netdev - Re: [PATCH] bpf: fix uapi bpf_prog

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Date:   Tue, 4 Jun 2019 21:13:56 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Baruch Siach <baruch@...s.co.il>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, "Dmitry V . Levin" <ldv@...linux.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] bpf: fix uapi bpf_prog_info fields alignment

On Tue, Jun 4, 2019 at 9:06 PM Baruch Siach <baruch@...s.co.il> wrote:
>
> Hi Alexei,
>
> (Adding Arnd and linux-arch to Cc)
>
> On Tue, Jun 04, 2019 at 08:30:29AM -0700, Alexei Starovoitov wrote:
> > On Tue, Jun 04, 2019 at 05:23:46PM +0200, Geert Uytterhoeven wrote:
> > > On Tue, Jun 4, 2019 at 5:17 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > > On Tue, Jun 4, 2019 at 4:40 AM Baruch Siach <baruch@...s.co.il> wrote:
> > > > > Merge commit 1c8c5a9d38f60 ("Merge
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next") undid the
> > > > > fix from commit 36f9814a494 ("bpf: fix uapi hole for 32 bit compat
> > > > > applications") by taking the gpl_compatible 1-bit field definition from
> > > > > commit b85fab0e67b162 ("bpf: Add gpl_compatible flag to struct
> > > > > bpf_prog_info") as is. That breaks architectures with 16-bit alignment
> > > > > like m68k. Widen gpl_compatible to 32-bit to restore alignment of the
> > > > > following fields.
> > > >
> > > > The commit log is misleading and incorrect.
> > > > Since compiler makes it into 16-bit field, it's a compiler bug.
> > > > u32 in C should stay as u32 regardless of architecture.
> > >
> > > C99 says (Section 6.7.2.1, Structure and union specifiers, Semantics)
> > >
> > >     10  An implementation may allocate any addressable storage unit
> > >         large enough to hold a bit-field.
> > >
> > > $ cat hello.c
> > > #include <stdio.h>
> > > #include <stdint.h>
> > > #include <stdlib.h>
> > >
> > > struct x {
> > >         unsigned int bit : 1;
> > >         unsigned char byte;
> > > };
> > >
> > > int main(int argc, char *argv[])
> > > {
> > >         struct x x;
> > >
> > >         printf("byte is at offset %zu\n", (uintptr_t)&x.byte - (uintptr_t)&x);
> > >         printf("sizeof(x) = %zu\n", sizeof(x));
> > >         exit(0);
> > > }
> > > $ gcc -Wall hello.c -o hello && ./hello
> > > byte is at offset 1
> > > sizeof(x) = 4
> > > $ uname -m
> > > x86_64
> > >
> > > So the compiler allocates a single byte, even on a 64-bit platform!
> > > The gap is solely determined by the alignment rule for the
> > > successive field.
> >
> > argh. then we need something like this:
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 7c6aef253173..a2ac0b961251 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3174,6 +3174,7 @@ struct bpf_prog_info {
> >         char name[BPF_OBJ_NAME_LEN];
> >         __u32 ifindex;
> >         __u32 gpl_compatible:1;
> > +       __u32 :31;
> >         __u64 netns_dev;
> >         __u64 netns_ino;
> >         __u32 nr_jited_ksyms;
>
> Is that guaranteed to work across platforms/compilers? Maybe an anonymous

it works on archs I have access to.
please try it on yours.

> union would be safer? Something like:
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 63e0cf66f01a..06c9fb314ea5 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3140,7 +3140,10 @@ struct bpf_prog_info {
>         __aligned_u64 map_ids;
>         char name[BPF_OBJ_NAME_LEN];
>         __u32 ifindex;
> -       __u32 gpl_compatible:1;
> +       union {
> +               __u32 gpl_compatible:1;
> +               __u32 pad;
> +       };

that's a potential uapi breakage.
We use anonymous bitfields in other places in uapi.
If some gcc on some arch doesn't understand them
it's a bigger breakage and such gcc would need
to be fixed regardless.