linux-kernel - Re: [PATCH] jffs2: use fs_umode_to

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Message-ID: <20260207073948.48144-1-adarshdas950@gmail.com>
Date: Sat,  7 Feb 2026 13:09:48 +0530
From: Adarsh Das <adarshdas950@...il.com>
To: Zhihao Cheng <chengzhihao1@...wei.com>
Cc: dwmw2@...radead.org,
	richard@....at,
	linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Adarsh Das <adarshdas950@...il.com>
Subject: Re: [PATCH] jffs2: use fs_umode_to_dtype() for dirent type

Hi Zhihao,

Thank you for taking the time to review my patch!

I'm trying to understand the issue you've identified. I looked at the 
fs_umode_to_dtype() implementation in fs/fs_types.c:

    unsigned char fs_umode_to_dtype(umode_t mode) {
        return fs_ftype_to_dtype(fs_umode_to_ftype(mode));
    }

>From what I can see, it seems like the function does produce FT_FIFO (5) 
as an intermediate value, but then converts it back before returning. 
Here's what I think happens for S_IFIFO (0010000):

Old bit-shift code:
    type = (mode & S_IFMT) >> 12
         = (0010000 & 00170000) >> 12
         = 0010000 >> 12
         = 1 (DT_FIFO)

New fs_umode_to_dtype() code:
    Step 1: fs_umode_to_ftype(0010000)
            = fs_ftype_by_dtype[S_DT(0010000)]
            = fs_ftype_by_dtype[1]
            = FT_FIFO = 5

    Step 2: fs_ftype_to_dtype(5)
            = fs_dtype_by_ftype[5]
            = DT_FIFO = 1

I'm not sure how FT_FIFO (5) would remain after the function returns, since 
it seems to get converted back to DT_FIFO (1) in step 2.

If there's an issue with this approach, I can edit my patch and keep changes in only jffs2_mknod.