linux-kernel - Re: [clang] stack protector and f1f029c7bf

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Message-ID: <A042BB2E-FACF-4AE0-8B1E-716061666C5F@zytor.com>
Date:   Fri, 25 May 2018 10:56:10 -0700
From:   hpa@...or.com
To:     Nick Desaulniers <ndesaulniers@...gle.com>
CC:     Alistair Strachan <astrachan@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Matthias Kaehlcke <mka@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>, sedat.dilek@...il.com,
        tstellar@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [clang] stack protector and f1f029c7bf

On May 25, 2018 10:31:51 AM PDT, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>On Fri, May 25, 2018 at 9:53 AM <hpa@...or.com> wrote:
>> On May 25, 2018 9:46:42 AM PDT, Nick Desaulniers
><ndesaulniers@...gle.com>
>wrote:
>> >On Fri, May 25, 2018 at 9:33 AM <hpa@...or.com> wrote:
>> >> On May 25, 2018 9:27:40 AM PDT, Nick Desaulniers
>> ><ndesaulniers@...gle.com> wrote:
>> >When you say
>> >
>> >> It still should be available as as inline, however, but now
>"extern
>> >inline".
>> >
>> >Am I understanding correctly that native_save_fl should be inlined
>into
>> >all
>> >call sites (modulo the problematic pv_irq_ops.save_fl case)? 
>Because
>> >for
>> >these two assembly implementations, it's not, but maybe there's
>> >something
>> >missing in my implementation?
>
>> Yes, that's what "extern inline" means. Maybe it needs a must inline
>annotation, but that's really messed up.
>
>I don't think it's possible to inline a function from an external
>translation unit without something like LTO.
>
>If I move the implementation of native_save_fl() to a separate .c (with
>out
>of line assembly) or .S, neither clang nor gcc will inline that
>assembly to
>any call sites, whether the declaration of native_save_fl() looks like:
>
>extern inline unsigned long native_save_fl(void);
>
>or
>
>__attribute__((always_inline))
>
>extern inline unsigned long native_save_fl(void);
>
>I think an external copy is the best approach for the paravirt code:
>
>diff --git a/arch/x86/kernel/irqflags.c b/arch/x86/kernel/irqflags.c
>new file mode 100644
>index 000000000000..e173ba8bee7b
>--- /dev/null
>+++ b/arch/x86/kernel/irqflags.c
>@@ -0,0 +1,24 @@
>+#include <asm/asm.h>
>+
>+extern unsigned long native_save_fl_no_stack_protector(void);
>+extern void native_restore_fl_no_stack_protector(unsigned long flags);
>+
>+asm(
>+".pushsection .text;"
>+".global native_save_fl_no_stack_protector;"
>+".type native_save_fl_no_stack_protector, @function;"
>+"native_save_fl_no_stack_protector:"
>+"pushf;"
>+"pop %" _ASM_AX ";"
>+"ret;"
>+".popsection");
>+
>+asm(
>+".pushsection .text;"
>+".global native_restore_fl_no_stack_protector;"
>+".type native_restore_fl_no_stack_protector, @function;"
>+"native_restore_fl_no_stack_protector:"
>+"push %" _ASM_DI ";"
>+"popf;"
>+"ret;"
>+".popsection");
>diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
>index 02d6f5cf4e70..8824d01c0c35 100644
>--- a/arch/x86/kernel/Makefile
>+++ b/arch/x86/kernel/Makefile
>@@ -61,6 +61,7 @@ obj-y                 += alternative.o i8253.o
>hw_breakpoint.o
>  obj-y                  += tsc.o tsc_msr.o io_delay.o rtc.o
>  obj-y                  += pci-iommu_table.o
>  obj-y                  += resource.o
>+obj-y                  += irqflags.o
>
>  obj-y                          += process.o
>  obj-y                          += fpu/
>--- a/arch/x86/kernel/paravirt.c
>+++ b/arch/x86/kernel/paravirt.c
>@@ -322,9 +322,12 @@ struct pv_time_ops pv_time_ops = {
>         .steal_clock = native_steal_clock,
>  };
>
>+extern unsigned long native_save_fl_no_stack_protector(void);
>+extern void native_restore_fl_no_stack_protector(unsigned long flags);
>+
>  __visible struct pv_irq_ops pv_irq_ops = {
>-       .save_fl = __PV_IS_CALLEE_SAVE(native_save_fl),
>-       .restore_fl = __PV_IS_CALLEE_SAVE(native_restore_fl),
>+       .save_fl =
>__PV_IS_CALLEE_SAVE(native_save_fl_no_stack_protector),
>+       .restore_fl =
>__PV_IS_CALLEE_SAVE(native_restore_fl_no_stack_protector),
>         .irq_disable = __PV_IS_CALLEE_SAVE(native_irq_disable),
>         .irq_enable = __PV_IS_CALLEE_SAVE(native_irq_enable),
>         .safe_halt = native_safe_halt,
>
>Thoughts?

You need the extern inline in the .h file and the out-of-line .S file both.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.