linux-kernel - Re: [PATCH] erofs: fix file-backed mounts over FUSE

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Message-ID: <20241114060434.GL3387508@ZenIV>
Date: Thu, 14 Nov 2024 06:04:34 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: linux-erofs@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
	syzbot+0b1279812c46e48bb0c1@...kaller.appspotmail.com
Subject: Re: [PATCH] erofs: fix file-backed mounts over FUSE

On Thu, Nov 14, 2024 at 01:19:57PM +0800, Gao Xiang wrote:

> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 6355866220ff..43c89194d348 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -38,7 +38,10 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset,
>  	}
>  	if (!folio || !folio_contains(folio, index)) {
>  		erofs_put_metabuf(buf);
> -		folio = read_mapping_folio(buf->mapping, index, NULL);
> +		folio = buf->use_fp ?
> +			read_mapping_folio(file_inode(buf->filp)->i_mapping,
> +				index, buf->filp) :
> +			read_mapping_folio(buf->mapping, index, NULL);

UGH...

1) 'filp' is an atrocious identifier.  Please, don't perpetuate
the piss-poor taste of AST - if you want to say 'file', say so.

2) there's ->f_mapping; no need to go through the file_inode().

3) AFAICS, (buf->kmap_type == EROFS_KMAP) == (buf->base != NULL).  What's
the point of having that as a separate field?

4) Why bother with union?  Just have buf->file serve as your buf->use_fp
and be done with that...