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Thu, 12 Nov 2026 12:04:53 -0800 (PST) Received: from localhost ([153.61.198.249]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2ee0f75c980sm536275ad.25.2026.11.12.12.04.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 12 Nov 2026 12:04:53 -0800 (PST) Content-Type: text/plain; charset=UTF-8 Date: Thu, 12 Nov 2026 20:04:52 +0000 Message-Id: Cc: , , "Sumit Semwal" , =?utf-8?q?Christian_K=C3=B6nig?= , "Philipp Stanner" , , , Subject: Re: [PATCH] bpf: make dmabuf iterator stay drained after NULL From: "Alexei Starovoitov" To: "Kang Chen" , "Yonghong Song" , "Daniel Borkmann" , "Andrii Nakryiko" , "Eduard Zingerman" , "Kumar Kartikeya Dwivedi" In-Reply-To: <20261110125106.2935677-1-k.chen@smail.nju.edu.cn> References: <20261110125106.2935677-1-k.chen@smail.nju.edu.cn> Content-Transfer-Encoding: 8bit X-Mailing-List: linux-media@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Tue, Nov 10, 2026 at 04:51 AM Kang Chen wrote: > +/* > + * The iterator state is one pointer wide because struct bpf_iter_dmabuf is > + * part of the generated vmlinux.h ABI. Use an invalid pointer value as the > + * "drained" sentinel instead of growing the state. > + */ > +#define DMABUF_ITER_DONE ((struct dma_buf *)1L) +1 to T.J.'s point re: DMABUF_ITER_START. bpf_iter_kmem_cache_next() does it that way. dma_buf_iter_next() leaves NULL at the end of the list. Let NULL mean drained. Then no need for the 2nd store in next() and for the long comments. diff --git a/kernel/bpf/dmabuf_iter.c b/kernel/bpf/dmabuf_iter.c index cd50024..1bef87b 100644 --- a/kernel/bpf/dmabuf_iter.c +++ b/kernel/bpf/dmabuf_iter.c @@ -156,6 +156,8 @@ struct bpf_iter_dmabuf_kern { struct dma_buf *dmabuf; } __aligned(8); +#define DMABUF_ITER_START ((struct dma_buf *)1L) + __bpf_kfunc_start_defs(); __bpf_kfunc int bpf_iter_dmabuf_new(struct bpf_iter_dmabuf *it) @@ -165,7 +167,7 @@ __bpf_kfunc int bpf_iter_dmabuf_new(struct bpf_iter_dmabuf *it) BUILD_BUG_ON(sizeof(*kit) > sizeof(*it)); BUILD_BUG_ON(__alignof__(*kit) != __alignof__(*it)); - kit->dmabuf = NULL; + kit->dmabuf = DMABUF_ITER_START; return 0; } @@ -173,10 +175,14 @@ __bpf_kfunc struct dma_buf *bpf_iter_dmabuf_next(struct bpf_iter_dmabuf *it) { struct bpf_iter_dmabuf_kern *kit = (void *)it; - if (kit->dmabuf) - kit->dmabuf = dma_buf_iter_next(kit->dmabuf); - else + /* Once NULL is returned, next() has to keep returning NULL */ + if (!kit->dmabuf) + return NULL; + + if (kit->dmabuf == DMABUF_ITER_START) kit->dmabuf = dma_buf_iter_begin(); + else + kit->dmabuf = dma_buf_iter_next(kit->dmabuf); return kit->dmabuf; } @@ -185,7 +191,7 @@ __bpf_kfunc void bpf_iter_dmabuf_destroy(struct bpf_iter_dmabuf *it) { struct bpf_iter_dmabuf_kern *kit = (void *)it; - if (kit->dmabuf) + if (kit->dmabuf && kit->dmabuf != DMABUF_ITER_START) dma_buf_put(kit->dmabuf); } Pls take T.J.'s selftest as patch 2 and tag the respin as [PATCH bpf v2]. Also you sent fixes for css and scx_dsq. bpf_iter_task_vma_next() has the same problem. It doesn't hold mmap_lock between the calls since commit bee9ef4a40a2 ("bpf: switch task_vma iterator from mmap_lock to per-VMA locks"): Pls fix it too. pw-bot: cr