Re: [PATCH bpf-next] selftests/bpf: fix list_del() in arena list

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-10-17 at 21:09 +0200, Puranjay Mohan wrote:

[...]

> [root@localhost bpf]# ./test_progs -a arena_list
> #5       arena_list:FAIL
> Caught signal #11!
> Stack trace:
> ./test_progs(crash_handler+0x1c)[0x956fd4]
> linux-vdso.so.1(__kernel_rt_sigreturn+0x0)[0xffff885b7820]
> ./test_progs[0x559f00]
> ./test_progs[0x55a728]
> ./test_progs(test_arena_list+0x28)[0x55aa7c]
> ./test_progs[0x957624]
> ./test_progs(main+0x6a0)[0x959298]
> /lib64/libc.so.6(+0x30558)[0xffff87e62558]
> /lib64/libc.so.6(__libc_start_main+0x9c)[0xffff87e6263c]
> ./test_progs(_start+0x30)[0x5522f0]
> 
> I pushed it to the CI so you can see it fail:
> https://github.com/kernel-patches/bpf/actions/runs/18602175717/job/53043507792
> 
> Another thing you can do in addition to commenting bpf_free() is to also comment
> 
> //n->next = LIST_POISON1;
> //n->pprev = LIST_POISON2;
> 
> and now the test will not crash but fail like:
> 
> test_arena_list_add_del:FAIL:sum of list elems after del unexpected
> sum of list elems after del: actual 499500 != expected 0

Ok, that was an error on my side. The test fails after clean rebuild,
sorry for the noise.

But my point is that if we care about list_del() correctness, the test
should be modified so that the bug is visible w/o commenting anything.

> This is because __list_del is a no-op, and after the poisoning logic
> is commented list_del() becomes a no-op.
> The list stays intact after arena_list_del() finishes.

Yes, I understand what you are fixing.