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Tue, 18 Aug 2026 14:34:11 -0700 (PDT) Date: Tue, 18 Aug 2026 17:34:10 -0400 From: Justin Suess To: Anastasios Papagiannis Cc: akpm@linux-foundation.org, andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, brauner@kernel.org, daniel@iogearbox.net, david@kernel.org, eddyz87@gmail.com, kpsingh@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, matt@bobrowski.net, memxor@gmail.com, song@kernel.org, viro@zeniv.linux.org.uk Subject: Re: [PATCH bpf-next 2/3] bpf: Add user memory access kfuncs for linux_binprm Message-ID: References: <20260818082441.4091-1-tasos.papagiannnis@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260818082441.4091-1-tasos.papagiannnis@gmail.com> X-Rspam-User: X-Stat-Signature: 8s49bb9oj6zzd3r6iw8q7aw3mbs19adb X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D7064A0009 X-HE-Tag: 1787088852-275766 X-HE-Meta: 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 cJJnTTrm 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 18, 2026 at 11:24:41AM +0300, Anastasios Papagiannis wrote: > Thanks for the review! > > > Would it be better to handle that case transparently rather than > > requiring introducing a new kfunc / leaving that gap open for NOMMU? > > > Either return an error or perform the copy from bprm->page[]. > > > Unless there's some reason I'm not seeing. > > My understanding is that there is currently no way to use these kfuncs > on a NOMMU system. Although CONFIG_BPF_LSM does not directly depend on > CONFIG_MMU, as far as I can tell, no current NOMMU architecture provides > the required support to use them. For this reason even if I write that > code, this cannot be tested. > > > It would also be better for portability across NOMMU / CONFIG_MMU > > systems (the exisiting kfunc is never registered, so a program using it > > would be rejected rather than able to handle the error). > > As you suggested, I would propose to register those kfuncs > unconditionally, move the ifdefs inside those kfuncs, in the case > of NOMMU return an error (e.g. -EOPNOTSUPP) and allow the programs to > handle those. > > Does this sound reasonable? > Yes that does. It's a small niche anyway (I don't think a lot of people are using BPF and NOMMU anyway, and BPF_LSM && !MMU is unsatisfiable unless RISC-V or some other architecture gets JIT support in nommu) And when/if (big if) it does, they wouldn't even need this kfunc and could bpf_probe_read_kernel() bprm->page[] directly. So an error is best. Justin > Thanks, > Anastasios