sym are those that
>>> +                      * belong to skipped/deduplicated sections (i.e.,
>>> +                      * license and version), so just skip them
>>> +                      */
>>> +                     if (!glob_sym)
>>> +                             continue;
>>> +
>>> +                     if (glob_sym->underlying_btf_id == 0)
>>> +                             glob_sym->underlying_btf_id = -t->type;
>>
>> Is this needed? If glob_sym->btf_id is not NULL, then
>> glob_sym->underlying_btf_id has been set by the previous object.
>> If it is NULL, it will set probably after this
>> if (btf_is_non_static(t)) { ...}, is this right?
> 
> I think it's still needed. Here's the scenario.
> 
> 1. Obj file A contains extern symbol X. We create corresponding
> glob_sym (with is_extern=true), and store btf_id to point to
> BTF_KIND_VAR, and btf_underlying_id to point to the type that
> BTF_KIND_VAR points to.
> 
> 2. Obj file B contains non-extern symbol X. At this point
> linker_append_elf_sym() will update glob_sym to is_extern = false, it
> will keep btf_id to re-use already appended BTF_KIND_VAR, but it will
> zero-out underlying_btf_id, because for externs type could be
> incomplete (e.g. for functions it won't contain function argument
> names, for maps it could differ even more drastically later). So then
> we get here, we see that glob_sym->underlying_btf_id is zero, so needs
> updating. We store it as -Y, because Y is BTF type ID in obj->btf, not
> in linker->btf (yet). Then the if (glob_sym->btf_id) below sees that
> glob_sym->btf_id is already set, so we just keep using already
> appended BTF_KIND_VAR (we already set its linkage to
> BTF_VAR_GLOBAL_ALLOCATED in complete_extern_btf_info(), called from
> linker_append_elf_sym(). So we'll skip appending another BTF_KIND_VAR.
> But we do want to point existing BTF_KIND_VAR to a new type that
> corresponds to ID -Y.

Thanks for explanation, I missed the code in
       /* request updating VAR's/FUNC's underlying BTF type when
  appending BTF type */
       glob_sym->underlying_btf_id = 0;

  in linker_append_elf_sym().

Maybe add a comment above the code with something like
  underlying_btf_id may have been reset to 0 due to the presence of
  a strong global variable.
?

> 
>>
>>> +
>>> +                     /* globals from previous object files that match our
>>> +                      * VAR/FUNC already have a corresponding associated
>>> +                      * BTF type, so just make sure to use it
>>> +                      */
>>> +                     if (glob_sym->btf_id) {
>>> +                             /* reuse existing BTF type for global var/func */
>>> +                             obj->btf_type_map[i] = glob_sym->btf_id;
>>> +                             continue;
>>> +                     }
>>> +             }
>>> +
>>>                id = btf__add_type(linker->btf, obj->btf, t);
>>>                if (id < 0) {
>>>                        pr_warn("failed to append BTF type #%d from file '%s'\n", i, obj->filename);
>>> @@ -1467,6 +2132,12 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>>>                }
>>>
>>>                obj->btf_type_map[i] = id;
>>> +
>>> +             /* record just appended BTF type for var/func */
>>> +             if (glob_sym) {
>>> +                     glob_sym->btf_id = id;
>>> +                     glob_sym->underlying_btf_id = -t->type;
>>> +             }
>>>        }
>>>
>>>        /* remap all the types except DATASECs */
>>> @@ -1478,6 +2149,22 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>>>                        return -EINVAL;
>>>        }
>>>
>>> +     /* Rewrite VAR/FUNC underlying types (i.e., FUNC's FUNC_PROTO and VAR's
>>> +      * actual type), if necessary
>>> +      */
>>> +     for (i = 0; i < linker->glob_sym_cnt; i++) {
>>> +             struct glob_sym *glob_sym = &linker->glob_syms[i];
>>> +             struct btf_type *glob_t;
>>> +
>>> +             if (glob_sym->underlying_btf_id >= 0)
>>> +                     continue;
>>> +
>>> +             glob_sym->underlying_btf_id = obj->btf_type_map[-glob_sym->underlying_btf_id];
>>
>> After this point, any new *extern* variables will hit the below in the
>> previous code:
> 
> Right, but we want to hit this for existing glob_syms that went from
> extern to non-extern or from weak to non-weak. See
> 
>      /* request updating VAR's/FUNC's underlying BTF type when
> appending BTF type */
>      glob_sym->underlying_btf_id = 0;
> 
> in linker_append_elf_sym().
> 
> And we'll use that even more extensively when extending __weak and
> extern map definitions later.
> 
>>   > +                    if (glob_sym->btf_id) {
>>   > +                            /* reuse existing BTF type for global var/func */
>>   > +                            obj->btf_type_map[i] = glob_sym->btf_id;
>>   > +                            continue;
>>   > +                    }
>>
>>> +
>>> +             glob_t = btf_type_by_id(linker->btf, glob_sym->btf_id);
>>> +             glob_t->type = glob_sym->underlying_btf_id;
>>> +     }
>>> +
>>>        /* append DATASEC info */
>>>        for (i = 1; i < obj->sec_cnt; i++) {
>>>                struct src_sec *src_sec;
>>> @@ -1505,6 +2192,42 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>> [...]

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