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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761676360; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=nf6l5EBnraIwk2fFm2crn/UFRlvBCZhPypt0K7UxZjY=; b=hssVsn3Q/lG0Ha8WD0NuxkPn5nIfYTE9daX6kZTPWtQkfcc3c5liTlrsaeobIK+I5u/Qsi 5u5PKy2Fn5+Y3sy756yujLmcWPZCVTncPzc64DvnC8OzMF/k3zCALlzXU5oKmoAsWHtJ7d ssChMGvkIZRChpOs5U70e0V6baHPiU0= From: Roman Gushchin To: Chris Mason Cc: Alexei Starovoitov , bot+bpf-ci@kernel.org, Andrew Morton , LKML , Alexei Starovoitov , Suren Baghdasaryan , Michal Hocko , Shakeel Butt , Johannes Weiner , Andrii Nakryiko , inwardvessel , linux-mm , "open list:CONTROL GROUP (CGROUP)" , bpf , Martin KaFai Lau , Song Liu , Kumar Kartikeya Dwivedi , Tejun Heo , Daniel Borkmann , Eduard , Yonghong Song , Ihor Solodrai Subject: Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg pointers In-Reply-To: (Chris Mason's message of "Tue, 28 Oct 2025 14:03:18 -0400") References: <20251027231727.472628-9-roman.gushchin@linux.dev> <2c91977fcab04be6305bf4be57e825f7e84005d16667adcdfad0585be506537c@mail.kernel.org> <87ldkv57nc.fsf@linux.dev> Date: Tue, 28 Oct 2025 11:32:31 -0700 Message-ID: <87y0ouri68.fsf@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 1285DA0008 X-Stat-Signature: fgbpyggkga9yo3ipm5cap7uatoik5kbe X-Rspam-User: X-HE-Tag: 1761676362-46774 X-HE-Meta: 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 dPviv7xH ctmG9jGXr/sudUMHu/lxc+1gonhEBBW1ECCanQVQzx/hwjvCYsCH6Vs2qB/JHODsxp3x2Re+81pHv/4RlonoPWTy7U8m3jP8iVY1h8lo2kGSJHhbL1+n9RK/8kE+Hwk2t4JOPb0h3WjbJy7tCsEfCLqSs9OhtG9FMZejdGl4Wzmp83NUAizQywOvzNn9AhI/uhk8Etwqqy6DdqDLNglxWOs5hfDluXhiieu2srKCekYMP7IAs/8R5FGoNw1TtnR2NLVX/CbXr7lrnHxAKsItfMpEZEfAa+BjVRWw70YvtJwnBa30Sol40pZj7SpYydSIPP++bSNUKmqbC8v5Zm3SitJHiZmugECHmadEp5SiBrvlbkmq79VhsCdEdRA== 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: Chris Mason writes: > On 10/28/25 1:12 PM, Alexei Starovoitov wrote: >> On Tue, Oct 28, 2025 at 9:11=E2=80=AFAM Roman Gushchin wrote: >>> >>> bot+bpf-ci@kernel.org writes: >>> >>>> ``` >>>> commit ec8e2e1769704b4f69261f36e6aaf035d47c5f0f >>>> Author: Roman Gushchin >>>> Can this dereference a NULL css pointer? The function checks css for >>>> NULL at line 42 with "if (css && css_tryget(css))", which suggests css >>>> can be NULL. If a BPF program calls this function with a NULL css, >>>> wouldn't it crash here before reaching that check? >>> >>> Here css passed as an argument to bpf_get_mem_cgroup() can't be NULL, >>> because the BPF verifier guarantees that it's a valid css pointer. >>> >>> However the result of rcu_dereference_raw(cgroup->subsys[ssid]) can be >>> NULL, this is why the "if (css && css_tryget(css))" check is required >>> down below. >>=20 >> Yeah. Not sure how feasible it is to teach AI about KF_RCU semantics. > > I pulled it down locally to try and w/semcode it is properly catching thi= s: > > False Positives Eliminated > > 1. EH-001 NULL dereference - css parameter dereferenced without check > > - Why false positive: BPF verifier ensures pointer parameters are > non-NULL. All kernel kfuncs follow the same pattern of not checking > parameters for NULL (css_rstat_updated, css_rstat_flush, > bpf_put_mem_cgroup, etc.). The KF_RET_NULL flag controls return value, > not parameter nullability. > > My plan is to just have the prompt read Documentation/bpf/kfuncs.rst, > which Eduard suggested. I'll make a bpf kfuncs pattern and do that. Awesome, thank you!