>From 8666468ac1a35f0678672de5c89a3f062d1aeb39 Mon Sep 17 00:00:00 2001 From: Michael Chang Date: Wed, 25 Mar 2020 14:28:15 +0800 Subject: [PATCH 2/2] zfs: Fix gcc10 error -Werror=zero-length-bounds 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 '' 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 to be best to explain what is going on here. "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 --- grub-core/fs/zfs/zfs.c | 7 ++++--- include/grub/zfs/zap_leaf.h | 1 - 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/grub-core/fs/zfs/zfs.c b/grub-core/fs/zfs/zfs.c index 2f72e42bf..f38f5b102 100644 --- a/grub-core/fs/zfs/zfs.c +++ b/grub-core/fs/zfs/zfs.c @@ -141,9 +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 - + (ZAP_LEAF_HASH_NUMENTRIES(bs) * 2) - / sizeof (grub_properly_aligned_t)))[idx]; + 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)))[idx]; } static inline struct zap_leaf_entry * 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