Re: [PATCH] mmap: add sysctl for controlling ~VM_MAYEXEC taint

[Will Drewry]

On Tue, Aug 16, 2011 at 2:50 PM, Will Drewry <wad@xxxxxxxxxxxx> wrote:
> On Tue, Aug 16, 2011 at 2:40 PM, Mel Gorman <mel@xxxxxxxxx> wrote:
>> On Tue, Aug 16, 2011 at 10:07:46AM -0700, Roland McGrath wrote:
>>> On Tue, Aug 16, 2011 at 2:33 AM, Mel Gorman <mel@xxxxxxxxx> wrote:
>>> > Is using shm_open()+mmap instead of open()+mmap() to open a file on
>>> > /dev/shm really that difficult?
>>> >
>>> > int shm_open(const char *name, int oflag, mode_t mode);
>>> > int open(const char *pathname, int flags, mode_t mode);
>>>
>>> I cannot figure out the rationale behind this question at all.
>>> Both of these library functions result in the same system call.
>>>
>>
>> They might result in the same system call but one of them creates
>> the file under /dev/shm which should not have the same permissions
>> problem. The library really appears to want to create a shared
>> executable object, using shm_open does not appear that unreasonable
>> to me.
>
> If /dev/shm is mounted noexec, the resulting file will have VM_MAYEXEC

Err VMA from mmaping the file from shm_open().

> stripped.  I don't believe it is capable of doing anything special
> that will cause the mmap code path to find a different containing
> mountpoint.  If it could, then that would certainly be preferable, but
> it would also make this VM_MAYEXEC calculation less effective in the
> default case.
>
> thanks!
>
>>> > An ordinary user is not going to know that a segfault from an
>>> > application can be fixed with this sysctl. This looks like something
>>> > that should be fixed in the library so that it can work on kernels
>>> > that do not have the sysctl.
>>>
>>> I think the expectation is that the administrator or system builder
>>> who decides to set the (non-default) noexec mount option will also
>>> set the sysctl at the same time.
>>>
>>
>> Which then needs to be copied in each distro wanting to do the same
>> thing and is not backwards compatible where as using shm_open is.
>>
>> --
>> Mel Gorman
>> SUSE Labs
>>
>
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