linux-kernel - Re: [PATCH, net-next 2/2] bpf: avoid -Wmaybe-uninitialized warning

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Message-ID: <1E0F5F01-EB03-4717-95E4-41944690B3E9@fb.com>
Date:   Fri, 25 May 2018 21:54:04 +0000
From:   Song Liu <songliubraving@...com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>, Martin Lau <kafai@...com>,
        Chenbo Feng <fengc@...gle.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH, net-next 2/2] bpf: avoid -Wmaybe-uninitialized warning


> On May 25, 2018, at 2:33 PM, Arnd Bergmann <arnd@...db.de> wrote:
> 
> The stack_map_get_build_id_offset() function is too long for gcc to track
> whether 'work' may or may not be initialized at the end of it, leading
> to a false-positive warning:
> 
> kernel/bpf/stackmap.c: In function 'stack_map_get_build_id_offset':
> kernel/bpf/stackmap.c:334:13: error: 'work' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> This removes the 'in_nmi_ctx' flag and uses the state of that variable
> itself to see if it got initialized.
> 
> Fixes: bae77c5eb5b2 ("bpf: enable stackmap with build_id in nmi context")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> kernel/bpf/stackmap.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index b59ace0f0f09..b675a3f3d141 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -285,11 +285,10 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> {
> 	int i;
> 	struct vm_area_struct *vma;
> -	bool in_nmi_ctx = in_nmi();
> 	bool irq_work_busy = false;
> -	struct stack_map_irq_work *work;
> +	struct stack_map_irq_work *work = NULL;
> 
> -	if (in_nmi_ctx) {
> +	if (in_nmi()) {
> 		work = this_cpu_ptr(&up_read_work);
> 		if (work->irq_work.flags & IRQ_WORK_BUSY)
> 			/* cannot queue more up_read, fallback */
> @@ -328,7 +327,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> 		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
> 	}
> 
> -	if (!in_nmi_ctx) {
> +	if (!work) {
> 		up_read(&current->mm->mmap_sem);
> 	} else {
> 		work->sem = &current->mm->mmap_sem;
> -- 
> 2.9.0
> 

Acked-by: Song Liu <songliubraving@...com>