linux-kernel - Re: [PATCH 1/2] kernel/sys.c: return the current gid when error occurs

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Date:	Fri, 09 Aug 2013 08:59:32 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>, holt@....com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] kernel/sys.c: return the current gid when error occurs

On 08/08/2013 09:37 PM, Michael Kerrisk (man-pages) wrote:
> On 08/07/13 18:21, Oleg Nesterov wrote:
>> On 08/06, Andy Lutomirski wrote:
>>>
>>> I assume that what the man page means is that the return value is
>>> whatever fsgid was prior to the call.  On error, fsgid isn't changed, so
>>> the return value is still "current".
>>
>> Probably... Still
>>
>> 	On success, the previous value of fsuid is returned.
>> 	On error, the current value of fsuid is returned.
>>
>> looks confusing. sys_setfsuid() always returns the old value.
>>
>>> (FWIW, this behavior is awful and is probably the cause of a security
>>> bug or three, since success and failure are indistinguishable.
>>
>> At least this all looks strange.
>>
>> I dunno if we can change this old behaviour. I won't be surprized
>> if someone already uses setfsuid(-1) as getfsuid().
>>
>> And perhaps the man page should be changed. Add Michael.
> 
> Thanks, Oleg. I've applied the following patch to setfsuid.2
> (and a similar patch to setfsgid.2).
> 
> Cheers,
> 
> Michael
> 
> --- a/man2/setfsuid.2
> +++ b/man2/setfsuid.2
> @@ -67,12 +67,8 @@ matches either the real user ID, effective user ID, saved set-user-ID, or
>  the current value of
>  .IR fsuid .
>  .SH RETURN VALUE
> -On success, the previous value of
> -.I fsuid
> -is returned.
> -On error, the current value of
> -.I fsuid
> -is returned.
> +On both success and failure,
> +this call returns the previous filesystem user ID of the caller.
>  .SH VERSIONS
>  This system call is present in Linux since version 1.2.
>  .\" This system call is present since Linux 1.1.44
> @@ -102,7 +98,16 @@ The glibc
>  .BR setfsuid ()
>  wrapper function transparently deals with the variation across kernel versions.
>  .SH BUGS
> -No error messages of any kind are returned to the caller.
> +No error indications of any kind are returned to the caller,
> +and the fact that both successful and unsuccessful calls return
> +the same value makes it impossible to directly determine
> +whether the call succeeded or failed.
> +Instead, the caller must resort to looking at the return value
> +from a further call such as
> +.IR setfsuid(\-1)
> +(which will always fail), in order to determine if a preceding call to
> +.BR setfsuid ()
> +changed the filesystem user ID.
>  At the very
>  least,
>  .B EPERM
> 

Is it suitable to mention this API is obsoleted and unneeded in man page
?  ;-)


Thanks.
-- 
Chen Gang
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