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Subject: Re: [PATCH 8/8] exec: open code copy_string_kernel
To:     Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     Arnd Bergmann <arnd@arndb.de>, linux-kernel@vger.kernel.org,
        Jeremy Kerr <jk@ozlabs.org>, linux-fsdevel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org,
        "Eric W . Biederman" <ebiederm@xmission.com>
References: <20200414070142.288696-1-hch@lst.de>
 <20200414070142.288696-9-hch@lst.de>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <ffea91ee-f386-9d19-0bc9-ab59eb7b9a41@c-s.fr>
Date:   Sat, 18 Apr 2020 10:15:42 +0200
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Le 14/04/2020 à 09:01, Christoph Hellwig a écrit :
> Currently copy_string_kernel is just a wrapper around copy_strings that
> simplifies the calling conventions and uses set_fs to allow passing a
> kernel pointer.  But due to the fact the we only need to handle a single
> kernel argument pointer, the logic can be sigificantly simplified while
> getting rid of the set_fs.


Instead of duplicating almost identical code, can you write a function 
that takes whether the source is from user or from kernel, then you just 
do things like:

	if (from_user)
		len = strnlen_user(str, MAX_ARG_STRLEN);
	else
		len = strnlen(str, MAX_ARG_STRLEN);


	if (from_user)
		copy_from_user(kaddr+offset, str, bytes_to_copy);
	else
		memcpy(kaddr+offset, str, bytes_to_copy);

> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   fs/exec.c | 43 ++++++++++++++++++++++++++++++++++---------
>   1 file changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index b2a77d5acede..ea90af1fb236 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -592,17 +592,42 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>    */
>   int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
>   {
> -	int r;
> -	mm_segment_t oldfs = get_fs();
> -	struct user_arg_ptr argv = {
> -		.ptr.native = (const char __user *const  __user *)&arg,
> -	};
> +	int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */;
> +	unsigned long pos = bprm->p;
> +
> +	if (len == 0)
> +		return -EFAULT;
> +	if (!valid_arg_len(bprm, len))
> +		return -E2BIG;
> +
> +	/* We're going to work our way backwards. */
> +	arg += len;
> +	bprm->p -= len;
> +	if (IS_ENABLED(CONFIG_MMU) && bprm->p < bprm->argmin)
> +		return -E2BIG;
> +
> +	while (len > 0) {
> +		unsigned int bytes_to_copy = min_t(unsigned int, len,
> +				min_not_zero(offset_in_page(pos), PAGE_SIZE));
> +		struct page *page;
> +		char *kaddr;
>   
> -	set_fs(KERNEL_DS);
> -	r = copy_strings(1, argv, bprm);
> -	set_fs(oldfs);
> +		pos -= bytes_to_copy;
> +		arg -= bytes_to_copy;
> +		len -= bytes_to_copy;
>   
> -	return r;
> +		page = get_arg_page(bprm, pos, 1);
> +		if (!page)
> +			return -E2BIG;
> +		kaddr = kmap_atomic(page);
> +		flush_arg_page(bprm, pos & PAGE_MASK, page);
> +		memcpy(kaddr + offset_in_page(pos), arg, bytes_to_copy);
> +		flush_kernel_dcache_page(page);
> +		kunmap_atomic(kaddr);
> +		put_arg_page(page);
> +	}
> +
> +	return 0;
>   }
>   EXPORT_SYMBOL(copy_string_kernel);
>   
> 

Christophe