Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function

Josh Poimboeuf
2021-08-20T19:31:20Z

On Fri, Aug 20, 2021 at 12:22:28PM -0700, Josh Poimboeuf wrote:
> On Thu, Jun 24, 2021 at 11:41:00AM +0200, Peter Zijlstra wrote:
> > The asm_cpu_bringup_and_idle() function is required to push the return
> > value on the stack in order to make ORC happy, but the only reason
> > objtool doesn't complain is because of a happy accident.
> >
> > The thing is that asm_cpu_bringup_and_idle() doesn't return, so
> > validate_branch() never terminates and falls through to the next
> > function, which in the normal case is the hypercall_page. And that, as
> > it happens, is 4095 NOPs and a RET.
> >
> > Make asm_cpu_bringup_and_idle() terminate on it's own, by making the
> > function it calls as a dead-end. This way we no longer rely on what
> > code happens to come after.
> >
> > Fixes: c3881eb58d56 ("x86/xen: Make the secondary CPU idle tasks reliable")
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> Looks right. Only problem is, with my assembler I get this:
>
> arch/x86/kernel/head_64.o: warning: objtool: .text+0x5: unreachable instruction
>
> Because gas insists on jumping over the page of nops...
>
> 0000000000000000 <asm_cpu_bringup_and_idle>:
> 0: e8 00 00 00 00 callq 5 <asm_cpu_bringup_and_idle+0x5>
> 1: R_X86_64_PLT32 cpu_bringup_and_idle-0x4
> 5: e9 f6 0f 00 00 jmpq 1000 <xen_hypercall_set_trap_table>
> a: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 11: 00 00 00 00
> 15: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 1c: 00 00 00 00
> 20: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 27: 00 00 00 00
> 2b: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 32: 00 00 00 00
> 36: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
> 3d: 00 00 00 00

Here's a fix:

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH] x86/xen: Move hypercall_page to top of the file

Because hypercall_page is page-aligned, the assembler inexplicably adds
an unreachable jump from after the end of the previous code to the
beginning of hypercall_page.

That confuses objtool, understandably. It also creates significant text
fragmentation. As a result, much of the object file is wasted text
(nops).

Move hypercall_page to the beginning of the file to both prevent the
text fragmentation and avoid the dead jump instruction.

$ size /tmp/head_64.before.o /tmp/head_64.after.o
text data bss dec hex filename
10924 307252 4096 322272 4eae0 /tmp/head_64.before.o
6823 307252 4096 318171 4dadb /tmp/head_64.after.o

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
arch/x86/xen/xen-head.S | 34 +++++++++++++++++-----------------
1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index cb6538ae2fe0..488944d6d430 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -20,6 +20,23 @@
#include <xen/interface/xen-mca.h>
#include <asm/xen/interface.h>

+.pushsection .text
+ .balign PAGE_SIZE
+SYM_CODE_START(hypercall_page)
+ .rept (PAGE_SIZE / 32)
+ UNWIND_HINT_FUNC
+ .skip 31, 0x90
+ ret
+ .endr
+
+#define HYPERCALL(n) \
+ .equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
+ .type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
+#include <asm/xen-hypercalls.h>
+#undef HYPERCALL
+SYM_CODE_END(hypercall_page)
+.popsection
+
#ifdef CONFIG_XEN_PV
__INIT
SYM_CODE_START(startup_xen)
@@ -64,23 +81,6 @@ SYM_CODE_END(asm_cpu_bringup_and_idle)
#endif
#endif

-.pushsection .text
- .balign PAGE_SIZE
-SYM_CODE_START(hypercall_page)
- .rept (PAGE_SIZE / 32)
- UNWIND_HINT_FUNC
- .skip 31, 0x90
- ret
- .endr
-
-#define HYPERCALL(n) \
- .equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
- .type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
-#include <asm/xen-hypercalls.h>
-#undef HYPERCALL
-SYM_CODE_END(hypercall_page)
-.popsection
-
ELFNOTE(Xen, XEN_ELFNOTE_GUEST_OS, .asciz "linux")
ELFNOTE(Xen, XEN_ELFNOTE_GUEST_VERSION, .asciz "2.6")
ELFNOTE(Xen, XEN_ELFNOTE_XEN_VERSION, .asciz "xen-3.0")
--
2.31.1