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

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Message-ID: <20181018043200.GE32577@ZenIV.linux.org.uk>
Date:   Thu, 18 Oct 2018 05:32:00 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Hongzhi, Song" <hongzhi.song@...driver.com>
Cc:     linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
        linux-mips@...ux-mips.org, Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: Question about mmap syscall and POSIX standard on mips arch

[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.
> 
> https://github.com/linux-test-project/ltp/blob/e816127e5d8efbff5ae53e9c2292fae22f36838b/testcases/open_posix_testsuite/conformance/interfaces/mmap/24-2.c#L94

[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:

[EINVAL]
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

[ENOMEM]
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,
AFAICS.