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Mon, 15 Jun 2026 18:23:55 -0700 (PDT) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Mon, 15 Jun 2026 21:23:53 -0400 Message-Id: To: "Alexei Starovoitov" , Cc: , , , Subject: Re: [PATCH bpf-next] bpf: Emit verbose message when prog-specific btf_struct_access rejects a write From: "Emil Tsalapatis" X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260615232146.5491-1-alexei.starovoitov@gmail.com> In-Reply-To: <20260615232146.5491-1-alexei.starovoitov@gmail.com> On Mon Jun 15, 2026 at 7:21 PM EDT, Alexei Starovoitov wrote: > From: Alexei Starovoitov > > When BPF_WRITE goes through a PTR_TO_BTF_ID register, check_ptr_to_btf_ac= cess() > delegates to env->ops->btf_struct_access(). Most implementations > (bpf_scx_btf_struct_access, tc_cls_act_btf_struct_access, etc.) return > -EACCES for disallowed fields without logging anything, so the verifier > rejects the program with an empty message. For example a scx program doin= g > > 1: R1=3Dtrusted_ptr_task_struct() > ... > 4: (7b) *(u64 *)(r1 +0) =3D r2 > verification time 83 usec > the program is rejected > > leaves the user guessing which field is off-limits. > Emit verbose message. > > Signed-off-by: Alexei Starovoitov Reviewed-by: Emil Tsalapatis > --- > kernel/bpf/verifier.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 2abc79dbf281..030505c8f3e2 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -5786,6 +5786,10 @@ static int check_ptr_to_btf_access(struct bpf_veri= fier_env *env, > return -EFAULT; > } > ret =3D env->ops->btf_struct_access(&env->log, reg, off, size); > + if (ret < 0) > + verbose(env, > + "%s cannot write into ptr_%s at off=3D%d size=3D%d\n", > + reg_arg_name(env, argno), tname, off, size); > } else { > /* Writes are permitted with default btf_struct_access for > * program allocated objects (which always have id > 0),