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

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Date:   Wed, 5 Jun 2019 07:06:11 +0300
From:   Baruch Siach <baruch@...s.co.il>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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@...r.kernel.org
Subject: Re: [PATCH] bpf: fix uapi bpf_prog_info fields alignment

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 
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;
+	};
 	__u64 netns_dev;
 	__u64 netns_ino;
 	__u32 nr_jited_ksyms;

baruch

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