[PATCH] Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT

["H.J. Lu" <hjl.tools-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

[-- Attachment #1: Type: text/plain, Size: 2086 bytes --]

On Tue, Dec 15, 2015 at 1:06 PM, Carlos O'Donell <carlos-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> On 12/15/2015 03:08 PM, H.J. Lu wrote:
>> On Tue, Dec 15, 2015 at 10:38 AM, Carlos O'Donell <carlos-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>>> > On 12/15/2015 01:27 PM, Carlos O'Donell wrote:
>>>> >> +  cpu_features->feature[index_Prefer_MAP_32BIT_EXEC]
>>>> >> +    |= get_prefer_map_32bit_exec ();
>>>> >>
>>>> >> You wouldn't need get_prefer_map_32bit_exec, since this is all part of
>>>> >> the code, like dl-librecon.h, which parses the extra env var.
>>> >
>>> > To be clear:
>>> >
>>> > * Add new bit flag definitions for cpu_features.
>>> > * Add a sysdeps/unix/sysv/linux/x86_64/dl-silvermont.h
>>> >   * Fill in EXTRA_LD_ENVVARS or add new ones.
>>> >   * Write to rtld's GLRO cpu_features the bit you need based
>>> >     on __libc_enable_secure.
>>> >
>>> > That should simplify and concentrate the Silvermont fixes to
>>> > just two files, making future maintenance and documentation
>>> > easier.
>>> >
>>> >
>> This is the updated patch.  I put EXTRA_LD_ENVVARS and
>> EXTRA_UNSECURE_ENVVARS in x86_64/64/dl-librecon.h
>> to be consistent with i386/dl-librecon.h.  If we ever need to
>> update EXTRA_LD_ENVVARS/EXTRA_UNSECURE_ENVVARS
>> in the future, we have a single file to change.
>>
>> Tested on x86-64.  OK for master?
>>
>> Thanks for all the feedbacks and suggestions.
>
> This looks much better and much cleaner. Looks good to me now. It also
> appears you have consesnsus with this last change.
>
> It needs a bug # please since you're fixing a user-visible performance
> problem on Silvermont.

I opened

https://sourceware.org/bugzilla/show_bug.cgi?id=19367

and checked in my patch.

> It appears to meet Zack's requirement to choose a security safe default
> at the expense of performance (I agree with that).
>
> I *strongly* urge you to immediately submit a patch to the linux man
> pages project to document that as of 2.23 this new flag exists and
> does what you describe it does.
>

Here is a patch for Linux man page.

Thanks.


H.J.

[-- Attachment #2: 0001-Document-LD_PREFER_MAP_32BIT_EXEC-for-ld.so-8.patch --]
[-- Type: text/x-patch, Size: 1292 bytes --]

From 6b8095fb2e123915ec2e586753d3d7ff7f14aee1 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Tue, 15 Dec 2015 13:29:37 -0800
Subject: [PATCH] Document LD_PREFER_MAP_32BIT_EXEC for ld.so (8)

Glibc 2.23 added LD_PREFER_MAP_32BIT_EXEC to improve branch prediction
performance on Silvermont.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
---
 man8/ld.so.8 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/man8/ld.so.8 b/man8/ld.so.8
index 608cf7c..da33404 100644
--- a/man8/ld.so.8
+++ b/man8/ld.so.8
@@ -472,6 +472,16 @@ environment variable has been set.
 (ELF only)(glibc since 2.1.3)
 If set to a nonempty string, warn about unresolved symbols.
 .TP
+.B LD_PREFER_MAP_32BIT_EXEC
+(x86-64 only)(glibc since 2.23)
+According to Silvermont software optimization guide, for 64-bit
+applications, branch prediction performance can be negatively impacted
+when the target of a branch is more than 4GB away from the branch.
+If set, ld.so will try to map executable pages with MAP_32BIT first.
+NB: MAP_32BIT will map to lower 2GB, not lower 4GB, address.  Since
+MAP_32BIT reduces bits available for address space layout randomization
+(ASLR), LD_PREFER_MAP_32BIT_EXEC is always disabled for SUID programs.
+.TP
 .B LDD_ARGV0
 (libc5)
 .IR argv [0]
-- 
2.5.0