[PATCH 2/2] zfs: Fix gcc10 error -Werror=zero-length-bounds

[Michael Chang]

We bumped into the build error while testing gcc-10 pre-release.

In file included from ../../include/grub/file.h:22,
                from ../../grub-core/fs/zfs/zfs.c:34:
../../grub-core/fs/zfs/zfs.c: In function 'zap_leaf_lookup':
../../grub-core/fs/zfs/zfs.c:2263:44: error: array subscript '<unknown>' is 
outside the bounds of an interior zero-length array 'grub_uint16_t[0]' {aka 
'short unsigned int[0]'} [-Werror=zero-length-bounds]
2263 |   for (chunk = grub_zfs_to_cpu16 (l->l_hash[LEAF_HASH (blksft, h, l)], 
endian);
../../include/grub/types.h:241:48: note: in definition of macro 
'grub_le_to_cpu16'
 241 | # define grub_le_to_cpu16(x) ((grub_uint16_t) (x))
     |                                                ^
../../grub-core/fs/zfs/zfs.c:2263:16: note: in expansion of macro 
'grub_zfs_to_cpu16'
2263 |   for (chunk = grub_zfs_to_cpu16 (l->l_hash[LEAF_HASH (blksft, h, l)], 
endian);
     |                ^~~~~~~~~~~~~~~~~
In file included from ../../grub-core/fs/zfs/zfs.c:48:
../../include/grub/zfs/zap_leaf.h:72:16: note: while referencing 'l_hash'
  72 |  grub_uint16_t l_hash[0];
     |                ^~~~~~

Here I'd like to quote from the gcc document [1] which seems best to
explain what is going on here.

"Although the size of a zero-length array is zero, an array member of
this kind may increase the size of the enclosing type as a result of
tail padding. The offset of a zero-length array member from the
beginning of the enclosing structure is the same as the offset of an
array with one or more elements of the same type. The alignment of a
zero-length array is the same as the alignment of its elements.

Declaring zero-length arrays in other contexts, including as interior
members of structure objects or as non-member objects, is discouraged.
Accessing elements of zero-length arrays declared in such contexts is
undefined and may be diagnosed."

The l_hash[0] is apparnetly an interior member to the enclosed structure
while l_entries[0] is the trailing member. And the offending code tries
to access members in l_hash[0] array that triggers the diagnose.

Given that the l_entries[0] is used to get proper alignment to access
leaf chunks, we can accomplish the same thing through the ALIGN_UP macro
thus eliminating l_entries[0] from the structure. In this way we can
pacify the warning as l_hash[0] now becomes the last member to the
enclosed structure.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Signed-off-by: Michael Chang <address@hidden>
---
 grub-core/fs/zfs/zfs.c      | 5 ++++-
 include/grub/zfs/zap_leaf.h | 1 -
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/grub-core/fs/zfs/zfs.c b/grub-core/fs/zfs/zfs.c
index 2f72e42bf..b5e10fd0b 100644
--- a/grub-core/fs/zfs/zfs.c
+++ b/grub-core/fs/zfs/zfs.c
@@ -141,7 +141,10 @@ ZAP_LEAF_NUMCHUNKS (int bs)
 static inline zap_leaf_chunk_t *
 ZAP_LEAF_CHUNK (zap_leaf_phys_t *l, int bs, int idx)
 {
-  return &((zap_leaf_chunk_t *) (l->l_entries 
+  grub_properly_aligned_t *l_entries;
+
+  l_entries = (grub_properly_aligned_t *) ALIGN_UP((grub_addr_t)l->l_hash, 
sizeof (grub_properly_aligned_t));
+  return &((zap_leaf_chunk_t *) (l_entries
                                 + (ZAP_LEAF_HASH_NUMENTRIES(bs) * 2)
                                 / sizeof (grub_properly_aligned_t)))[idx];
 }
diff --git a/include/grub/zfs/zap_leaf.h b/include/grub/zfs/zap_leaf.h
index 95c67dcba..11447c166 100644
--- a/include/grub/zfs/zap_leaf.h
+++ b/include/grub/zfs/zap_leaf.h
@@ -70,7 +70,6 @@ typedef struct zap_leaf_phys {
         */
 
        grub_uint16_t l_hash[0];
-        grub_properly_aligned_t l_entries[0];
 } zap_leaf_phys_t;
 
 typedef union zap_leaf_chunk {
-- 
2.16.4