Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved

[John Hubbard]

On 04/12/2018 08:39 AM, Jann Horn wrote:
> Clarify that MAP_FIXED is appropriate if the specified address range has
> been reserved using an existing mapping, but shouldn't be used otherwise.
> 
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  man2/mmap.2 | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/man2/mmap.2 b/man2/mmap.2
> index bef8b4432..80c9ec285 100644
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -253,8 +253,9 @@ Software that aspires to be portable should use this option with care,
>  keeping in mind that the exact layout of a process's memory mappings
>  is allowed to change significantly between kernel versions,
>  C library versions, and operating system releases.
> -Furthermore, this option is extremely hazardous (when used on its own),
> -because it forcibly removes preexisting mappings,
> +This option should only be used when the specified memory region has
> +already been reserved using another mapping; otherwise, it is extremely
> +hazardous because it forcibly removes preexisting mappings,
>  making it easy for a multithreaded process to corrupt its own address space.

Yes, that's clearer and provides more information than before.

>  .IP
>  For example, suppose that thread A looks through
> @@ -284,13 +285,15 @@ and the PAM libraries
>  .UR http://www.linux-pam.org
>  .UE .
>  .IP
> -Newer kernels
> -(Linux 4.17 and later) have a
> +For cases in which the specified memory region has not been reserved using an
> +existing mapping, newer kernels (Linux 4.17 and later) provide an option
>  .B MAP_FIXED_NOREPLACE
> -option that avoids the corruption problem; if available,
> -.B MAP_FIXED_NOREPLACE
> -should be preferred over
> -.BR MAP_FIXED .
> +that should be used instead; older kernels require the caller to use
> +.I addr
> +as a hint (without
> +.BR MAP_FIXED )

Here, I got lost: the sentence suddenly jumps into explaining non-MAP_FIXED
behavior, in the MAP_FIXED section. Maybe if you break up the sentence, and
possibly omit non-MAP_FIXED discussion, it will help. 

> +and take appropriate action if the kernel places the new mapping at a
> +different address.
>  .TP
>  .BR MAP_FIXED_NOREPLACE " (since Linux 4.17)"
>  .\" commit a4ff8e8620d3f4f50ac4b41e8067b7d395056843
> 

thanks,
-- 
John Hubbard
NVIDIA