bpf: Use kallsyms to find the function name of a struct_ops's stub function

In commit 1611603537a4 ("bpf: Create argument information for nullable arguments."),
it introduced a "__nullable" tagging at the argument name of a
stub function. Some background on the commit:
it requires to tag the stub function instead of directly tagging
the "ops" of a struct. This is because the btf func_proto of the "ops"
does not have the argument name and the "__nullable" is tagged at
the argument name.

To find the stub function of a "ops", it currently relies on a naming
convention on the stub function "st_ops__ops_name".
e.g. tcp_congestion_ops__ssthresh. However, the new kernel
sub system implementing bpf_struct_ops have missed this and
have been surprised that the "__nullable" and the to-be-landed
"__ref" tagging was not effective.

One option would be to give a warning whenever the stub function does
not follow the naming convention, regardless if it requires arg tagging
or not.

Instead, this patch uses the kallsyms_lookup approach and removes
the requirement on the naming convention. The st_ops->cfi_stubs has
all the stub function kernel addresses. kallsyms_lookup() is used to
lookup the function name. With the function name, BTF can be used to
find the BTF func_proto. The existing "__nullable" arg name searching
logic will then fall through.

One notable change is,
if it failed in kallsyms_lookup or it failed in looking up the stub
function name from the BTF, the bpf_struct_ops registration will fail.
This is different from the previous behavior that it silently ignored
the "st_ops__ops_name" function not found error.

The "tcp_congestion_ops", "sched_ext_ops", and "hid_bpf_ops" can still be
registered successfully after this patch. There is struct_ops_maybe_null
selftest to cover the "__nullable" tagging.

Other minor changes:
1. Removed the "%s__%s" format from the pr_warn because the naming
   convention is removed.
2. The existing bpf_struct_ops_supported() is also moved earlier
   because prepare_arg_info needs to use it to decide if the
   stub function is NULL before calling the prepare_arg_info.

Cc: Tejun Heo <tj@kernel.org>
Cc: Benjamin Tissoires <bentiss@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20250127222719.2544255-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 040fb1cd840b653148b0941bf525841494025fcb..9b7f3b9c52622d71891a80ab3725e90fbe4d039e 100644 (file)
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -146,39 +146,6 @@ void bpf_struct_ops_image_free(void *image)
 }
 
 #define MAYBE_NULL_SUFFIX "__nullable"
-#define MAX_STUB_NAME 128
-
-/* Return the type info of a stub function, if it exists.
- *
- * The name of a stub function is made up of the name of the struct_ops and
- * the name of the function pointer member, separated by "__". For example,
- * if the struct_ops type is named "foo_ops" and the function pointer
- * member is named "bar", the stub function name would be "foo_ops__bar".
- */
-static const struct btf_type *
-find_stub_func_proto(const struct btf *btf, const char *st_op_name,
-                    const char *member_name)
-{
-       char stub_func_name[MAX_STUB_NAME];
-       const struct btf_type *func_type;
-       s32 btf_id;
-       int cp;
-
-       cp = snprintf(stub_func_name, MAX_STUB_NAME, "%s__%s",
-                     st_op_name, member_name);
-       if (cp >= MAX_STUB_NAME) {
-               pr_warn("Stub function name too long\n");
-               return NULL;
-       }
-       btf_id = btf_find_by_name_kind(btf, stub_func_name, BTF_KIND_FUNC);
-       if (btf_id < 0)
-               return NULL;
-       func_type = btf_type_by_id(btf, btf_id);
-       if (!func_type)
-               return NULL;
-
-       return btf_type_by_id(btf, func_type->type); /* FUNC_PROTO */
-}
 
 /* Prepare argument info for every nullable argument of a member of a
  * struct_ops type.
@@ -203,27 +170,42 @@ find_stub_func_proto(const struct btf *btf, const char *st_op_name,
 static int prepare_arg_info(struct btf *btf,
                            const char *st_ops_name,
                            const char *member_name,
-                           const struct btf_type *func_proto,
+                           const struct btf_type *func_proto, void *stub_func_addr,
                            struct bpf_struct_ops_arg_info *arg_info)
 {
        const struct btf_type *stub_func_proto, *pointed_type;
        const struct btf_param *stub_args, *args;
        struct bpf_ctx_arg_aux *info, *info_buf;
        u32 nargs, arg_no, info_cnt = 0;
+       char ksym[KSYM_SYMBOL_LEN];
+       const char *stub_fname;
+       s32 stub_func_id;
        u32 arg_btf_id;
        int offset;
 
