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[2a01:cb08:8949:7e00:d448:ffe2:5653:721a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-458705bcbb7sm84285275e9.18.2025.07.26.03.07.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 26 Jul 2025 03:07:32 -0700 (PDT) Date: Sat, 26 Jul 2025 12:07:31 +0200 From: Paul Chaignon To: Eduard Zingerman Cc: bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Yonghong Song , Shung-Hsi Yu Subject: Re: [PATCH bpf-next v2 3/4] selftests/bpf: Test cross-sign 64bits range refinement Message-ID: References: <8f1297bcbfaeebff55215d57f488570152ebb05f.1753364265.git.paul.chaignon@gmail.com> <905853bfc266a6969953b4de8433ef9ca7e7a34c.camel@gmail.com> <6d75ad3a05ebf56ab2f68e677264e8142c372fbc.camel@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: On Fri, Jul 25, 2025 at 12:15:18AM -0700, Eduard Zingerman wrote: [...] > So, going back to the question of the test cases, here is a relevant > part with debug prints [1]: > > 7: (1f) r0 -= r6 > reg_bounds_sync entry: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146) > reg_bounds_sync __update_reg_bounds #1: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146) > __reg32_deduce_bounds #8: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146,umin32=0xfffffcf1,umax32=0xffffff6e) > __reg_deduce_mixed_bounds #1: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1, umax=0xffffffffffffff6e,smin32=-783,smax32=-146, umax32=0xffffff6e) > reg_bounds_sync __reg_deduce_bounds #1: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1, umax=0xffffffffffffff6e,smin32=-783,smax32=-146, umax32=0xffffff6e) > __reg32_deduce_bounds #7: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1, umax=0xffffffffffffff6e,smin32=-783,smax32=-146, umax32=0xffffff6e) > __reg32_deduce_bounds #8: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1, umax=0xffffffffffffff6e,smin32=-783,smax32=-146, umax32=0xffffff6e) > __reg64_deduce_bounds #4: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e) > __reg_deduce_mixed_bounds #1: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e) > reg_bounds_sync __reg_deduce_bounds #2: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e) > reg_bounds_sync __reg_bound_offset: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff)) > reg_bounds_sync __update_reg_bounds #2: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff)) > > 8: R0=scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff)) > R6=scalar(smin=umin=smin32=umin32=400,smax=umax=smax32=umax32=527,var_off=(0x0; 0x3ff)) > > Important parts are: > a. "__reg32_deduce_bounds #8" updates umin32 and umax32 > b. "__reg_deduce_mixed_bounds #1" updates umin and umax (uses values from a) > c. "__reg64_deduce_bounds #4" updates smax and umin (enabled by b) > > Only at this point there is an opportunity to refine smin32 from smin > using rule "__reg32_deduce_bounds #2", because of the conditions for > umin and umax (umin refinement by (c) is crucial). > Your new check is (c). > > So, it looks like adding third call to __reg_deduce_bounds() in > reg_bounds_sync() is not wrong and the change is linked to this > patch-set. Thanks a lot for the full analysis! I've added a patch in the v3 to call __reg_deduce_bounds a third time. I reused your analysis and trace from above in the patch description. Note I added you as a co-author; give me a shout if I shouldn't have. > > As you say, whether there is a better way to organize all these rules > requires further analysis, and is a bit out of scope for this > patch-set. > > [1] https://github.com/kernel-patches/bpf/commit/f68d4957204f21caac67d24de40fb66e4618f354