Messages in this thread Patch in this message | | | Date | Thu, 8 Feb 2018 10:13:02 +0100 | From | Peter Zijlstra <> | Subject | Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f |
| |
On Wed, Feb 07, 2018 at 11:43:37AM -0800, Linus Torvalds wrote: > On Wed, Feb 7, 2018 at 11:28 AM, Borislav Petkov <bp@alien8.de> wrote: > > On Wed, Feb 07, 2018 at 08:14:51PM +0100, Peter Zijlstra wrote: > >> Then someone went and wrecked it. > > > > Yeah, note says UD0 didn't eat a ModRM byte on old CPUs. But then that > > changed too. Fun stuff changing insn encoding underway. > > > > So if we opt for adding a ModRM byte, could a 0x90 NOP work so that it > > doesn't shit itself on those old CPUs? > > We could just also decide that the only thing that the modrm bytes of > UD0 actually *affect* is how the CPU might act for a page-crossing > instruction. > > Because I think that's the only semantic difference: if it's a > page-crosser, the instruction could take a page fault before raising > the #UD. > > Is there any other decode issue we might want to look out for?
_The_ problem is that new binutils cannot sanely decode any function that has a WARN in (this very much includes perf annotate):
old:
00000000000016a0 <copy_overflow>: 16a0: 48 89 f2 mov %rsi,%rdx 16a3: 89 fe mov %edi,%esi 16a5: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 16a8: R_X86_64_32S .rodata.str1.8+0x288 16ac: e8 00 00 00 00 callq 16b1 <copy_overflow+0x11> 16ad: R_X86_64_PC32 __warn_printk-0x4 16b1: 0f ff (bad) 16b3: c3 retq 16b4: 66 90 xchg %ax,%ax 16b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 16bd: 00 00 00
new:
00000000000016a0 <copy_overflow>: 16a0: 48 89 f2 mov %rsi,%rdx 16a3: 89 fe mov %edi,%esi 16a5: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 16a8: R_X86_64_32S .rodata.str1.8+0x288 16ac: e8 00 00 00 00 callq 16b1 <copy_overflow+0x11> 16ad: R_X86_64_PC32 __warn_printk-0x4 16b1: 0f ff c3 ud0 %ebx,%eax 16b4: 66 90 xchg %ax,%ax 16b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 16bd: 00 00 00
I went through the register opcodes and matched it against the ModR/M encoding, and the best option I've found so far is using 0xd6 as the next byte.
That yields:
old:
0000000000001690 <copy_overflow>: 1690: 48 89 f2 mov %rsi,%rdx 1693: 89 fe mov %edi,%esi 1695: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 1698: R_X86_64_32S .rodata.str1.8+0x270 169c: e8 00 00 00 00 callq 16a1 <copy_overflow+0x11> 169d: R_X86_64_PC32 __warn_printk-0x4 16a1: 0f ff (bad) 16a3: d6 (bad) 16a4: c3 retq 16a5: 90 nop 16a6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
new:
0000000000001690 <copy_overflow>: 1690: 48 89 f2 mov %rsi,%rdx 1693: 89 fe mov %edi,%esi 1695: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 1698: R_X86_64_32S .rodata.str1.8+0x270 169c: e8 00 00 00 00 callq 16a1 <copy_overflow+0x11> 169d: R_X86_64_PC32 __warn_printk-0x4 16a1: 0f ff d6 ud0 %esi,%edx 16a4: c3 retq 16a5: 90 nop 16a6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 16ad: 00 00 00
And only grows a defconfig image by 91 bytes, purely for the purpose of being able to disassemble it :/
text data bss dec hex filename 17307211 4890808 1052880 23250899 162c7d3 defconfig-build/vmlinux 17307302 4890808 1052880 23250990 162c82e defconfig-build/vmlinux
--- arch/x86/include/asm/bug.h | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h index 34d99af43994..f0d5b4a1512d 100644 --- a/arch/x86/include/asm/bug.h +++ b/arch/x86/include/asm/bug.h @@ -12,16 +12,21 @@ * (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas * our kernel decoder thinks it takes a ModRM byte, which seems consistent * with various things like the Intel SDM instruction encoding rules) + * + * And now someone went and taught binutils about UD0 as taking a ModR/M too + * and it generates crap when disassembling the kernel. Stick a pointless 0xD6 + * ModR/M on, which the old binutils decodes as (bad) and the new binutils sees + * as a valid single byte ModR/M. */ -#define ASM_UD0 ".byte 0x0f, 0xff" +#define ASM_UD0 ".byte 0x0f, 0xff, 0xd6" #define ASM_UD1 ".byte 0x0f, 0xb9" /* + ModRM */ #define ASM_UD2 ".byte 0x0f, 0x0b" #define INSN_UD0 0xff0f #define INSN_UD2 0x0b0f -#define LEN_UD0 2 +#define LEN_UD0 3 #ifdef CONFIG_GENERIC_BUG
| |