On Fri, May 18, 2018 at 09:36:44AM +0200, Ingo Molnar wrote: > > * Alexey Dobriyan <adobri...@gmail.com> wrote: > > > 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 <adobri...@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 > > So the question is, without objtool support, how will we find INT3-padding > related > crash bugs on 32-bit kernels?
Is the INT3 padding really worth it, even on x86-64? What problem are we trying to solve? I've seen cases with GCC functions falling through, but with asm code, falling through could just be working as designed. -- Josh
