linux-kernel - Re: Remaining randconfig objtool warnings, linux-next-20200428

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Date:   Tue, 28 Apr 2020 23:55:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: Remaining randconfig objtool warnings, linux-next-20200428

On Tue, Apr 28, 2020 at 03:38:55PM -0500, Josh Poimboeuf wrote:
> This one makes no sense to me.  It looks like the assembler is inserting
> a jump as part of the alignment padding???  WTH.
> 
> 0000000000000980 <common_spurious>:
>      980:	48 83 04 24 80       	addq   $0xffffffffffffff80,(%rsp)
>      985:	e8 00 00 00 00       	callq  98a <common_spurious+0xa>
> 			986: R_X86_64_PLT32	interrupt_entry-0x4
>      98a:	e8 00 00 00 00       	callq  98f <common_spurious+0xf>
> 			98b: R_X86_64_PLT32	smp_spurious_interrupt-0x4
>      98f:	eb 7e                	jmp    a0f <ret_from_intr>
>      991:	eb 6d                	jmp    a00 <common_interrupt>
>      993:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      99a:	00 00 00 00 
>      99e:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9a5:	00 00 00 00 
>      9a9:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9b0:	00 00 00 00 
>      9b4:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9bb:	00 00 00 00 
>      9bf:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9c6:	00 00 00 00 
>      9ca:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9d1:	00 00 00 00 
>      9d5:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9dc:	00 00 00 00 
>      9e0:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9e7:	00 00 00 00 
>      9eb:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>      9f2:	00 00 00 00 
>      9f6:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
>      9fd:	00 00 00 

binutils.git/gas/configure/tc-i386.c:i386_generate_nops

When there's too many NOPs (as here) it generates a JMP across the NOPS.
It makes some sort of sense, at some point executing NOPs is going to be
more expensive than a branch.. But shees..