linux-kernel - Re: [merged] exec-open-code-copy_string

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Date:   Tue, 16 Jun 2020 15:14:44 +0200
From:   Vegard Nossum <vegard.nossum@...cle.com>
To:     linux-kernel@...r.kernel.org, hch@....de,
        mm-commits@...r.kernel.org, viro@...iv.linux.org.uk
Subject: Re: [merged] exec-open-code-copy_string_kernel.patch removed from -mm
 tree


On 2020-06-05 22:19, akpm@...ux-foundation.org wrote:
> The patch titled
>       Subject: exec: open code copy_string_kernel
> has been removed from the -mm tree.  Its filename was
>       exec-open-code-copy_string_kernel.patch
> 
> This patch was dropped because it was merged into mainline or a subsystem tree
> 
> ------------------------------------------------------
> From: Christoph Hellwig <hch@....de>
> Subject: exec: open code copy_string_kernel
> 
> 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.
> 
> Link: http://lkml.kernel.org/r/20200501104105.2621149-3-hch@lst.de
> Signed-off-by: Christoph Hellwig <hch@....de>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>   fs/exec.c |   45 +++++++++++++++++++++++++++++++++++----------
>   1 file changed, 35 insertions(+), 10 deletions(-)
> 
> --- a/fs/exec.c~exec-open-code-copy_string_kernel
> +++ a/fs/exec.c
> @@ -592,17 +592,42 @@ out:
>    */
>   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,
> -	};
> -
> -	set_fs(KERNEL_DS);
> -	r = copy_strings(1, argv, bprm);
> -	set_fs(oldfs);
> +	int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */;
> +	unsigned long pos = bprm->p;
>   
> -	return r;
> +	if (len == 0)
> +		return -EFAULT;

Just a quick question, how can len ever be 0 here when len was set to
strnlen() + 1? Should the test be different?

The old version (i.e. copy_strings()) seems to return -EFAULT when
strnlen() returns 0.


Vegard

> +	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;
> +
> +		pos -= bytes_to_copy;
> +		arg -= bytes_to_copy;
> +		len -= bytes_to_copy;
> +
> +		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);
>