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

From: Andy Lutomirski
Date: Wed Aug 07 2013 - 12:58:35 EST

On Wed, Aug 7, 2013 at 9:21 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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().

I suspect that changing this without introducing security or other
bugs is impossible. If someone wanted to add new_setfsuid that
returned an error when it failed, that would be a different story.
(I'm not going to do that myself.)

> And perhaps the man page should be changed. Add Michael.

Agreed. The text is a bit odd.

> Oleg.

Andy Lutomirski
AMA Capital Management, LLC
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