Re: [PATCH] zfs: fix compilation failure with clang due to alignment

[Leif Lindholm]

On Fri, Jul 03, 2015 at 10:05:47PM +0300, Andrei Borzenkov wrote:
> I do not claim I understand why clang complains, but this patch does
> fix it.
> 
> fs/xfs.c:452:25: error: cast from 'struct grub_xfs_btree_node *' to
>       'grub_uint64_t *' (aka 'unsigned long long *') increases required
>       alignment from 1 to 8 [-Werror,-Wcast-align]
>   grub_uint64_t *keys = (grub_uint64_t *)(leaf + 1);
>                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> 1 error generated.
> 
> ---
> 
> Jan, do you have any idea what's wrong and whether this is proper fix?
> Or should I raise it with clang?

Well, the problem is that struct grub_xfs_btree_node is defined with
GRUB_PACKED - forcing a 1-byte alignment requirement as opposed to the
8-byte requirement that would naturally be enforced by the struct
contents. And apparently clang objects to this, whereas gcc thinks
everything is fine ... even though -Wcast-align is explicitly used.

Now, grub_xfs_btree_keys() is only called by grub_xfs_read_block(),
where it is immediately stuffed back into another 8-byte aligned
pointer. And then the alignment is immediately discarded again by
casting it to a (char *) for an arithmetic operation.

If the alignment is indeed not required, it may be worth explicitly
marking that pointer as one to a potentially unaligned location.
But we don't currently appear to have a GRUB_UNALIGNED macro, to match
the GRUB_PACKED for structs. Should we?

If so, something like the following could be added to your patch for a
more complete fix:
--- a/grub-core/fs/xfs.c
+++ b/grub-core/fs/xfs.c
@@ -488,7 +488,7 @@ grub_xfs_read_block (grub_fshelp_node_t node,
grub_disk_addr_t fileblock)
   if (node->inode.format == XFS_INODE_FORMAT_BTREE)
     {
       struct grub_xfs_btree_root *root;
-      const grub_uint64_t *keys;
+      const grub_uint64_t *keys GRUB_UNALIGNED;
       int recoffset;
 
       leaf = grub_malloc (node->data->bsize);
diff --git a/include/grub/types.h b/include/grub/types.h
index e732efb..720e236 100644
--- a/include/grub/types.h
+++ b/include/grub/types.h
@@ -30,6 +30,8 @@
 #define GRUB_PACKED __attribute__ ((packed))
 #endif
 
+#define GRUB_UNALIGNED __attribute__ ((aligned (1)))
+
 #ifdef GRUB_BUILD
 # define GRUB_CPU_SIZEOF_VOID_P        BUILD_SIZEOF_VOID_P
 # define GRUB_CPU_SIZEOF_LONG  BUILD_SIZEOF_LONG

/
    Leif

>  grub-core/fs/xfs.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/grub-core/fs/xfs.c b/grub-core/fs/xfs.c
> index 7249291..ea8cf7e 100644
> --- a/grub-core/fs/xfs.c
> +++ b/grub-core/fs/xfs.c
> @@ -445,14 +445,14 @@ grub_xfs_next_de(struct grub_xfs_data *data, struct 
> grub_xfs_dir2_entry *de)
>    return (struct grub_xfs_dir2_entry *)(((char *)de) + ALIGN_UP(size, 8));
>  }
>  
> -static grub_uint64_t *
> +static void *
>  grub_xfs_btree_keys(struct grub_xfs_data *data,
>                   struct grub_xfs_btree_node *leaf)
>  {
> -  grub_uint64_t *keys = (grub_uint64_t *)(leaf + 1);
> +  char *keys = (char *)leaf + sizeof (*leaf);
>  
>    if (data->hascrc)
> -    keys += 6;       /* skip crc, uuid, ... */
> +    keys += 6 * sizeof (grub_uint64_t);      /* skip crc, uuid, ... */
>    return keys;
>  }
>  
> -- 
> tg: (7a21030..) u/xfs-clang-align (depends on: master)
> 
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