As Stefan pointed out, I misremembered what clang can do specifically,
and it turns out that the variable-length array at the end of the
structure did not work (a flexible array would have worked here
but not solved the problem):

fs/ext4/mballoc.c:2303:17: error: fields must have a constant size:
'variable length array in structure' extension will never be supported
                ext4_grpblk_t counters[blocksize_bits + 2];

This reverts part of my previous patch, using a fixed-size array
again, but keeping the check for the array overflow.

Fixes: 2df2c3402fc8 ("ext4: fix warning about stack corruption")
Reported-by: Stefan Agner <ste...@agner.ch>
Signed-off-by: Arnd Bergmann <a...@arndb.de>
---
 fs/ext4/mballoc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 5a1052627a81..701085620cd8 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2300,7 +2300,7 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, 
void *v)
                                             EXT4_MAX_BLOCK_LOG_SIZE);
        struct sg {
                struct ext4_group_info info;
-               ext4_grpblk_t counters[blocksize_bits + 2];
+               ext4_grpblk_t counters[EXT4_MAX_BLOCK_LOG_SIZE + 2];
        } sg;
 
        group--;
@@ -2309,6 +2309,9 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, 
void *v)
                              " 2^0   2^1   2^2   2^3   2^4   2^5   2^6  "
                              " 2^7   2^8   2^9   2^10  2^11  2^12  2^13  ]\n");
 
+       i = (blocksize_bits + 2) * sizeof(sg.info.bb_counters[0]) +
+               sizeof(struct ext4_group_info);
+
        grinfo = ext4_get_group_info(sb, group);
        /* Load the group info in memory only if not already loaded. */
        if (unlikely(EXT4_MB_GRP_NEED_INIT(grinfo))) {
@@ -2320,7 +2323,7 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, 
void *v)
                buddy_loaded = 1;
        }
 
-       memcpy(&sg, ext4_get_group_info(sb, group), sizeof(sg));
+       memcpy(&sg, ext4_get_group_info(sb, group), i);
 
        if (buddy_loaded)
                ext4_mb_unload_buddy(&e4b);
-- 
2.9.0

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