Re: [PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Hongbo Li]

On 2024/4/25 11:34, Kent Overstreet wrote:

On Fri, Apr 19, 2024 at 03:48:50PM +0800, Hongbo Li wrote:

[BUG]
When compiling the bcachefs-tools, the following compilation warning
is reported:
     libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
     libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used 
uninitialized in this function [-Wmaybe-uninitialized]
       915 |  snapshot->v.tree = cpu_to_le32(tree_id);
     libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
       903 |  u32 tree_id;
        |      ^~~~~~~

[CAUSE]
This is a false alert, because @tree_id is changed in
bch2_snapshot_tree_create after it returns 0. And if this function
returns other value, @tree_id wouldn't be used. Thus there should
be nothing wrong in logical.

[FIX]
Although the report itself is a false alert, we can still make it more
explicit by:
   - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
   - initialize @tree_id to U32_MAX
   - add extra WARN_ON_ONCE to make sure @tree_id is updated

Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
Signed-off-by: Hongbo Li <lihongb...@huawei.com>
---
  fs/bcachefs/snapshot.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
index 0b26dee17a5a..cad3408903b2 100644
--- a/fs/bcachefs/snapshot.c
+++ b/fs/bcachefs/snapshot.c
@@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
  static int bch2_snapshot_tree_create(struct btree_trans *trans,
                                u32 root_id, u32 subvol_id, u32 *tree_id)
  {
+       WARN_ON_ONCE(!tree_id);

There's no point checking for a null pointer like that; the oops from a
null ptr deref gives us exactly the same information.

        struct bkey_i_snapshot_tree *n_tree =
                __bch2_snapshot_tree_create(trans);

@@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)

        if (bch2_snapshot_equiv(c, id))
                return 0;

- u32 tree_id;

+       u32 tree_id = U32_MAX;

Just initialize it to 0. 0 is an invalid tree ID, so it'll be caught by
snapshot_tree_invalid() if it's not set.

        int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
        if (ret)
                return ret;
@@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, 
u32 id)
        if (ret)
                return ret;

+ /* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */

+       WARN_ON_ONCE(tree_id == U32_MAX);

Nix this as well.

This is to ensure that the tree_id is modified.

        bkey_snapshot_init(&snapshot->k_i);
        snapshot->k.p                = POS(0, id);
        snapshot->v.tree     = cpu_to_le32(tree_id);
--
2.34.1