https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84108
--- Comment #3 from Arnd Bergmann <arnd at linaro dot org> ---
(In reply to Jakub Jelinek from comment #1)
> I vaguely remember the behavior of packed + aligned(N) kept changing in the
> past, some versions of GCC treated it just like packed, others as aligned.
> Is this in the kernel?
Yes. The example I showed corresponds to include/linux/sysv_fs.h, which
probably has very few users, if any. If this was inconsistent in the past, it
may have gone unnoticed. OTOH, there is a compile-time assertion in the code to
ensure that the structure has the right size:
#define __packed2__ __attribute__((packed,aligned(2)))
#define XENIX_NICINOD 100 /* number of inode cache entries */
#define XENIX_NICFREE 100 /* number of free block list chunk entries */
struct xenix_super_block {
__fs16 s_isize; /* index of first data zone */
__fs32 s_fsize __packed2__; /* total number of zones of this
fs */
/* the start of the free block list: */
__fs16 s_nfree; /* number of free blocks in s_free, <=
XENIX_NICFREE */
sysv_zone_t s_free[XENIX_NICFREE]; /* first free block list chunk
*/
/* the cache of free inodes: */
__fs16 s_ninode; /* number of free inodes in s_inode, <=
XENIX_NICINOD */
sysv_ino_t s_inode[XENIX_NICINOD]; /* some free inodes */
/* locks, not used by Linux: */
char s_flock; /* lock during free block list
manipulation */
char s_ilock; /* lock during inode cache manipulation
*/
char s_fmod; /* super-block modified flag */
char s_ronly; /* flag whether fs is mounted read-only
*/
__fs32 s_time __packed2__; /* time of last super block update
*/
__fs32 s_tfree __packed2__; /* total number of free zones */
__fs16 s_tinode; /* total number of free inodes */
__fs16 s_dinfo[4]; /* device information ?? */
char s_fname[6]; /* file system volume name */
char s_fpack[6]; /* file system pack name */
char s_clean; /* set to 0x46 when filesystem is
properly unmounted */
char s_fill[371];
s32 s_magic; /* version of file system */
__fs32 s_type; /* type of file system: 1 for 512 byte
blocks
2 for 1024 byte
blocks
3 for 2048 byte
blocks */
};
BUILD_BUG_ON(1024 != sizeof (struct xenix_super_block));
Two other variations of this warning that I ran into are slightly different:
fs/ubifs/ubifs-media.h and include/scsi/libsas.h define a structure with
__attribute__((packed)) and use that structure as a member in another
structure, with that member being marked __attribute__((aligned)). I would hope
that this behavior has never changed.
