Re: [PATCH v3 bpf-next] selftests/bpf: Fix selftests broken by mitigations=off

[Yonghong Song <yonghong.song@xxxxxxxxx>]

On 10/26/23 6:46 AM, Daniel Borkmann wrote:
> On 10/25/23 6:56 AM, Yonghong Song wrote:
> > On 10/24/23 8:11 PM, Yafang Shao wrote:
> > > When we configure the kernel command line with 'mitigations=off' and 
> > > set
> > > the sysctl knob 'kernel.unprivileged_bpf_disabled' to 0, the commit
> > > bc5bc309db45 ("bpf: Inherit system settings for CPU security 
> > > mitigations")
> > > causes issues in the execution of `test_progs -t verifier`. This is 
> > > because
> > > 'mitigations=off' bypasses Spectre v1 and Spectre v4 protections.
> > >
> > > Currently, when a program requests to run in unprivileged mode
> > > (kernel.unprivileged_bpf_disabled = 0), the BPF verifier may prevent it
> > > from running due to the following conditions not being enabled:
> > >
> > >    - bypass_spec_v1
> > >    - bypass_spec_v4
> > >    - allow_ptr_leaks
> > >    - allow_uninit_stack
> > >
> > > While 'mitigations=off' enables the first two conditions, it does not
> > > enable the latter two. As a result, some test cases in
> > > 'test_progs -t verifier' that were expected to fail to run may run
> > > successfully, while others still fail but with different error 
> > > messages.
> > > This makes it challenging to address them comprehensively.
> > >
> > > Moreover, in the future, we may introduce more fine-grained control 
> > > over
> > > CPU mitigations, such as enabling only bypass_spec_v1 or 
> > > bypass_spec_v4.
> > >
> > > Given the complexity of the situation, rather than fixing each 
> > > broken test
> > > case individually, it's preferable to skip them when 
> > > 'mitigations=off' is
> > > in effect and introduce specific test cases for the new 
> > > 'mitigations=off'
> > > scenario. For instance, we can introduce new BTF declaration tags like
> > > '__failure__nospec', '__failure_nospecv1' and '__failure_nospecv4'.
> > >
> > > In this patch, the approach is to simply skip the broken test cases 
> > > when
> > > 'mitigations=off' is enabled. The result of `test_progs -t verifier` as
> > > follows after this commit,
> > >
> > > Before this commit
> > > ==================
> > > - without 'mitigations=off'
> > >    - kernel.unprivileged_bpf_disabled = 2
> > >      Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
> > >    - kernel.unprivileged_bpf_disabled = 0
> > >      Summary: 74/1336 PASSED, 0 SKIPPED, 0 FAILED <<<<
> > > - with 'mitigations=off'
> > >    - kernel.unprivileged_bpf_disabled = 2
> > >      Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
> > >    - kernel.unprivileged_bpf_disabled = 0
> > >      Summary: 63/1276 PASSED, 0 SKIPPED, 11 FAILED <<<< 11 FAILED
> > >
> > > After this commit
> > > =================
> > > - without 'mitigations=off'
> > >    - kernel.unprivileged_bpf_disabled = 2
> > >      Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
> > >    - kernel.unprivileged_bpf_disabled = 0
> > >      Summary: 74/1336 PASSED, 0 SKIPPED, 0 FAILED <<<<
> > > - with this patch, with 'mitigations=off'
> > >    - kernel.unprivileged_bpf_disabled = 2
> > >      Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
> > >    - kernel.unprivileged_bpf_disabled = 0
> > >      Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED <<<< SKIPPED
> > >
> > > Fixes: bc5bc309db45 ("bpf: Inherit system settings for CPU security 
> > > mitigations")
> > > Reported-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
> > > Closes: 
> > > https://lore.kernel.org/bpf/CAADnVQKUBJqg+hHtbLeeC2jhoJAWqnmRAzXW3hmUCNSV9kx4sQ@xxxxxxxxxxxxxx
> > > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> >
> > Ack with a nit below.
> > Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> [...]
> > >       }
> > > -    return disabled;
> > > +    return disabled ? true : get_mitigations_off();
> >
> > Above code is correct. But you could slightly simplify it with
> >      return disabled ? : get_mitigations_off();
> >
> > I guess maintainer can decide whether simplification is needed
> > or not.
>
> Turns out if you omit, then compiler will complain with a warning :)
>
>   [...]
>   GEN      vmlinux.h
> unpriv_helpers.c: In function ‘get_unpriv_disabled’:
> unpriv_helpers.c:56:27: error: the omitted middle operand in ‘?:’ will 
> always be ‘true’, suggest explicit middle operand [-Werror=parentheses]
>    56 |         return disabled ? : get_mitigations_off();
>       |                           ^
> cc1: all warnings being treated as errors
> make: *** [Makefile:615: 
> /root/linux/tools/testing/selftests/bpf/unpriv_helpers.o] Error 1

clang compiler is okay with '?:' change while gcc compiler issued errors. So yes,
existing code is good for both compilers. Thanks!


> So it's okay as is, applied, thanks!