Re: [PATCH] fs: fix possible extra iput() in do_unlinkat()

[Mateusz Guzik <mjguzik@gmail.com>]

On Thu, Sep 28, 2023 at 02:11:29PM +0100, Luís Henriques wrote:
> Because inode is being initialised before checking if dentry is negative,
> and the ihold() is only done if the dentry is *not* negative, the cleanup
> code may end-up doing an extra iput() on that inode.
> 
> Fixes: b18825a7c8e3 ("VFS: Put a small type field into struct dentry::d_flags")
> Signed-off-by: Luís Henriques <lhenriques@suse.de>
> ---
> Hi!
> 
> I was going to also remove the 'if (inode)' before the 'iput(inode)',
> because 'iput()' already checks for NULL anyway.  But since I probably
> wouldn't have caught this bug if it wasn't for that 'if', I decided to
> keep it there.  But I can send v2 with that change too if you prefer.
> 
> Cheers,
> --
> Luís
> 
>  fs/namei.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 567ee547492b..156a570d7831 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4386,11 +4386,9 @@ int do_unlinkat(int dfd, struct filename *name)
>  	if (!IS_ERR(dentry)) {
>  
>  		/* Why not before? Because we want correct error value */
> -		if (last.name[last.len])
> +		if (last.name[last.len] || d_is_negative(dentry))
>  			goto slashes;
>  		inode = dentry->d_inode;
> -		if (d_is_negative(dentry))
> -			goto slashes;
>  		ihold(inode);
>  		error = security_path_unlink(&path, dentry);
>  		if (error)

I ran into this myself, but I'm pretty sure there is no bug here. The
code is just incredibly misleading and it became this way from the
sweeping change introducing d_is_negative. I could not be bothered to
argue about patching it so I did not do anything. ;)

AFAICS it is an invariant that d_is_negative passes iff d_inode is NULL.

Personally I support the patch, but commit message needs to stop
claiming a bug.