linux-kernel - Re: [PATCH v4 2/5] fs: Add fchmodat2()

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Date:   Tue, 11 Jul 2023 19:05:44 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Alexey Gladkov <legion@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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        Palmer Dabbelt <palmer@...ive.com>
Subject: Re: [PATCH v4 2/5] fs: Add fchmodat2()

On Tue, Jul 11, 2023 at 06:16:04PM +0200, Alexey Gladkov wrote:
> 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 fchmodat2() 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
> 
> Co-developed-by: Palmer Dabbelt <palmer@...ive.com>
> Signed-off-by: Palmer Dabbelt <palmer@...ive.com>
> Signed-off-by: Alexey Gladkov <legion@...nel.org>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> ---
>  fs/open.c                | 18 ++++++++++++++----
>  include/linux/syscalls.h |  2 ++
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 0c55c8e7f837..39a7939f0d00 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -671,11 +671,11 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
>  	return err;
>  }
>  
> -static int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
> +static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int lookup_flags)

Should all be unsigned instead of int here for flags. We also had a
documentation update to that effect but smh never sent it.
user_path_at() itself takes an unsigned as well.

I'll fix that up though.