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

[Hongbo Li <lihongbo22@huawei.com>]

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 <lihongbo22@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
>>