linux-kernel - Re: [RFC PATCH 2/3] add statmnt(2) syscall

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Date:   Mon, 25 Sep 2023 14:57:31 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Miklos Szeredi" <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-man@...r.kernel.org, linux-security-module@...r.kernel.org,
        "Karel Zak" <kzak@...hat.com>, "Ian Kent" <raven@...maw.net>,
        "David Howells" <dhowells@...hat.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Alexander Viro" <viro@...iv.linux.org.uk>,
        "Christian Brauner" <christian@...uner.io>,
        "Amir Goldstein" <amir73il@...il.com>
Subject: Re: [RFC PATCH 2/3] add statmnt(2) syscall

On Wed, Sep 13, 2023, at 17:22, Miklos Szeredi wrote:

>  asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz,
>  				struct statfs64 __user *buf);
> +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask,
> +			    struct statmnt __user *buf, size_t bufsize,
> +			    unsigned int flags);

This definition is problematic on 32-bit architectures for two
reasons:

- 64-bit register arguments are passed in pairs of registers
  on two architectures, so anything passing those needs to
  have a separate entry point for compat syscalls on 64-bit
  architectures. I would suggest also using the same one on
  32-bit ones, so you don't rely on the compiler splitting
  up the long arguments into pairs.

- There is a limit of six argument registers for system call
  entry points, but with two pairs and three single registers
  you end up with seven of them.

The listmnt syscall in patch 3 also has the first problem,
but not the second.

      Arnd