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[27.51.48.151]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-7ebcae1dce3sm14429042a34.3.2026.07.13.22.01.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2026 22:01:42 -0700 (PDT) Date: Tue, 14 Jul 2026 13:01:32 +0800 From: Shung-Hsi Yu To: sun jian , Eduard Zingerman Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com, andrii@kernel.org, memxor@gmail.com, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, emil@etsalapatis.com, shuah@kernel.org, mmullins@fb.com, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH bpf v4 1/2] bpf: Reject negative const offsets for buffer pointers Message-ID: References: <1f8aad47f61fbf85eb55276e4bb01bfcff61393e.camel@gmail.com> <3430dc0a2a141769a596ab21d7abdd86a0a804db.camel@gmail.com> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jul 14, 2026 at 12:31:38PM +0800, Shung-Hsi Yu wrote: ... > > > var_off = (s64)reg->var_off.value; > > > if (var_off >= BPF_MAX_VAR_OFF || var_off <= -BPF_MAX_VAR_OFF) > > > > This should hold already. > > FWIW, I had thought so, but after checking I realize that the verifier > only ensure that > > -BPF_MAX_VAR_OFF < smin < BPF_MAX_VAR_OFF > > holds true in check_reg_sane_offset_{scalar,ptr}(), and does not check > smax, so technically var_off.value can be greater than BPF_MAX_VAR_OFF > when we reach here... Scratch that, it cannot. Below just shows that (var_off.value | var_off.mask) can be greater than BPF_MAX_VAR_OFF, not var_off.value. So what Eduard said is correct, and -BPF_MAX_VAR_OFF < var_off.value < BPF_MAX_VAR_OFF does always hold. > ..., just that it doesn't really matter much as it is > rejected anyway. And if it is a constant then smin=smax, so for a > constant value it does always falls between +-BPF_MAX_VAR_OFF. > > 0: (79) r6 = *(u64 *)(r1 +0) ; R1=ctx() R6=tp_buffer() > 1: (85) call bpf_get_prandom_u32#7 ; R0=scalar() > 2: (bf) r2 = r0 ; R0=scalar(id=1) R2=scalar(id=1) > 3: (67) r2 <<= 32 ; R2=scalar(smax=0x7fffffff00000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xffffffff00000000)) > 4: (77) r2 >>= 32 ; R2=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff)) > 5: (07) r2 += 536870911 ; R2=scalar(smin=umin=0x1fffffff,smax=umax=0x11ffffffe,var_off=(0x0; 0x1ffffffff)) > /* r2 is now [2^29 - 1, 2^32 + 2^29 - 2] */ > 6: (0f) r6 += r2 > 7: R2=scalar(smin=umin=0x1fffffff,smax=umax=0x11ffffffe,var_off=(0x0; 0x1ffffffff)) R6=tp_buffer(smin=umin=0x1fffffff,smax=umax=0x11ffffffe,var_off=(0x0; 0x1ffffffff)) > 7: (79) r0 = *(u64 *)(r6 +0) > R6 invalid variable buffer offset: off=0, var_off=(0x0; 0x1ffffffff) > [...]