On Mon, Mar 06, 2017 at 05:38:07PM +0100, Pavel Machek wrote:
> Sorry for the delay. This is on v4.11-rc1, but that should be similar.
>
> pavel@duo:~$ gcc --version
> gcc (Debian 4.9.2-10) 4.9.2
>
> And here's the disassemble:
>
> c402d200 <start_secondary>:
> c402d200: 57 push %edi
> c402d201: 8d 7c 24 08 lea 0x8(%esp),%edi
> c402d205: 83 e4 f8 and $0xfffffff8,%esp
> c402d208: ff 77 fc pushl -0x4(%edi)
> c402d20b: 55 push %ebp
> c402d20c: 89 e5 mov %esp,%ebp
> c402d20e: 57 push %edi
> c402d20f: 56 push %esi
> c402d210: 83 ec 10 sub $0x10,%esp
Thanks. This confirms what I was thinking, the function prologue is
wack. It's realigning the stack, but it's not the "normal" realign
pattern. Instead it makes a fake frame header, which saves a duplicate
copy of the return address ("pushl -0x4(%edi)") in a place the unwinder
wasn't expecting.
I did some digging in gcc to figure out why this can happen. gcc uses
something called a Dynamic Realign Argument Pointer (DRAP), which, when
enabled, makes a prologue like the above. It's almost always enabled
for aligned stacks when -maccumulate-outgoing-args isn't set.
So I'm thinking we should have -maccumulate-outgoing-args always enabled
on x86_32 just like we already do on x86_64.
Can you verify the warning is fixed with the following patch?
-----
diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
index 6647ed4..53ec1e4 100644
--- a/arch/x86/Makefile_32.cpu
+++ b/arch/x86/Makefile_32.cpu
@@ -61,7 +61,7 @@ ifeq ($(CONFIG_JUMP_LABEL), y)
ADD_ACCUMULATE_OUTGOING_ARGS := y
endif
-cflags-$(ADD_ACCUMULATE_OUTGOING_ARGS) += $(call
cc-option,-maccumulate-outgoing-args)
+cflags-y += $(call cc-option,-maccumulate-outgoing-args)
# Bug fix for binutils: this option is required in order to keep
# binutils from generating NOPL instructions against our will.
