Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged

[Jann Horn]

On Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@xxxxxxxxx wrote:
>> MAP_FIXED has been widely used for a very long time, yet the man
>> page still claims that "the use of this option is discouraged".
>
> I think we should continue to discourage the use of this option, but
> I'm going to include some of your text in my replacement paragraph ...
>
> -Because requiring a fixed address for a mapping is less portable,
> -the use of this option is discouraged.
> +The use of this option is discouraged because it forcibly unmaps any
> +existing mapping at that address.  Programs which use this option need
> +to be aware that their memory map may change significantly from one run to
> +the next, depending on library versions, kernel versions and random numbers.

How about adding something explicit about when it's okay to use MAP_FIXED?
"This option should only be used to displace an existing mapping that is
controlled by the caller, or part of such a mapping." or something like that?

> +In a threaded process, checking the existing mappings can race against
> +a new dynamic library being loaded

malloc() and its various callers can also cause mmap() calls, which is probably
more relevant than library loading.