linux-kernel - Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED

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Date:   Mon, 20 Nov 2017 10:45:56 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-api@...r.kernel.org, Khalid Aziz <khalid.aziz@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE

On 11/20/2017 10:33 AM, Michal Hocko wrote:
> On Mon 20-11-17 10:10:32, Florian Weimer wrote:
>> On 11/20/2017 09:55 AM, Michal Hocko wrote:
>>> On Fri 17-11-17 08:30:48, Florian Weimer wrote:
>>>> On 11/16/2017 11:18 AM, Michal Hocko wrote:
>>>>> +	if (flags & MAP_FIXED_SAFE) {
>>>>> +		struct vm_area_struct *vma = find_vma(mm, addr);
>>>>> +
>>>>> +		if (vma && vma->vm_start <= addr)
>>>>> +			return -ENOMEM;
>>>>> +	}
>>>>
>>>> Could you pick a different error code which cannot also be caused by a an
>>>> unrelated, possibly temporary condition?  Maybe EBUSY or EEXIST?
>>>
>>> Hmm, none of those are described in the man page. I am usually very
>>> careful to not add new and potentially unexpected error codes but it is
>>
>> I think this is a bad idea.  It leads to bizarre behavior, like open failing
>> with EOVERFLOW with certain namespace configurations (which have nothing to
>> do with file sizes).
> 
> Ohh, I agree but breaking userspace is, you know, no-no. And an
> unexpected error codes can break things terribly.

On the glibc side, we see a lot of changes in error codes depending on 
kernel version, build and run-time configuration.  It never occurred to 
me that you guys think the precise error code is part of the userspace 
ABI.  Personally, I even assume that failure itself can disappear at any 
time (evidence: the f* functions which accept O_PATH in their non-*at 
variants).

Thanks,
Florian