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

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Date:   Wed, 17 Jun 2020 09:56:36 +0200
From:   Christoph Hellwig <hch@....de>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     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 Tue, Jun 16, 2020 at 03:14:44PM +0200, Vegard Nossum wrote:
>
> 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.

So, the nasty part here is that strnlen_user has different semantics
from strnlen:

 - strlen excludes the terminating null byte and never returns error
   codes
 - strnlen_user includes the terminating null byte, and a 0 return
   means it could not access the user address (a condition that can't
   happen for strlen).

Now with that back to your original question:  I think then len == 0
check can just be removed without replacement.