[PATCH -v2] ext4: handle wraparound when searching for blocks for indirect mapped blocks

["Theodore Ts'o" <tytso@mit.edu>]

Commit 4865c768b563 ("ext4: always allocate blocks only from groups
inode can use") restricts what blocks will be allocated for indirect
block based files to block numbers that fit within 32-bit block
numbers.

However, when using a review bot running on the latest Gemini LLM to
check this commit when backporting into an LTS based kernel, it raised
this concern:

   If ac->ac_g_ex.fe_group is >= ngroups (for instance, if the goal
   group was populated via stream allocation from s_mb_last_groups),
   then start will be >= ngroups.

   Does this allow allocating blocks beyond the 32-bit limit for
   indirect block mapped files? The commit message mentions that
   ext4_mb_scan_groups_linear() takes care to not select unsupported
   groups. However, its loop uses group = *start, and the very first
   iteration will call ext4_mb_scan_group() with this unsupported
   group because next_linear_group() is only called at the end of the
   iteration.

After reviewing the code paths involved and considering the LLM
review, I determined that this can happen when there is a file system
where some files/directories are extent-mapped and others are
indirect-block mapped.  To address this, add a safety clamp in
ext4_mb_scan_groups().

Fixes: 4865c768b563 ("ext4: always allocate blocks only from groups inode can use")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Jan Kara <jack@suse.cz>
---
v2:
  * Remove extra checks that were not needed once we add the clamp
    in ext4_mb_scan_groups().

 fs/ext4/mballoc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 20e9fdaf4301..b10db5d7545b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1199,6 +1199,8 @@ static int ext4_mb_scan_groups(struct ext4_allocation_context *ac)
 
 	/* searching for the right group start from the goal value specified */
 	start = ac->ac_g_ex.fe_group;
+	if (start >= ngroups)
+		start = 0;
 	ac->ac_prefetch_grp = start;
 	ac->ac_prefetch_nr = 0;
 
-- 
2.51.0