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

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

[Steven Rostedt]

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.

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

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

[Alexei Starovoitov]

On Fri, Aug 1, 2025 at 4:16 AM 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.
>
> 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;

Sorry I hate this __free() style.
It's not a simplification, but an obfuscation of code and logic.

--
pw-bot: cr

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

[Steven Rostedt]

On Fri, 1 Aug 2025 08:02:24 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> > 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;  
> 
> Sorry I hate this __free() style.
> It's not a simplification, but an obfuscation of code and logic.

Well, it's becoming more common. But you are in control of this, so it's
your decision. I wanted this just to get rid of the gotos in my code.

-- Steve

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

[Alexei Starovoitov]

On Fri, Aug 1, 2025 at 8:06 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 1 Aug 2025 08:02:24 -0700
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>
> > > 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;
> >
> > Sorry I hate this __free() style.
> > It's not a simplification, but an obfuscation of code and logic.
>
> Well, it's becoming more common. But you are in control of this, so it's
> your decision. I wanted this just to get rid of the gotos in my code.

You can use it in kernel/trace/trace_output.c, of course,
but I really think it's a step back in maintainability.
All this cleanup.h is not a silver bullet. It needs to be used sparingly.

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

[Steven Rostedt]

On Fri, 1 Aug 2025 08:12:08 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> 
> You can use it in kernel/trace/trace_output.c, of course,

I guess that means I should just use the DEFINE_FREE() in that file
directly and not in the btf.h header file?

> but I really think it's a step back in maintainability.
> All this cleanup.h is not a silver bullet. It needs to be used sparingly.

I have my reservations about the cleanup.h code too. But the more I use it,
the more I like it. My biggest worry is guard() leak. That is, having a
lock or interrupt/preemption disabled for longer than they need to be,
because code was added at the end of the function after the protection is
needed.

-- Steve

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

[Alexei Starovoitov]

On Fri, Aug 1, 2025 at 8:19 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 1 Aug 2025 08:12:08 -0700
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>
> >
> > You can use it in kernel/trace/trace_output.c, of course,
>
> I guess that means I should just use the DEFINE_FREE() in that file
> directly and not in the btf.h header file?

yes. I don't want this to proliferate and people spam us
with this kind of "cleanups".

> > but I really think it's a step back in maintainability.
> > All this cleanup.h is not a silver bullet. It needs to be used sparingly.
>
> I have my reservations about the cleanup.h code too. But the more I use it,
> the more I like it. My biggest worry is guard() leak. That is, having a
> lock or interrupt/preemption disabled for longer than they need to be,
> because code was added at the end of the function after the protection is
> needed.

guard() is ok. We use it too,
but __free() is imo garbage. It's essence of what's wrong with C++

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

[Tomas Glozar]

pá 1. 8. 2025 v 17:29 odesílatel Alexei Starovoitov
<alexei.starovoitov@gmail.com> napsal:
>
> but __free() is imo garbage. It's essence of what's wrong with C++
>

Here, you at least can read the beginning of the function though, and
see that a free will be done at the end, like Go's defer, right?

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

[Steven Rostedt]

On Fri, 1 Aug 2025 17:34:51 +0200
Tomas Glozar <tglozar@redhat.com> wrote:

> pá 1. 8. 2025 v 17:29 odesílatel Alexei Starovoitov
> <alexei.starovoitov@gmail.com> napsal:
> >
> > but __free() is imo garbage. It's essence of what's wrong with C++
> >  
> 
> Here, you at least can read the beginning of the function though, and
> see that a free will be done at the end, like Go's defer, right?

I don't know Go, but I'll assume "yes".

I prefer the __free() over goto end; where you need to free up a temporary
variable on error paths.

But regardless. This will just be a difference of opinion, and I respect
that Alexei doesn't want to use it in his code.

-- Steve

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

[Tomas Glozar]

pá 1. 8. 2025 v 17:56 odesílatel Steven Rostedt <rostedt@goodmis.org> napsal:
>
> But regardless. This will just be a difference of opinion, and I respect
> that Alexei doesn't want to use it in his code.
>

Of course.

I just wanted to point out this is (in my opinion not nearly) as bad
as what C++ is doing.

Tomas

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

[Masami Hiramatsu]

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>

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

[Steven Rostedt]

On Mon, 4 Aug 2025 22:26:45 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> 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>

Thanks but Alexei already declined it.

I'm gonna respin where it is only used in the tracing code.

Thanks!

-- Steve