Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE

From: Florian Weimer
Date: Mon Nov 20 2017 - 04:46:05 EST

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).