linux-kernel - Re: [PATCH 3/4] exec: simplify the compat syscall handling

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Date:   Fri, 26 Mar 2021 16:12:23 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Arnd Bergmann <arnd@...db.de>, Brian Gerst <brgerst@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] exec: simplify the compat syscall handling

On Fri, Mar 26, 2021 at 03:38:30PM +0100, Christoph Hellwig wrote:

> -static const char __user *get_user_arg_ptr(struct user_arg_ptr argv, int nr)
> +static const char __user *
> +get_user_arg_ptr(const char __user *const __user *argv, int nr)
>  {
> -	const char __user *native;
> -
> -#ifdef CONFIG_COMPAT
> -	if (unlikely(argv.is_compat)) {
> +	if (in_compat_syscall()) {
> +		const compat_uptr_t __user *compat_argv =
> +			compat_ptr((unsigned long)argv);
>  		compat_uptr_t compat;
>  
> -		if (get_user(compat, argv.ptr.compat + nr))
> +		if (get_user(compat, compat_argv + nr))
>  			return ERR_PTR(-EFAULT);
> -
>  		return compat_ptr(compat);
> -	}
> -#endif
> -
> -	if (get_user(native, argv.ptr.native + nr))
> -		return ERR_PTR(-EFAULT);
> +	} else {
> +		const char __user *native;
>  
> -	return native;
> +		if (get_user(native, argv + nr))
> +			return ERR_PTR(-EFAULT);
> +		return native;
> +	}
>  }

Yecchhh....  So you have in_compat_syscall() called again and again, for
each argument in the list?  I agree that current version is fucking ugly,
but I really hate that approach ;-/