structure layout using __aligned__ attribute is incorrect

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

Reproduce the issue with input.h based on linux/target_core_user.h using bindgen input.h, then inspect bindgen's handling of __aligned__ on packed structs. Compare the generated tcmu_mailbox layout with the C offsets, especially cmd_tail at offset 64; done means the Rust binding places the field at the same offset.