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[34.147.59.14]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b842a2bc6bbsm29697666b.27.2026.01.05.13.04.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Jan 2026 13:04:21 -0800 (PST) Date: Mon, 5 Jan 2026 21:04:17 +0000 From: Matt Bobrowski To: Alexei Starovoitov Cc: Kumar Kartikeya Dwivedi , Tejun Heo , Roman Gushchin , bpf , linux-mm , LKML , JP Kobryn , Alexei Starovoitov , Daniel Borkmann , Shakeel Butt , Michal Hocko , Johannes Weiner Subject: Re: [PATCH bpf-next v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Message-ID: References: <20251223044156.208250-1-roman.gushchin@linux.dev> <20251223044156.208250-4-roman.gushchin@linux.dev> <7ia4ms2zwuqb.fsf@castle.c.googlers.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam02 X-Stat-Signature: 1qwdoxe5cgtej6u9g9sffj4fuq9ye5bt X-Rspam-User: X-Rspamd-Queue-Id: 54D3814001C X-HE-Tag: 1767647063-599498 X-HE-Meta: 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 saKz+hBh 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jan 05, 2026 at 08:05:54AM -0800, Alexei Starovoitov wrote: > On Sun, Jan 4, 2026 at 11:49 PM Matt Bobrowski wrote: > > > > > > > > No need for a new KF flag. Any struct returned by kfunc should be > > > trusted or trusted_or_null if KF_RET_NULL was specified. > > > I don't remember off the top of my head, but this behavior > > > is already implemented or we discussed making it this way. > > > > Hm, I do not see any evidence of this kind of semantic currently > > implemented, so perhaps it was only discussed at some point. Would you > > like me to put forward a patch that introduces this kind of implicit > > trust semantic for BPF kfuncs returning pointer to struct types? > > Hmm. What about these: > BTF_ID_FLAGS(func, scx_bpf_cpu_rq) > BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_RET_NULL) > BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_RET_NULL | KF_RCU_PROTECTED) > > I thought they're returning a trusted pointer without acquiring it. > iirc the last one returns trusted in RCU CS, > but the first two return just a legacy ptr_to_btf_id ? > This is something to fix asap then. No, AFAIU they do not. These simply return a regular pointer to BTF ID (PTR_TO_BTF_ID), rather than a formally "trusted" pointer (which would carry the PTR_TRUSTED flag or a ref_obj_id). scx_bpf_cpu_curr returns a MEM_RCU pointer (via KF_RCU_PROTECTED), which is somewhat considered to be trusted within a RCU read-side critical section *ONLY*. Kumar/Tejun, Please keep me honest here.