Re: [patch v2] mremap.2: Add description of old_size == 0 functionality

[Michael Kerrisk (man-pages)]

Hello Mike,

On 09/19/2017 11:42 PM, Mike Kravetz wrote:
> v2: Fix incorrect wording noticed by Jann Horn.
>     Remove deprecated and memfd_create discussion as suggested
>     by Florian Weimer.
> 
> Since at least the 2.6 time frame, mremap would create a new mapping
> of the same pages if 'old_size == 0'.  It would also leave the original
> mapping.  This was used to create a 'duplicate mapping'.
> 
> A recent change was made to mremap so that an attempt to create a
> duplicate a private mapping will fail.
> 
> Document the 'old_size == 0' behavior and new return code from
> below commit.
> 
> commit dba58d3b8c5045ad89c1c95d33d01451e3964db7
> Author: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Date:   Wed Sep 6 16:20:55 2017 -0700
> 
>     mm/mremap: fail map duplication attempts for private mappings
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  man2/mremap.2 | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/man2/mremap.2 b/man2/mremap.2
> index 98643c640..235984a96 100644
> --- a/man2/mremap.2
> +++ b/man2/mremap.2
> @@ -58,6 +58,20 @@ may be provided; see the description of
>  .B MREMAP_FIXED
>  below.
>  .PP
> +If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to
> +a shareable mapping (see
> +.BR mmap (2)
> +.BR MAP_SHARED )
> +, then
> +.BR mremap ()
> +will create a new mapping of the same pages. \fInew_size\fP
> +will be the size of the new mapping and the location of the new mapping
> +may be specified with \fInew_address\fP, see the description of
> +.B MREMAP_FIXED
> +below.  If a new mapping is requested via this method, then the
> +.B MREMAP_MAYMOVE
> +flag must also be specified.
> +.PP
>  In Linux the memory is divided into pages.
>  A user process has (one or)
>  several linear virtual memory segments.
> @@ -174,7 +188,12 @@ and
>  or
>  .B MREMAP_FIXED
>  was specified without also specifying
> -.BR MREMAP_MAYMOVE .
> +.BR MREMAP_MAYMOVE ;
> +or \fIold_size\fP was zero and \fIold_address\fP does not refer to a
> +shareable mapping;
> +or \fIold_size\fP was zero and the
> +.BR MREMAP_MAYMOVE
> +flag was not specified.
>  .TP
>  .B ENOMEM
>  The memory area cannot be expanded at the current virtual address, and the

I've applied this, and added Reviewed-by tags for Florian and Jann.
But, I think it's also worth noting the older, now disallowed, behavior,
and why the behavior was changed. So I added a note in BUGS:

    BUGS
       Before Linux 4.14, if old_size was zero and the  mapping  referred
       to  by  old_address  was  a private mapping (mmap(2) MAP_PRIVATE),
       mremap() created a new private mapping unrelated to  the  original
       mapping.   This behavior was unintended and probably unexpected in
       user-space applications (since the intention  of  mremap()  is  to
       create  a new mapping based on the original mapping).  Since Linux
       4.14, mremap() fails with the error EINVAL in this scenario.

Does that seem okay?

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/