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

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Message-ID: <20230711-demolieren-nilpferd-80ffe47563ad@brauner>
Date:   Tue, 11 Jul 2023 13:52:01 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Alexey Gladkov <legion@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
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Subject: Re: [PATCH v3 2/5] fs: Add fchmodat4()

On Tue, Jul 11, 2023 at 01:42:19PM +0200, Arnd Bergmann wrote:
> 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>

The system call itself is useful afaict. But please,

s/fchmodat4/fchmodat2/

With very few exceptions we don't version by argument number but by
revision and we should stick to one scheme:

openat()->openat2()
eventfd()->eventfd2()
clone()/clone2()->clone3()
dup()->dup2()->dup3() // coincides with nr of arguments
pipe()->pipe2() // coincides with nr of arguments
renameat()->renameat2()