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Tue, 13 Jan 2026 17:35:12 -0800 (PST) Message-ID: Subject: Re: [PATCH bpf-next v1 03/10] bpf: Verifier support for KF_IMPLICIT_ARGS From: Eduard Zingerman To: Ihor Solodrai , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau Cc: Mykyta Yatsenko , Tejun Heo , Alan Maguire , Benjamin Tissoires , Jiri Kosina , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-input@vger.kernel.org, sched-ext@lists.linux.dev Date: Tue, 13 Jan 2026 17:35:09 -0800 In-Reply-To: References: <20260109184852.1089786-1-ihor.solodrai@linux.dev> <20260109184852.1089786-4-ihor.solodrai@linux.dev> <952853dd064d5303a7e7ec8e58028e9ee88f2fad.camel@gmail.com> <93ecdc25-aa5e-485b-8ff4-a9db3b585861@linux.dev> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.2 (3.58.2-1.fc43) Precedence: bulk X-Mailing-List: linux-input@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-01-13 at 15:48 -0800, Ihor Solodrai wrote: [...] > A follow up after a chat with Eduard. >=20 > This change in check_kfunc_call() appears to be working: >=20 > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 092003cc7841..ff743335111c 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -13958,8 +13958,11 @@ static int check_kfunc_call(struct bpf_verifier_= env *env, struct bpf_insn *insn, > regs =3D branch->frame[branch->curframe]->regs; > =20 > /* Clear r0-r5 registers in forked state */ > - for (i =3D 0; i < CALLER_SAVED_REGS; i++) > - mark_reg_not_init(env, regs, caller_saved[i]); > + for (i =3D 0; i < CALLER_SAVED_REGS; i++) { > + u32 regno =3D caller_saved[i]; > + mark_reg_not_init(env, regs, regno); > + regs[regno].subreg_def =3D DEF_NOT_SUBREG; > + } > =20 > mark_reg_unknown(env, regs, BPF_REG_0); > err =3D __mark_reg_s32_range(env, regs, BPF_REG_0, -MAX_E= RRNO, -1); >=20 > https://github.com/kernel-patches/bpf/actions/runs/20975419422 >=20 > Apparently, doing .subreg_def =3D DEF_NOT_SUBREG in mark_reg_not_init() > breaks zero-extension tracking somewhere else. But this is not > directly relevant to the series. >=20 > Eduard, Alexei, any concerns with this diff? Should I send a separate > patch? Imo this is acceptable to land this series but follow up investigation is definitely needed. Either there is a bug and mark_reg_not_init() is called in a context where upper 32-bits are still significant, or zero extension related code can be improved to avoid patching in some cases. Additional context for other reviewers, Ihor did two experiments: - added '.subreg_def =3D DEF_NOT_SUBREG' to mark_reg_not_init(), which resulted in selftests failure; - added '.subreg_def =3D DEF_NOT_SUBREG' as above, which worked fine. Meaning that code in check_kfunc_call() is not a culprit.