From: Theodore Ts'o Date: Fri, 3 Apr 2026 13:35:36 +0000 (-0400) Subject: ext4: handle wraparound when searching for blocks for indirect mapped blocks X-Git-Tag: v6.1.168~21 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=f89bba144938921a2249237ad04a0183ff3f8930;p=thirdparty%2Fkernel%2Fstable.git ext4: handle wraparound when searching for blocks for indirect mapped blocks [ Upstream commit bb81702370fad22c06ca12b6e1648754dbc37e0f ] 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") Cc: Jan Kara Reviewed-by: Baokun Li Reviewed-by: Jan Kara Signed-off-by: Theodore Ts'o Link: https://patch.msgid.link/20260326045834.1175822-1-tytso@mit.edu Signed-off-by: Theodore Ts'o Cc: stable@kernel.org [ adapted fix from ext4_mb_scan_groups() to inline equivalent in ext4_mb_regular_allocator() ] Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 3b8d35a98d4d1..df6fa0a568559 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -2733,6 +2733,8 @@ repeat: * from the goal value specified */ group = ac->ac_g_ex.fe_group; + if (group >= ngroups) + group = 0; ac->ac_groups_linear_remaining = sbi->s_mb_max_linear_groups; prefetch_grp = group;