linux-kernel - Re: [PATCH v3 2/5] fs: Add fchmodat4()

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Date:   Tue, 11 Jul 2023 13:42:19 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Alexey Gladkov" <legion@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        "Alexander Viro" <viro@...iv.linux.org.uk>
Cc:     "Palmer Dabbelt" <palmer@...ive.com>,
        "James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
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        "Yoshinori Sato" <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH v3 2/5] fs: Add fchmodat4()

On Tue, Jul 11, 2023, at 13:25, Alexey Gladkov wrote:
> From: Palmer Dabbelt <palmer@...ive.com>
>
> On the userspace side fchmodat(3) is implemented as a wrapper
> function which implements the POSIX-specified interface. This
> interface differs from the underlying kernel system call, which does not
> have a flags argument. Most implementations require procfs [1][2].
>
> There doesn't appear to be a good userspace workaround for this issue
> but the implementation in the kernel is pretty straight-forward.
>
> The new fchmodat4() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag,
> unlike existing fchmodat.
>
> [1] 
> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35
> [2] 
> https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28
>
> Signed-off-by: Palmer Dabbelt <palmer@...ive.com>
> Signed-off-by: Alexey Gladkov <legion@...nel.org>

I don't know the history of why we ended up with the different
interface, or whether this was done intentionally in the kernel
or if we want this syscall.

Assuming this is in fact needed, I double-checked that the
implementation looks correct to me and is portable to all the
architectures, without the need for a compat wrapper.

Acked-by: Arnd Bergmann <arnd@...db.de>