Re: [PATCH] selftests/bpf: Fix bind{4,6} tcp/socket header type conflict

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Fri, Aug 26, 2022 at 7:19 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
>>
>>
>> > On Thu, Aug 25, 2022 at 11:49 PM Martin KaFai Lau <kafai@fb.com> wrote:
>> >>
>> >> On Thu, Aug 25, 2022 at 11:31:15PM -0600, James Hilliard wrote:
>> >> > On Thu, Aug 25, 2022 at 11:16 PM Martin KaFai Lau <kafai@fb.com> wrote:
>> >> > >
>> >> > > On Thu, Aug 25, 2022 at 04:17:49PM -0600, James Hilliard wrote:
>> >> > > > There is a potential for us to hit a type conflict when including
>> >> > > > netinet/tcp.h with sys/socket.h, we can replace both of these includes
>> >> > > > with linux/tcp.h to avoid this conflict.
>> >> > > >
>> >> > > > Fixes errors like:
>> >> > > > In file included from /usr/include/netinet/tcp.h:91,
>> >> > > >                  from progs/bind4_prog.c:10:
>> >> > > > /home/buildroot/opt/cross/lib/gcc/bpf/13.0.0/include/stdint.h:34:23: error: conflicting types for 'int8_t'; have 'char'
>> >> > > >    34 | typedef __INT8_TYPE__ int8_t;
>> >> > > >       |                       ^~~~~~
>> >> > > > In file included from /usr/include/x86_64-linux-gnu/sys/types.h:155,
>> >> > > >                  from /usr/include/x86_64-linux-gnu/bits/socket.h:29,
>> >> > > >                  from /usr/include/x86_64-linux-gnu/sys/socket.h:33,
>> >> > > >                  from progs/bind4_prog.c:9:
>> >> > > > /usr/include/x86_64-linux-gnu/bits/stdint-intn.h:24:18: note: previous declaration of 'int8_t' with type 'int8_t' {aka 'signed char'}
>> >> > > >    24 | typedef __int8_t int8_t;
>> >> > > >       |                  ^~~~~~
>> >> > > > /home/buildroot/opt/cross/lib/gcc/bpf/13.0.0/include/stdint.h:43:24:
>> >> > > > error: conflicting types for 'int64_t'; have 'long int'
>> >> > > >    43 | typedef __INT64_TYPE__ int64_t;
>> >> > > >       |                        ^~~~~~~
>> >> > > > /usr/include/x86_64-linux-gnu/bits/stdint-intn.h:27:19: note:
>> >> > > > previous declaration of 'int64_t' with type 'int64_t' {aka
>> >> > > > 'long long int'}
>> >> > > >    27 | typedef __int64_t int64_t;
>> >> > > >       |                   ^~~~~~~
>> >> > > > make: *** [Makefile:537:
>> >> > > > /home/buildroot/bpf-next/tools/testing/selftests/bpf/bpf_gcc/bind4_prog.o]
>> >> > > > Error 1
>> >> > > >
>> >> > > > Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
>> >> > > > ---
>> >> > > >  tools/testing/selftests/bpf/progs/bind4_prog.c | 3 +--
>> >> > > >  tools/testing/selftests/bpf/progs/bind6_prog.c | 3 +--
>> >> > > >  2 files changed, 2 insertions(+), 4 deletions(-)
>> >> > > >
>> >> > > > diff --git a/tools/testing/selftests/bpf/progs/bind4_prog.c
>> >> > > > b/tools/testing/selftests/bpf/progs/bind4_prog.c
>> >> > > > index 474c6a62078a..6bd20042fd53 100644
>> >> > > > --- a/tools/testing/selftests/bpf/progs/bind4_prog.c
>> >> > > > +++ b/tools/testing/selftests/bpf/progs/bind4_prog.c
>> >> > > > @@ -6,8 +6,7 @@
>> >> > > >  #include <linux/bpf.h>
>> >> > > >  #include <linux/in.h>
>> >> > > >  #include <linux/in6.h>
>> >> > > > -#include <sys/socket.h>
>> >> > > > -#include <netinet/tcp.h>
>> >> > > These includes look normal to me.  What environment is hitting this.
>> >> >
>> >> > I was hitting this error with GCC 13(GCC master branch).
>> >> These two includes (<sys/socket.h> and <netinet/tcp.h>) are normal,
>> >> so does it mean all existing programs need to change to use gcc 13 ?
>> >
>> > Well I think it's mostly just an issue getting hit with GCC-BPF as it
>> > looks to me like a cross compilation host/target header conflict.
>>
>> This is an interesting issue.
>>
>> Right now the BPF GCC target is a sort of a bare-metal target.  As such,
>> it provides a set of header files that implement ISO C types and other
>> machinery (i.e. it doesn't rely on a C library to provide them):
>>
>>   iso646.h
>>   stdalign.h
>>   stdarg.h
>>   stdatomic.h
>>   stdbool.h
>>   stddef.h
>>   stdfix.h
>>   stdint.h
>>   stdnoreturn.h
>>   tgmath.h
>>   unwind.h
>>   varargs.h
>>
>> This is because we were expecting this to be used like:
>>
>>        <compiler-provided std C headers>
>>                   |        |
>>                   v        |
>>         <kernel headers>   |
>>                   |        |
>>                   v        v
>>                <BPF C program>
>>
>> However, if it is expected/intended for C BPF programs to include libc
>> headers, such as sys/socket.h, this can quickly go sour as you have
>> found with that conflict.
>>
>> So this leads to the question: should we turn the BPF target into a
>> target that assumes a libc?  This basically means we will be assuming
>> BPF programs are always compiled in an environment that provides a
>> standard stdint.h, stdbool.h and friends.
>
> Well for a normal GCC BPF setup we're basically cross compiling for the
> BPF bare metal target while sharing headers with the build host(for libbpf
> and any other libc headers that get included).
>
> On the other hand when using GCC BPF as part of a full cross toolchain
> we actually end up sharing headers with our real cross target architecture
> sysroot(which would provide a libc), essentially in that case BPF is a bare
> metal cross target which shares headers with the real cross target(which
> is not a bare metal target). For this libbpf is installed to the real
> cross target
> sysroot which is used by both GCC BPF(for bpf progs) and the real cross
> target GCC compiler(for userspace side). From my understanding with this
> setup GCC BPF will pick up the real cross target libc headers as a fallback
> which may sometimes have conflict/compatibility issues with the kernel
> headers.
>
> I think it's probably best to avoid depending on libc headers as things may
> otherwise get even more complex. You would essentially have 2 libc's
> in a normal GCC BPF setup and 3 libc's in a full cross toolchain setup(you'd
> have one for the build host, one for the real cross target arch and one for
> the BPF target arch).
>
> Cross build systems will typically allow a libc choice as
> well(glibc/musl/uclibc)
> and we don't really want the bpf programs to have to care about the specific
> libc being used as they are bare metal programs which shouldn't depend on
> a libc.
>

