Re: Question about mmap syscall and POSIX standard on mips arch

From: Paul Burton
Date: Thu Oct 18 2018 - 19:09:17 EST

Hi Al,

On Thu, Oct 18, 2018 at 05:32:00AM +0100, Al Viro wrote:
> [mips folks Cc'd]
> On Thu, Oct 18, 2018 at 11:26:02AM +0800, Hongzhi, Song wrote:
> > Hi all,
> >
> > Ltp has a POSIX teatcase about mmap, 24-2.c.
> >
> >
> [basically, MAP_FIXED mmap with addr + len > TASK_SIZE fails with
> -EINVAL on mips and -ENOMEM elsewhere]
> > Under POSIX standard, the expected errno should be ENOMEM
> >
> > when the specific [addr+len] exceeds the bound of memory.
> The mmap() function may fail if:
> The addr argument (if MAP_FIXED was specified) or off is not a multiple
> of the page size as returned by sysconf(), or is considered invalid by
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the implementation.
> ^^^^^^^^^^^^^^^^^^^
> So that behaviour gets past POSIX. That part is mostly about the
> things like cache aliasing constraints, etc., but it leaves enough
> space to weasel out. Said that, this
> MAP_FIXED was specified, and the range [addr,addr+len) exceeds that allowed
> for the address space of a process; or, if MAP_FIXED was not specified and
> there is insufficient room in the address space to effect the mapping.
> is a lot more specific, so switching to -ENOMEM there might be a good idea,
> especially since on other architectures we do get -ENOMEM in that case,

Thanks for the heads up - that does sound like reasonably clear
non-compliance. I'll make a note to put together a patch & test it out,
likely next week, if nobody submits one first.