-       stub_func_proto = find_stub_func_proto(btf, st_ops_name, member_name);
-       if (!stub_func_proto)
-               return 0;
+       stub_fname = kallsyms_lookup((unsigned long)stub_func_addr, NULL, NULL, NULL, ksym);
+       if (!stub_fname) {
+               pr_warn("Cannot find the stub function name for the %s in struct %s\n",
+                       member_name, st_ops_name);
+               return -ENOENT;
+       }
+
+       stub_func_id = btf_find_by_name_kind(btf, stub_fname, BTF_KIND_FUNC);
+       if (stub_func_id < 0) {
+               pr_warn("Cannot find the stub function %s in btf\n", stub_fname);
+               return -ENOENT;
+       }
+
+       stub_func_proto = btf_type_by_id(btf, stub_func_id);
+       stub_func_proto = btf_type_by_id(btf, stub_func_proto->type);
 
        /* Check if the number of arguments of the stub function is the same
         * as the number of arguments of the function pointer.
         */
        nargs = btf_type_vlen(func_proto);
        if (nargs != btf_type_vlen(stub_func_proto)) {
-               pr_warn("the number of arguments of the stub function %s__%s does not match the number of arguments of the member %s of struct %s\n",
-                       st_ops_name, member_name, member_name, st_ops_name);
+               pr_warn("the number of arguments of the stub function %s does not match the number of arguments of the member %s of struct %s\n",
+                       stub_fname, member_name, st_ops_name);
                return -EINVAL;
        }
 
@@ -253,21 +235,21 @@ static int prepare_arg_info(struct btf *btf,
                                                    &arg_btf_id);
                if (!pointed_type ||
                    !btf_type_is_struct(pointed_type)) {
-                       pr_warn("stub function %s__%s has %s tagging to an unsupported type\n",
-                               st_ops_name, member_name, MAYBE_NULL_SUFFIX);
+                       pr_warn("stub function %s has %s tagging to an unsupported type\n",
+                               stub_fname, MAYBE_NULL_SUFFIX);
                        goto err_out;
                }
 
                offset = btf_ctx_arg_offset(btf, func_proto, arg_no);
                if (offset < 0) {
-                       pr_warn("stub function %s__%s has an invalid trampoline ctx offset for arg#%u\n",
-                               st_ops_name, member_name, arg_no);
+                       pr_warn("stub function %s has an invalid trampoline ctx offset for arg#%u\n",
+                               stub_fname, arg_no);
                        goto err_out;
                }
 
                if (args[arg_no].type != stub_args[arg_no].type) {
-                       pr_warn("arg#%u type in stub function %s__%s does not match with its original func_proto\n",
-                               arg_no, st_ops_name, member_name);
+                       pr_warn("arg#%u type in stub function %s does not match with its original func_proto\n",
+                               arg_no, stub_fname);
                        goto err_out;
                }
 
@@ -324,6 +306,13 @@ static bool is_module_member(const struct btf *btf, u32 id)
        return !strcmp(btf_name_by_offset(btf, t->name_off), "module");
 }
 
+int bpf_struct_ops_supported(const struct bpf_struct_ops *st_ops, u32 moff)
+{
+       void *func_ptr = *(void **)(st_ops->cfi_stubs + moff);
+
+       return func_ptr ? 0 : -ENOTSUPP;
+}
+
 int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
                             struct btf *btf,
                             struct bpf_verifier_log *log)
@@ -387,7 +376,10 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
 
        for_each_member(i, t, member) {
                const struct btf_type *func_proto;
+               void **stub_func_addr;
+               u32 moff;
 
+               moff = __btf_member_bit_offset(t, member) / 8;
                mname = btf_name_by_offset(btf, member->name_off);
                if (!*mname) {
                        pr_warn("anon member in struct %s is not supported\n",
@@ -413,7 +405,11 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
                func_proto = btf_type_resolve_func_ptr(btf,
                                                       member->type,
                                                       NULL);
-               if (!func_proto)
+
+               /* The member is not a function pointer or
+                * the function pointer is not supported.
+                */
+               if (!func_proto || bpf_struct_ops_supported(st_ops, moff))
                        continue;
 
                if (btf_distill_func_proto(log, btf,
@@ -425,8 +421,9 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
                        goto errout;
                }
 
+               stub_func_addr = *(void **)(st_ops->cfi_stubs + moff);
                err = prepare_arg_info(btf, st_ops->name, mname,
-                                      func_proto,
+                                      func_proto, stub_func_addr,
                                       arg_info + i);
                if (err)
                        goto errout;
@@ -1152,13 +1149,6 @@ void bpf_struct_ops_put(const void *kdata)
        bpf_map_put(&st_map->map);
 }
 
-int bpf_struct_ops_supported(const struct bpf_struct_ops *st_ops, u32 moff)
-{
-       void *func_ptr = *(void **)(st_ops->cfi_stubs + moff);
-
-       return func_ptr ? 0 : -ENOTSUPP;
-}
-
 static bool bpf_struct_ops_valid_to_reg(struct bpf_map *map)
 {
        struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map *)map;