nilfs2: fix failure to detect DAT corruption in btree and direct mappings

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix failure to detect DAT corruption in btree and direct mappings

Patch series "nilfs2: fix kernel bug at submit_bh_wbc()".

This resolves a kernel BUG reported by syzbot. Since there are two

flaws involved, I've made each one a separate patch.

The first patch alone resolves the syzbot-reported bug, but I think

both fixes should be sent to stable, so I've tagged them as such.

This patch (of 2):

Syzbot has reported a kernel bug in submit_bh_wbc() when writing file data

to a nilfs2 file system whose metadata is corrupted.

There are two flaws involved in this issue.

The first flaw is that when nilfs_get_block() locates a data block using

btree or direct mapping, if the disk address translation routine

nilfs_dat_translate() fails with internal code -ENOENT due to DAT metadata

corruption, it can be passed back to nilfs_get_block(). This causes

nilfs_get_block() to misidentify an existing block as non-existent,

causing both data block lookup and insertion to fail inconsistently.

The second flaw is that nilfs_get_block() returns a successful status in

this inconsistent state. This causes the caller __block_write_begin_int()

or others to request a read even though the buffer is not mapped,

resulting in a BUG_ON check for the BH_Mapped flag in submit_bh_wbc()

failing.

This fixes the first issue by changing the return value to code -EINVAL

when a conversion using DAT fails with code -ENOENT, avoiding the

conflicting condition that leads to the kernel bug described above. Here,

code -EINVAL indicates that metadata corruption was detected during the

block lookup, which will be properly handled as a file system error and

converted to -EIO when passing through the nilfs2 bmap layer.