I don't understand what do you mean with "real cross target".

From the toolchain perspective, the compiler is targetted to just one
platform: bpf-unknown-none.  As is usual for bare-metal targets, the
compiler provides headers to implement the C standard with things like
floating-point types and standard integer types, `bool', etc.

If you then -I directories in order to "share headers with the build
host" or with that "real cross target", or to use any other header that
may implement the same types (typically a libc) then well, thats when
the problem arises.

I don't know how much sense does it makes to include glibc headers like
sys/socket.h in BPF C programs: I'm no BPF programmer.  But if it is
something to be supported, we will have to change the compiler to not
provide the standard headers.

>> Thoughts?
>>
>> >> > > I don't prefer the selftest writers need to remember this rule.
>> >> > >
>> >> > > Beside, afaict, tcp.h should be removed because
>> >> > > I don't see this test needs it.  I tried removing it
>> >> > > and it works fine.  It should be removed instead of replacing it
>> >> > > with another unnecessary tcp.h.
>> >> >
>> >> > Oh, that does also appear to work, thought I had tried that already but I guess
>> >> > I hadn't, sent a v2 with them removed:
>> >> > https://lore.kernel.org/bpf/20220826052925.980431-1-james.hilliard1@gmail.com/T/#u
>> >> >
>> >> > >
>> >> > > > +#include <linux/tcp.h>
>> >> > > >  #include <linux/if.h>
>> >> > > >  #include <errno.h>
>> >> > > >
>> >> > > > diff --git a/tools/testing/selftests/bpf/progs/bind6_prog.c
>> >> > > > b/tools/testing/selftests/bpf/progs/bind6_prog.c
>> >> > > > index c19cfa869f30..f37617b35a55 100644
>> >> > > > --- a/tools/testing/selftests/bpf/progs/bind6_prog.c
>> >> > > > +++ b/tools/testing/selftests/bpf/progs/bind6_prog.c
>> >> > > > @@ -6,8 +6,7 @@
>> >> > > >  #include <linux/bpf.h>
>> >> > > >  #include <linux/in.h>
>> >> > > >  #include <linux/in6.h>
>> >> > > > -#include <sys/socket.h>
>> >> > > > -#include <netinet/tcp.h>
>> >> > > > +#include <linux/tcp.h>
>> >> > > >  #include <linux/if.h>
>> >> > > >  #include <errno.h>
>> >> > > >
>> >> > > > --
>> >> > > > 2.34.1
>> >> > > >