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Subject[PATCH v2] x86/asm: Pad assembly functions with INT3 instructions
Use INT3 instead of NOP. All that padding between functions is
an illegal area, no legitimate code should jump into it.

I've checked x86_64 allyesconfig disassembly, all changes looks sane:
INT3 is only used after RET or unconditional JMP.

On i386:
* promote ret_from_exception into ENTRY as it has corresponding END,
* demote "resume_userspace" -- unused,
* delete ALIGN directive in page_fault. It is leftover from x86 assembly
cleanups.

commit d211af055d0c12dc3416c2886e6fbdc6eb74a381
i386: get rid of the use of KPROBE_ENTRY / KPROBE_END

has ALIGN directive before branch target which makes sense.
All the code after ALIGN disappeared later.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

arch/x86/entry/entry_32.S | 6 +-----
arch/x86/include/asm/linkage.h | 2 +-
2 files changed, 2 insertions(+), 6 deletions(-)

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -320,8 +320,7 @@ END(ret_from_fork)
*/

# userspace resumption stub bypassing syscall exit tracing
- ALIGN
-ret_from_exception:
+ENTRY(ret_from_exception)
preempt_stop(CLBR_ANY)
ret_from_intr:
#ifdef CONFIG_VM86
@@ -337,8 +336,6 @@ ret_from_intr:
#endif
cmpl $USER_RPL, %eax
jb resume_kernel # not returning to v8086 or userspace
-
-ENTRY(resume_userspace)
DISABLE_INTERRUPTS(CLBR_ANY)
TRACE_IRQS_OFF
movl %esp, %eax
@@ -910,7 +907,6 @@ BUILD_INTERRUPT3(hv_stimer0_callback_vector, HYPERV_STIMER0_VECTOR,
ENTRY(page_fault)
ASM_CLAC
pushl $do_page_fault
- ALIGN
jmp common_exception
END(page_fault)

--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -18,7 +18,7 @@
name:

#if defined(CONFIG_X86_64) || defined(CONFIG_X86_ALIGNMENT_16)
-#define __ALIGN .p2align 4, 0x90
+#define __ALIGN .p2align 4, 0xCC
#define __ALIGN_STR __stringify(__ALIGN)
#endif

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