Re: [PATCH] btf: Simplify BTF logic with use of __free(btf_put)

Masami Hiramatsu
2025-08-04T13:26:50Z

On Fri, 1 Aug 2025 07:16:22 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
>
> Several functions need to call btf_put() on the btf pointer before it
> returns leading to using "goto" branches to jump to the end to call
> btf_put(btf). This can be simplified by introducing DEFINE_FREE() to allow
> functions to define the btf descriptor with:
>
> struct btf *btf __free(btf_put) = NULL;
>
> Then the btf descriptor will always have btf_put() called on it if it
> isn't NULL or ERR before exiting the function.
>
> Where needed, no_free_ptr(btf) is used to assign the btf descriptor to a
> pointer that will be used outside the function.
>

Yeah, this looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> include/linux/btf.h | 4 +++
> kernel/bpf/btf.c | 56 +++++++++++--------------------------
> kernel/trace/trace_btf.c | 15 +++++-----
> kernel/trace/trace_output.c | 8 ++----
> 4 files changed, 31 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index b2983706292f..e118c69c4996 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -8,6 +8,7 @@
> #include <linux/bpfptr.h>
> #include <linux/bsearch.h>
> #include <linux/btf_ids.h>
> +#include <linux/cleanup.h>
> #include <uapi/linux/btf.h>
> #include <uapi/linux/bpf.h>
>
> @@ -150,6 +151,9 @@ struct btf *btf_get_by_fd(int fd);
> int btf_get_info_by_fd(const struct btf *btf,
> const union bpf_attr *attr,
> union bpf_attr __user *uattr);
> +
> +DEFINE_FREE(btf_put, struct btf *, if (!IS_ERR_OR_NULL(_T)) btf_put(_T))
> +
> /* Figure out the size of a type_id. If type_id is a modifier
> * (e.g. const), it will be resolved to find out the type with size.
> *
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 1d2cf898e21e..480657912c96 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3788,7 +3788,7 @@ static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
> /* If a matching btf type is found in kernel or module BTFs, kptr_ref
> * is that BTF, otherwise it's program BTF
> */
> - struct btf *kptr_btf;
> + struct btf *kptr_btf __free(btf_put) = NULL;
> int ret;
> s32 id;
>
> @@ -3827,23 +3827,17 @@ static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
> * the same time.
> */
> dtor_btf_id = btf_find_dtor_kfunc(kptr_btf, id);
> - if (dtor_btf_id < 0) {
> - ret = dtor_btf_id;
> - goto end_btf;
> - }
> + if (dtor_btf_id < 0)
> + return dtor_btf_id;
>
> dtor_func = btf_type_by_id(kptr_btf, dtor_btf_id);
> - if (!dtor_func) {
> - ret = -ENOENT;
> - goto end_btf;
> - }
> + if (!dtor_func)
> + return -ENOENT;
>
> if (btf_is_module(kptr_btf)) {
> mod = btf_try_get_module(kptr_btf);
> - if (!mod) {
> - ret = -ENXIO;
> - goto end_btf;
> - }
> + if (!mod)
> + return -ENXIO;
> }
>
> /* We already verified dtor_func to be btf_type_is_func
> @@ -3860,13 +3854,11 @@ static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
>
> found_dtor:
> field->kptr.btf_id = id;
> - field->kptr.btf = kptr_btf;
> + field->kptr.btf = no_free_ptr(kptr_btf);
> field->kptr.module = mod;
> return 0;
> end_mod:
> module_put(mod);
> -end_btf:
> - btf_put(kptr_btf);
> return ret;
> }
>
> @@ -8699,7 +8691,7 @@ u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id,
> static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
> const struct btf_kfunc_id_set *kset)
> {
> - struct btf *btf;
> + struct btf *btf __free(btf_put) = NULL;
> int ret, i;
>
> btf = btf_get_module_btf(kset->owner);
> @@ -8712,14 +8704,10 @@ static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
> ret = btf_check_kfunc_protos(btf, btf_relocate_id(btf, kset->set->pairs[i].id),
> kset->set->pairs[i].flags);
> if (ret)
> - goto err_out;
> + return ret;
> }
>
