linux-kernel - [PATCH] [v2] bpf: work around -Wuninitialized warning

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Date:   Tue, 25 Jul 2023 22:26:40 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hou Tao <houtao1@...wei.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] [v2] bpf: work around -Wuninitialized warning

From: Arnd Bergmann <arnd@...db.de>

Splitting these out into separate helper functions means that we
actually pass an uninitialized variable into another function call
if dec_active() happens to not be inlined, and CONFIG_PREEMPT_RT
is disabled:

kernel/bpf/memalloc.c: In function 'add_obj_to_free_list':
kernel/bpf/memalloc.c:200:9: error: 'flags' is used uninitialized [-Werror=uninitialized]
  200 |         dec_active(c, flags);

Avoid this by passing the flags by reference, so they either get
initialized and dereferenced through a pointer, or the pointer never
gets accessed at all.

Fixes: 18e027b1c7c6d ("bpf: Factor out inc/dec of active flag into helpers.")
Suggested-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 kernel/bpf/memalloc.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 51d6389e5152e..14d9b1a9a4cac 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -183,11 +183,11 @@ static void inc_active(struct bpf_mem_cache *c, unsigned long *flags)
 	WARN_ON_ONCE(local_inc_return(&c->active) != 1);
 }
 
-static void dec_active(struct bpf_mem_cache *c, unsigned long flags)
+static void dec_active(struct bpf_mem_cache *c, unsigned long *flags)
 {
 	local_dec(&c->active);
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_irq_restore(flags);
+		local_irq_restore(*flags);
 }
 
 static void add_obj_to_free_list(struct bpf_mem_cache *c, void *obj)
@@ -197,7 +197,7 @@ static void add_obj_to_free_list(struct bpf_mem_cache *c, void *obj)
 	inc_active(c, &flags);
 	__llist_add(obj, &c->free_llist);
 	c->free_cnt++;
-	dec_active(c, flags);
+	dec_active(c, &flags);
 }
 
 /* Mostly runs from irq_work except __init phase. */
@@ -344,7 +344,7 @@ static void free_bulk(struct bpf_mem_cache *c)
 			cnt = --c->free_cnt;
 		else
 			cnt = 0;
-		dec_active(c, flags);
+		dec_active(c, &flags);
 		if (llnode)
 			enque_to_free(tgt, llnode);
 	} while (cnt > (c->high_watermark + c->low_watermark) / 2);
@@ -384,7 +384,7 @@ static void check_free_by_rcu(struct bpf_mem_cache *c)
 		llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra_rcu))
 			if (__llist_add(llnode, &c->free_by_rcu))
 				c->free_by_rcu_tail = llnode;
-		dec_active(c, flags);
+		dec_active(c, &flags);
 	}
 
 	if (llist_empty(&c->free_by_rcu))
@@ -408,7 +408,7 @@ static void check_free_by_rcu(struct bpf_mem_cache *c)
 	inc_active(c, &flags);
 	WRITE_ONCE(c->waiting_for_gp.first, __llist_del_all(&c->free_by_rcu));
 	c->waiting_for_gp_tail = c->free_by_rcu_tail;
-	dec_active(c, flags);
+	dec_active(c, &flags);
 
 	if (unlikely(READ_ONCE(c->draining))) {
 		free_all(llist_del_all(&c->waiting_for_gp), !!c->percpu_size);
-- 
2.39.2