structure layout using __aligned__ attribute is incorrect

[toshipp]

Input C/C++ Header

The original header is linux kernel source header; linux/target_core_user.h.

#define ALIGN_SIZE 64 /* Should be enough for most CPUs */

typedef unsigned short __u16;
typedef unsigned int __u32;

struct tcmu_mailbox {
	__u16 version;
	__u16 flags;
	__u32 cmdr_off;
	__u32 cmdr_size;

	__u32 cmd_head;

	/* Updated by user. On its own cacheline */
	__u32 cmd_tail __attribute__((__aligned__(ALIGN_SIZE)));

} __attribute__((packed));

Bindgen Invocation

$ bindgen input.h

Actual Results

pub type __u16 = ::std::os::raw::c_ushort;
pub type __u32 = ::std::os::raw::c_uint;
#[repr(C, packed)]
#[derive(Copy)]
pub struct tcmu_mailbox {
    pub version: __u16,
    pub flags: __u16,
    pub cmdr_off: __u32,
    pub cmdr_size: __u32,
    pub cmd_head: __u32,
    pub cmd_tail: __u32,
    pub __bindgen_padding_0: [u8; 108usize],
}

Expected Results

pub type __u16 = ::std::os::raw::c_ushort;
pub type __u32 = ::std::os::raw::c_uint;
#[repr(C, packed)]
#[derive(Copy)]
pub struct tcmu_mailbox {
    pub version: __u16,
    pub flags: __u16,
    pub cmdr_off: __u32,
    pub cmdr_size: __u32,
    pub cmd_head: __u32,
    pub __bindgen_padding_0: [u8; 48usize],
    pub cmd_tail: __u32,
}

In C, the offset of cmd_tail is 64, so generated rust code should also have same offset.

To check offset, I wrote C code like as;

#include <stddef.h>
#include <stdio.h>
#include "mini.h"

int main(int n, char**v) {
    printf("version %ld\n", offsetof(struct tcmu_mailbox, version));
    printf("flags %ld\n", offsetof(struct tcmu_mailbox, flags));
    printf("cmdr_off %ld\n", offsetof(struct tcmu_mailbox, cmdr_off));
    printf("cmdr_size %ld\n", offsetof(struct tcmu_mailbox, cmdr_size));
    printf("cmd_head %ld\n", offsetof(struct tcmu_mailbox, cmd_head));
    printf("cmd_tail %ld\n", offsetof(struct tcmu_mailbox, cmd_tail));
}

The outpu is;

version 0
flags 2
cmdr_off 4
cmdr_size 8
cmd_head 12
cmd_tail 64

[emilio]

Thanks a lot for the report :)

[kawamuray]

FYI this issue seems to have solved with newer clang/llvm version.

I was trying to bindgen for some linux kernel header files like https://github.com/torvalds/linux/blob/master/include/uapi/linux/taskstats.h#L41 and hit the same issue, but on different machine with newer clang/llvm version the padding field is inserted in correct place and it worked well.

The older clang and llvm version is indeed lower than the recommended version at: https://rust-lang.github.io/rust-bindgen/requirements.html#clang

With old clang and llvm:

$ clang --version 
clang version 3.4.2 (tags/RELEASE_34/dot2-final)
$ llvm-config --version 
3.4.2

Generated:

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct taskstats {
(snip)
    pub ac_pad: [__u8; 3usize],
    pub ac_uid: __u32,
(snip)
    pub freepages_delay_total: __u64,
    pub __bindgen_padding_0: u64,
}

With new clang and llvm:

$ clang --version
clang version 7.0.1-8 (tags/RELEASE_701/final)
$ llvm-config --version
7.0.1

Generated:

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct taskstats {
(snip)
    pub ac_pad: [__u8; 3usize],
    pub __bindgen_padding_0: u32,
    pub ac_uid: __u32,
(snip)
    pub freepages_delay_total: __u64,
}

[toshipp]

Thank you for your information.
I checked my current environment, my clang version is 10.0.0.

clang --version
clang version 10.0.0
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/sbin

I try to generate rust code from the header with bindgen 0.53.2.
I got following code.

pub const ALIGN_SIZE: u32 = 64;
pub type __u16 = ::std::os::raw::c_ushort;
pub type __u32 = ::std::os::raw::c_uint;
#[repr(C, packed(64))]
#[repr(align(64))]
#[derive(Copy, Clone)]
pub struct tcmu_mailbox {
    pub version: __u16,
    pub flags: __u16,
    pub cmdr_off: __u32,
    pub cmdr_size: __u32,
    pub cmd_head: __u32,
    pub cmd_tail: __u32,
    pub __bindgen_padding_0: [u8; 108usize],
}
<snip test code>

It seems works well, but it can not be compiled.

$ rustc -o foo --test foo.rs
warning: type `__u16` should have an upper camel case name
 --> foo.rs:4:10
  |
4 | pub type __u16 = ::std::os::raw::c_ushort;
  |          ^^^^^ help: convert the identifier to upper camel case: `U16`
  |
  = note: `#[warn(non_camel_case_types)]` on by default

warning: type `__u32` should have an upper camel case name
 --> foo.rs:5:10
  |
5 | pub type __u32 = ::std::os::raw::c_uint;
  |          ^^^^^ help: convert the identifier to upper camel case: `U32`

error[E0587]: type has conflicting packed and align representation hints
  --> foo.rs:9:1
   |
9  | / pub struct tcmu_mailbox {
10 | |     pub version: __u16,
11 | |     pub flags: __u16,
12 | |     pub cmdr_off: __u32,
...  |
16 | |     pub __bindgen_padding_0: [u8; 108usize],
17 | | }
   | |_^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0587`.

[pvdrz]

This became an instance of #1538 if I understand.