> - ret = btf_populate_kfunc_set(btf, hook, kset);
> -
> -err_out:
> - btf_put(btf);
> - return ret;
> + return btf_populate_kfunc_set(btf, hook, kset);
> }
>
> /* This function must be invoked only from initcalls/module init functions */
> @@ -8807,7 +8795,7 @@ int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dtors, u32 add_c
> struct module *owner)
> {
> struct btf_id_dtor_kfunc_tab *tab;
> - struct btf *btf;
> + struct btf *btf __free(btf_put) = NULL;
> u32 tab_cnt, i;
> int ret;
>
> @@ -8873,7 +8861,6 @@ int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dtors, u32 add_c
> end:
> if (ret)
> btf_free_dtor_kfunc_tab(btf);
> - btf_put(btf);
> return ret;
> }
> EXPORT_SYMBOL_GPL(register_btf_id_dtor_kfuncs);
> @@ -9484,9 +9471,8 @@ bpf_struct_ops_find(struct btf *btf, u32 type_id)
>
> int __register_bpf_struct_ops(struct bpf_struct_ops *st_ops)
> {
> - struct bpf_verifier_log *log;
> - struct btf *btf;
> - int err = 0;
> + struct bpf_verifier_log *log __free(kfree) = NULL;
> + struct btf *btf __free(btf_put) = NULL;
>
> btf = btf_get_module_btf(st_ops->owner);
> if (!btf)
> @@ -9495,20 +9481,12 @@ int __register_bpf_struct_ops(struct bpf_struct_ops *st_ops)
> return PTR_ERR(btf);
>
> log = kzalloc(sizeof(*log), GFP_KERNEL | __GFP_NOWARN);
> - if (!log) {
> - err = -ENOMEM;
> - goto errout;
> - }
> + if (!log)
> + return -ENOMEM;
>
> log->level = BPF_LOG_KERNEL;
>
> - err = btf_add_struct_ops(btf, st_ops, log);
> -
> -errout:
> - kfree(log);
> - btf_put(btf);
> -
> - return err;
> + return btf_add_struct_ops(btf, st_ops, log);
> }
> EXPORT_SYMBOL_GPL(__register_bpf_struct_ops);
> #endif
> diff --git a/kernel/trace/trace_btf.c b/kernel/trace/trace_btf.c
> index 5bbdbcbbde3c..1c9111ab538b 100644
> --- a/kernel/trace/trace_btf.c
> +++ b/kernel/trace/trace_btf.c
> @@ -13,26 +13,25 @@
> const struct btf_type *btf_find_func_proto(const char *func_name, struct btf **btf_p)
> {
> const struct btf_type *t;
> + struct btf *btf __free(btf_put) = NULL;
> s32 id;
>
> - id = bpf_find_btf_id(func_name, BTF_KIND_FUNC, btf_p);
> + id = bpf_find_btf_id(func_name, BTF_KIND_FUNC, &btf);
> if (id < 0)
> return NULL;
>
> /* Get BTF_KIND_FUNC type */
> - t = btf_type_by_id(*btf_p, id);
> + t = btf_type_by_id(btf, id);
> if (!t || !btf_type_is_func(t))
> - goto err;
> + return NULL;
>
> /* The type of BTF_KIND_FUNC is BTF_KIND_FUNC_PROTO */
> - t = btf_type_by_id(*btf_p, t->type);
> + t = btf_type_by_id(btf, t->type);
> if (!t || !btf_type_is_func_proto(t))
> - goto err;
> + return NULL;
>
> + *btf_p = no_free_ptr(btf);
> return t;
> -err:
> - btf_put(*btf_p);
> - return NULL;
> }
>
> /*
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 0b3db02030a7..c11adfa83d5c 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -698,7 +698,7 @@ void print_function_args(struct trace_seq *s, unsigned long *args,
> const char *param_name;
> char name[KSYM_NAME_LEN];
> unsigned long arg;
> - struct btf *btf;
> + struct btf *btf __free(btf_put) = NULL;
> s32 tid, nr = 0;
> int a, p, x;
>
> @@ -716,7 +716,7 @@ void print_function_args(struct trace_seq *s, unsigned long *args,
>
> param = btf_get_func_param(t, &nr);
> if (!param)
> - goto out_put;
> + goto out;
>
> for (a = 0, p = 0; p < nr; a++, p++) {
> if (p)
> @@ -756,7 +756,7 @@ void print_function_args(struct trace_seq *s, unsigned long *args,
> trace_seq_putc(s, ':');
> if (++a == FTRACE_REGS_MAX_ARGS) {
> trace_seq_puts(s, "...]");
> - goto out_put;
> + goto out;
> }
> trace_seq_printf(s, "0x%lx", args[a]);
> }
> @@ -764,8 +764,6 @@ void print_function_args(struct trace_seq *s, unsigned long *args,
> break;
> }
> }
> -out_put:
> - btf_put(btf);
> out:
> trace_seq_printf(s, ")");
> }
> --
> 2.47.2
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>