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

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Date:   Mon, 25 Sep 2023 15:04:40 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, 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 Mon, Sep 25, 2023 at 02:57:31PM +0200, Arnd Bergmann wrote:
> 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.

Both fields could also just be moved into the struct itself just like we
did for clone3() and others.