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Re: PING^2: [PATCH] i386; Add -mmanual-endbr and cf_check function attribute


On 12/11/18 9:03 AM, H.J. Lu wrote:
> On Mon, Dec 3, 2018 at 5:45 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Jun 18, 2018 at 2:20 AM Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>> On Fri, Jun 15, 2018 at 2:59 PM H.J. Lu <hongjiu.lu@intel.com> wrote:
>>>> Currently GCC inserts ENDBR instruction at entries of all non-static
>>>> functions, unless LTO compilation is used.  Marking all functions,
>>>> which are not called indirectly with nocf_check attribute, is not
>>>> ideal since 99% of functions in a program may be of this kind.
>>>>
>>>> This patch adds -mmanual-endbr and cf_check function attribute.  They
>>>> can be used together with -fcf-protection such that ENDBR instruction
>>>> is inserted only at entries of functions with cf_check attribute.  It
>>>> can limit number of ENDBR instructions to reduce program size.
>>>>
>>>> OK for trubk?
>>> I wonder if the linker could assist with ENDBR creation by
>>> redirecting all non-direct call relocs to a linker-generated
>>> stub with ENBR and a direct branch?
>>>
>> The goal of this patch is to add as few as ENDBR as possible
>> to reduce program size as much as possible.   Also there is no
>> relocation for indirect branch via register.
>>
> Hi Honza, Jakub, Jeff, Richard,
> 
> Here is the rebased patch.  Can you guys take a look?
> 
> Thanks.
> 
> 
> -- H.J.
> 
> 
> 0001-i386-Add-mmanual-endbr-and-cf_check-function-attribu.patch
> 
> From 5934c6be6495b2d6f278646e25f9e684f6610e2b Mon Sep 17 00:00:00 2001
> From: "H.J. Lu" <hjl.tools@gmail.com>
> Date: Thu, 14 Jun 2018 09:19:27 -0700
> Subject: [PATCH] i386; Add -mmanual-endbr and cf_check function attribute
> 
> Currently GCC inserts ENDBR instruction at entries of all non-static
> functions, unless LTO compilation is used.  Marking all functions,
> which are not called indirectly with nocf_check attribute, is not
> ideal since 99% of functions in a program may be of this kind.
> 
> This patch adds -mmanual-endbr and cf_check function attribute.  They
> can be used together with -fcf-protection such that ENDBR instruction
> is inserted only at entries of functions with cf_check attribute.  It
> can limit number of ENDBR instructions to reduce program size.
> 
> gcc/
> 
> 	* config/i386/i386.c (rest_of_insert_endbranch): Insert ENDBR
> 	at the function entry only when -mmanual-endbr isn't used or
> 	there is cf_check function attribute.
> 	(ix86_attribute_table): Add cf_check.
> 	* config/i386/i386.opt: Add -mmanual-endbr.
> 	* doc/extend.texi: Document cf_check attribute.
> 	* doc/invoke.texi: Document -mmanual-endbr.
> 
> gcc/testsuite/
> 
> 	* gcc.target/i386/cf_check-1.c: New test.
> 	* gcc.target/i386/cf_check-2.c: Likewise.
> 	* gcc.target/i386/cf_check-3.c: Likewise.
> 	* gcc.target/i386/cf_check-4.c: Likewise.
> 	* gcc.target/i386/cf_check-5.c: Likewise.
OK.

Though I'm not sure how valuable this is in practice.  Yea, it saves
some space at the start of functions, but I find myself wondering more
and more if we should be pushing folks towards LTO for a variety of reasons.

jeff


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