[PATCH] Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT

[Carlos O'Donell]

On 12/16/2015 09:02 AM, Michael Kerrisk (man-pages) wrote:
> (Thanks, Carlos.)
> 
>> Here is a patch for Linux man page.
> 
> Thanks, H.J. I applied the patch and tweaked your text somewhat.
> Does the following look okay?
> 
>        LD_PREFER_MAP_32BIT_EXEC
>               (x86-64 only)(glibc since 2.23)  According  to  the  Intel
>               Silvermont  software optimization guide, for 64-bit appli‐
>               cations, branch prediction performance can  be  negatively
>               impacted when the target of a branch is more than 4GB away
>               from the branch.  If this environment variable is set  (to
>               any  value),  ld.so will first try to map executable pages
>               using the mmap(2) MAP_32BIT flag, and fall back to mapping
>               without  that  flag  if that attempt fails.  NB: MAP_32BIT
>               will map to the low 2GB (not 2GB) of  the  address  space.
>               Because  MAP_32BIT reduces the address range available for
>               address  space  layout   randomization   (ASLR),   LD_PRE‐
>               FER_MAP_32BIT_EXEC  is always disabled in secure-execution
>               mode.

Looks great. Thanks for the quick turnaround Michael!

Cheers,
Carlos.