Re: [PATCH] bpf: fix uapi bpf_prog_info fields alignment

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Tue, Jun 04, 2019 at 05:23:46PM +0200, Geert Uytterhoeven wrote:
> Hi Alexei,
> 
> On Tue, Jun 4, 2019 at 5:17 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> > On Tue, Jun 4, 2019 at 4:40 AM Baruch Siach <baruch@xxxxxxxxxx> 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;