On Sun, Oct 29, 2023 at 2:35 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Fri, Oct 27, 2023 at 2:09 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote: > > > > Hi, > > > > The following C repro contains a bpf program that can cause rcu > > stall/soft lockup during running in bpf_prog_run(). Seems the verifier > > incorrectly passed the program with an infinite loop. > > > > C repro: https://pastebin.com/raw/ymzAxjeU > > Thanks for the report. > Did you debug what exactly caused this bug? > Are you planning to work on the fix? This bug is really hard to debug. Here is a simplified view of the original program: loop: 0: r4 = r8 1: r1 = 0x1f 2: r8 -= -8 3: if r1 > r7 goto pc+1 4: r7 <<= r1 ; LSH r7 by 31 5: r5 = r0 6: r5 *= 2 7: if r5 < r0 goto pc+1 8: r8 s>>= 6 9: w7 &= w7 ; r7 = 0 after the first iter 10: r8 -= r7 11: r8 -= -1 12: if r4 >= 0x9 goto loop 13: exit At runtime, r7 is updated to 0 through #4 and #9 at the first iteration, so the following iteration will not take #3 to #4, so #3 can be ignored after the first iteration. r0 is init by get_current_task, and r5 is always smaller than r0 at runtime, so #7 to #8 will never run. So, the update to r8 is only #2 and #11, which together add 9 to r8. Since r4 is set to r8 at the start of each iteration, so it's an infinite loop at runtime. Based on the log, the verifier keeps tracking #7 to #8 and to #9, and at some point, the verifier prunes states and path from #7 to #9, so it stops checking. The log is huge and hard to follow, the issue is likely in pruning logic, but I don't have much knowledge about that part. > > > Verifier's log: https://pastebin.com/raw/thZDTFJc > > log is trimmed. Full log: https://pastebin.com/raw/cTC8wmDH
