Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers

[Alexei Starovoitov]

On Tue, Jul 12, 2022 at 6:29 PM James Hilliard
<james.hilliard1@xxxxxxxxx> wrote:
>
> On Tue, Jul 12, 2022 at 7:18 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Tue, Jul 12, 2022 at 07:10:27PM -0600, James Hilliard wrote:
> > > On Tue, Jul 12, 2022 at 10:48 AM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jul 12, 2022 at 4:20 AM Jose E. Marchesi
> > > > <jose.marchesi@xxxxxxxxxx> wrote:
> > > > >
> > > > >
> > > > > > CC Quentin as well
> > > > > >
> > > > > > On Mon, Jul 11, 2022 at 5:11 PM James Hilliard
> > > > > > <james.hilliard1@xxxxxxxxx> wrote:
> > > > > >>
> > > > > >> On Mon, Jul 11, 2022 at 5:36 PM Yonghong Song <yhs@xxxxxx> wrote:
> > > > > >> >
> > > > > >> >
> > > > > >> >
> > > > > >> > On 7/6/22 10:28 AM, James Hilliard wrote:
> > > > > >> > > The current bpf_helper_defs.h helpers are llvm specific and don't work
> > > > > >> > > correctly with gcc.
> > > > > >> > >
> > > > > >> > > GCC appears to required kernel helper funcs to have the following
> > > > > >> > > attribute set: __attribute__((kernel_helper(NUM)))
> > > > > >> > >
> > > > > >> > > Generate gcc compatible headers based on the format in bpf-helpers.h.
> > > > > >> > >
> > > > > >> > > This adds conditional blocks for GCC while leaving clang codepaths
> > > > > >> > > unchanged, for example:
> > > > > >> > >       #if __GNUC__ && !__clang__
> > > > > >> > >       void *bpf_map_lookup_elem(void *map, const void *key)
> > > > > >> > > __attribute__((kernel_helper(1)));
> > > > > >> > >       #else
> > > > > >> > >       static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
> > > > > >> > >       #endif
> > > > > >> >
> > > > > >> > It does look like that gcc kernel_helper attribute is better than
> > > > > >> > '(void *) 1' style. The original clang uses '(void *) 1' style is
> > > > > >> > just for simplicity.
> > > > > >>
> > > > > >> Isn't the original style going to be needed for backwards compatibility with
> > > > > >> older clang versions for a while?
> > > > > >
> > > > > > I'm curious, is there any added benefit to having this special
> > > > > > kernel_helper attribute vs what we did in Clang for a long time?
> > > > > > Did GCC do it just to be different and require workarounds like this
> > > > > > or there was some technical benefit to this?
> > > > >
> > > > > We did it that way so we could make trouble and piss you off.
> > > > >
> > > > > Nah :)
> > > > >
> > > > > We did it that way because technically speaking the clang construction
> > > > > works relying on particular optimizations to happen to get correct
> > > > > compiled programs, which is not guaranteed to happen and _may_ break in
> > > > > the future.
> > > > >
> > > > > In fact, if you compile a call to such a function prototype with clang
> > > > > with -O0 the compiler will try to load the function's address in a
> > > > > register and then emit an invalid BPF instruction:
> > > > >
> > > > >   28:   8d 00 00 00 03 00 00 00         *unknown*
> > > > >
> > > > > On the other hand the kernel_helper attribute is bullet-proof: will work
> > > > > with any optimization level, with any version of the compiler, and in
> > > > > our opinion it is also more readable, more tidy and more correct.
> > > > >
> > > > > Note I'm not saying what you do in clang is not reasonable; it may be,
> > > > > obviously it works well enough for you in practice.  Only that we have
> > > > > good reasons for doing it differently in GCC.
> > > >
> > > > Not questioning the validity of the reasons, but they created
> > > > the unnecessary difference between compilers.
> > >
> > > Sounds to me like clang is relying on an unreliable hack that may
> > > be difficult to implement in GCC, so let's see what's the best option
> > > moving forwards in terms of a migration path for both GCC and clang.
> >
> > The following is a valid C code:
> > static long (*foo) (void) = (void *) 1234;
> > foo();
> >
> > and GCC has to generate correct assembly assuming it runs at -O1 or higher.
>
> Providing -O1 or higher with gcc-bpf does not seem to work at the moment.

Let's fix gcc first.

> > There is no indirect call insn defined in BPF ISA yet,
> > so the -O0 behavior is undefined.
>
> Well GCC at least seems to be able to compile BPF programs with -O0 using
> kernel_helper. I assume -O0 is probably just targeting the minimum BPF ISA
> optimization level or something like that which avoids indirect calls.

There are other reasons why -O0 compiled progs will
fail in the verifier.

> >
> > > Or we can just feature detect kernel_helper and leave the (void *)1 style
> > > fallback in place until we drop support for clang variants that don't support
> > > kernel_helper. This would provide GCC compatibility and a better migration
> > > path for clang as well as clang will then automatically use the new variant
> > > whenever support for kernel_helper is introduced.
> >
> > Support for valid C code will not be dropped from clang.
>
> That wasn't what I was suggesting, I was suggesting adding support for
> kernel_helper to clang, and then in the future libbpf(not clang) can
> drop support
> for the (void *)1 style in the future if desired(or can just keep the
> fallback). By
> feature detecting kernel_helper and providing a fallback we get a nice clean
> migration path.

Makes sense. That deprecation step is far away though.
Assuming that kernel_helper attr is actually necessary
we have to add its support to clang as well.
We have to keep compilers in sync.
gcc-bpf is a niche. If gcc devs want it to become a real
alternative to clang they have to always aim for feature parity
instead of inventing their own ways of doing things.