[PATCH] Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT

[H.J. Lu]

On Tue, Dec 15, 2015 at 1:06 PM, Carlos O'Donell <carlos@redhat.com> 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@redhat.com> 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.
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