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[2a01:cb09:b041:9c77:7d42:8aa4:583d:b37e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43cf21f1749sm5885241f8f.14.2026.03.28.09.20.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 28 Mar 2026 09:20:47 -0700 (PDT) Date: Sat, 28 Mar 2026 17:20:44 +0100 From: Paul Chaignon To: Mykyta Yatsenko Cc: bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Harishankar Vishwanathan , Shung-Hsi Yu , Srinivas Narayana , Santosh Nagarakatte Subject: Re: [PATCH v2 bpf-next 5/6] selftests/bpf: Cover invariant violation cases from syzbot Message-ID: References: <87ikamjv5q.fsf@gmail.com> Precedence: bulk X-Mailing-List: bpf@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: <87ikamjv5q.fsf@gmail.com> On Mon, Mar 23, 2026 at 05:46:41PM +0000, Mykyta Yatsenko wrote: > Paul Chaignon writes: [...] > > +/* Last jump can be detected as always taken because the intersection of R5 and > > + * R7 32bit tnums produces a constant that isn't within R7's s32 bounds. > > + */ > > +SEC("socket") > > +__description("dead branch: tnums give impossible constant if equal") > > +__success > > +__flag(BPF_F_TEST_REG_INVARIANTS) > > +__naked void tnums_equal_impossible_constant(void *ctx) > > +{ > > + asm volatile(" \ > > + call %[bpf_get_prandom_u32]; \ > > + r5 = r0; \ > > + r5 &= 0xfffffffffffffffc; /* var_off32=(0; 0xfffffffc) */ \ > > + r7 = r0; \ > > + r7 &= 0x1; /* var_off32=(0x0; 0x1) */ \ > > + r7 += -43; /* s32=[-43; -42] & var_off32=(0xffffffd4; 0x3) */ \ > > + if w5 != w7 goto +1; /* on fallthrough var_off32=-44, not in s32 */ \ > w5 has bits 0,1 zero, rest unknown > w7 has top bits known as 0xffffffd4, and bits 0,1 unknown > If w5 == w7, their tnums must intersect to a single > value = 0xffffffd4(-44). Which is outside of w7's range [-43; 42]. So > the r10 = 0 branch should be unreachable. > I guess the comment should be updated to the kernel style as well. Thanks for the review! We already have a lot of end-of-line comments on the BPF assembly in selftests (including two tests above) so I assumed that was fine. (Unrelated: On some mail clients, your comments are hard to spot. It might help if you could include a blank line before and after them :) ) > Acked-by: Mykyta Yatsenko > > + r10 = 0; \ > > + exit; \ > > +" : > > + : __imm(bpf_get_prandom_u32) > > + : __clobber_all); > > +} > > + > > char _license[] SEC("license") = "GPL"; > > -- > > 2.43.0