Re: [PATCH] mmap.2: offset behavior for MAP_ANONYMOUS

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Jann,

On 08/09/2016 11:12 AM, Jann Horn wrote:
> The manpage claimed that the offset argument is ignored, and when I
> interpreted that as "I don't need to set that register when doing the
> syscall", I got failures. I was able to spot two reasons for that:
>
> What I probably ran into:
> At least on x86-64, sys_mmap (in arch/x86/kernel/sys_x86_64.c) always
> checks that the offset is page-aligned, even for MAP_ANONYMOUS.
>
> Another one, could probably trigger on 32-bit x86:
> In do_mmap(), there is a check to ensure that pgoff together with the
> allocation length won't cause an overflow, even for MAP_ANONYMOUS.
>
> Document that userspace should pass in zero, since that's probably what
> everyone is doing already. (It would also be possible to describe the
> constraints on the offset more carefully, but zero works, and nobody
> should need to pass in anything else.)

I think the level of detail in the patch is just right, and the commit
message contains all the useful background. Thanks for that! Patch
applied.

Cheers,

Michael


> Signed-off-by: Jann Horn <jann-XZ1E9jl8jIdeoWH0uzbU5w@public.gmane.org>
> ---
>  man2/mmap.2 | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/man2/mmap.2 b/man2/mmap.2
> index 3bcfffd..751dd36 100644
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -164,9 +164,7 @@ The mapping is not backed by any file;
>  its contents are initialized to zero.
>  The
>  .I fd
> -and
> -.I offset
> -arguments are ignored;
> +argument is ignored;
>  however, some implementations require
>  .I fd
>  to be \-1 if
> @@ -175,6 +173,11 @@ to be \-1 if
>  .BR MAP_ANON )
>  is specified,
>  and portable applications should ensure this.
> +The
> +.I offset
> +argument should be zero.
> +.\" See the pgoff overflow check in do_mmap().
> +.\" See the offset check in sys_mmap in arch/x86/kernel/sys_x86_64.c.
>  The use of
>  .B MAP_ANONYMOUS
>  in conjunction with
>


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