Re: [PATCH v4 2/5] tools: add xdp-forwarder

[Yureka <yuka@yuka.dev>]

On 9/23/25 17:31, Alyssa Ross wrote:
> Yureka <yuka@yuka.dev> writes:
>
>> On 9/23/25 17:14, Alyssa Ross wrote:
>>> Yureka Lilian <yureka@cyberchaos.dev> writes:
>>>> @@ -88,12 +94,15 @@ stdenv.mkDerivation (finalAttrs: {
>>>>      mesonFlags = [
>>>>        (lib.mesonBool "app" appSupport)
>>>>        (lib.mesonBool "host" hostSupport)
>>>> +    (lib.mesonBool "driver" driverSupport)
>>>>        "-Dhostfsrootdir=/run/virtiofs/virtiofs0"
>>>>        "-Dtests=false"
>>>>        "-Dunwind=false"
>>>>        "-Dwerror=true"
>>>>      ];
>>>>    
>>>> +  hardeningDisable = lib.optionals driverSupport [ "zerocallusedregs" ];
>>>> +
>>> Could we instead do this in bpf_o_cmd, so it's not disabled for
>>> userspace programs?
>> This environment variable works on the stdenv level, so it is difficult
>> to mix it in from the meson recipe. Any way to do this would add NixOS
>> specifics to the meson recipe and doesn't feel quite right.
> The environment variable in stdenv just adds
> -fzero-call-used-regs=used-gpr to the compiler flags, before the ones
> given on the command line, so I was thinking we could just add
> -fzero-call-used-regs=skip (the default) to bpf_o_cmd, to explicitly say
> we don't want it for these compiler invocations.  It'll override the
> option given by the compiler wrapper, and won't do anything Nix-specific
> — it would be the right thing for other distros that change compiler
> defaults as well, which I think is not that uncommon.

Sadly this does not work, because passing -fzero-call-used-regs=skip 
results in

clang: error: unsupported option '-fzero-call-used-regs=skip' for target 
'bpf'


>>>> diff --git a/tools/xdp-forwarder/meson.build b/tools/xdp-forwarder/meson.build
>>>> new file mode 100644
>>>> index 0000000..e6d91ca
>>>> --- /dev/null
>>>> +++ b/tools/xdp-forwarder/meson.build
>>>> @@ -0,0 +1,48 @@
>>>> +# SPDX-License-Identifier: EUPL-1.2+
>>>> +# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
>>>> +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>
>>>> +
>>>> +libbpf = dependency('libbpf', version : '1.6.2')
>>>> +
>>>> +executable('set-router-iface', 'set_router_iface.c',
>>>> +  dependencies : libbpf,
>>>> +  install : true)
>>>> +
>>>> +clang = find_program('clang')
>>> Should be native: true I think.
>> I can't find a parameter 'native' for find_program() in the meson docs.
>> Can you explain why this option is needed? Is it to prevent passing two
>> --target args when cross-compiling?
> It's to prevent it trying to execute clang for the system you're
> building for.  You'd use find_program(…, native: false) (the default) if
> you wanted to embed the path to that program in your binary, for
> example.
>
> Documentation is here:
> https://mesonbuild.com/Reference-manual_functions.html#find_program_native