ext4: prevent the normalized size from exceeding EXT_MAX_BLOCKS

commit 2dcf5fd upstream.

For files with logical blocks close to EXT_MAX_BLOCKS, the file size
predicted in ext4_mb_normalize_request() may exceed EXT_MAX_BLOCKS.
This can cause some blocks to be preallocated that will not be used.
And after [Fixes], the following issue may be triggered:

=========================================================
 kernel BUG at fs/ext4/mballoc.c:4653!
 Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
 CPU: 1 PID: 2357 Comm: xfs_io 6.7.0-rc2-00195-g0f5cc96c367f
 Hardware name: linux,dummy-virt (DT)
 pc : ext4_mb_use_inode_pa+0x148/0x208
 lr : ext4_mb_use_inode_pa+0x98/0x208
 Call trace:
  ext4_mb_use_inode_pa+0x148/0x208
  ext4_mb_new_inode_pa+0x240/0x4a8
  ext4_mb_use_best_found+0x1d4/0x208
  ext4_mb_try_best_found+0xc8/0x110
  ext4_mb_regular_allocator+0x11c/0xf48
  ext4_mb_new_blocks+0x790/0xaa8
  ext4_ext_map_blocks+0x7cc/0xd20
  ext4_map_blocks+0x170/0x600
  ext4_iomap_begin+0x1c0/0x348
=========================================================

Here is a calculation when adjusting ac_b_ex in ext4_mb_new_inode_pa():

	ex.fe_logical = orig_goal_end - EXT4_C2B(sbi, ex.fe_len);
	if (ac->ac_o_ex.fe_logical >= ex.fe_logical)
		goto adjust_bex;

The problem is that when orig_goal_end is subtracted from ac_b_ex.fe_len
it is still greater than EXT_MAX_BLOCKS, which causes ex.fe_logical to
overflow to a very small value, which ultimately triggers a BUG_ON in
ext4_mb_new_inode_pa() because pa->pa_free < len.

The last logical block of an actual write request does not exceed
EXT_MAX_BLOCKS, so in ext4_mb_normalize_request() also avoids normalizing
the last logical block to exceed EXT_MAX_BLOCKS to avoid the above issue.

The test case in [Link] can reproduce the above issue with 64k block size.

Link: https://patchwork.kernel.org/project/fstests/list/?series=804003
Cc:  <[email protected]> # 6.4
Fixes: 93cdf49 ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/ext4/mballoc.c

@@ -4137,6 +4137,10 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
 	start = max(start, rounddown(ac->ac_o_ex.fe_logical,
 			(ext4_lblk_t)EXT4_BLOCKS_PER_GROUP(ac->ac_sb)));
 
+	/* avoid unnecessary preallocation that may trigger assertions */
+	if (start + size > EXT_MAX_BLOCKS)
+		size = EXT_MAX_BLOCKS - start;
+
 	/* don't cover already allocated blocks in selected range */
 	if (ar->pleft && start <= ar->lleft) {
 		size -= ar->lleft + 1 - start;