On Fri, Apr 29, 2016 at 12:39:16PM -0700, Andy Lutomirski wrote: > On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > Thanks to all the recent x86 entry code refactoring, most tasks' kernel > > stacks start at the same offset right above their saved pt_regs, > > regardless of which syscall was used to enter the kernel. That creates > > a nice convention which makes it straightforward to identify the > > "bottom" of the stack, which can be useful for stack walking code which > > needs to verify the stack is sane. > > > > However there are still a few types of tasks which don't yet follow that > > convention: > > > > 1) CPU idle tasks, aka the "swapper" tasks > > > > 2) freshly forked TIF_FORK tasks which don't have a stack at all > > > > Make the idle tasks conform to the new stack bottom convention by > > starting their stack at a sizeof(pt_regs) offset from the end of the > > stack page. > > > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > > --- > > arch/x86/kernel/head_64.S | 7 ++++--- > > 1 file changed, 4 insertions(+), 3 deletions(-) > > > > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S > > index 6dbd2c0..0b12311 100644 > > --- a/arch/x86/kernel/head_64.S > > +++ b/arch/x86/kernel/head_64.S > > @@ -296,8 +296,9 @@ ENTRY(start_cpu) > > * REX.W + FF /5 JMP m16:64 Jump far, absolute indirect, > > * address given in m16:64. > > */ > > - movq initial_code(%rip),%rax > > - pushq $0 # fake return address to stop unwinder > > + call 1f # put return address on stack for unwinder > > +1: xorq %rbp, %rbp # clear frame pointer > > + movq initial_code(%rip), %rax > > pushq $__KERNEL_CS # set correct cs > > pushq %rax # target address in negative space > > lretq > > @@ -325,7 +326,7 @@ ENDPROC(start_cpu0) > > GLOBAL(initial_gs) > > .quad INIT_PER_CPU_VAR(irq_stack_union) > > GLOBAL(initial_stack) > > - .quad init_thread_union+THREAD_SIZE-8 > > + .quad init_thread_union + THREAD_SIZE - SIZEOF_PTREGS > > As long as you're doing this, could you also set orig_ax to -1? I > remember running into some oddities resulting from orig_ax containing > garbage at some point. I assume you mean to initialize the orig_rax value in the pt_regs at the bottom of the stack of the idle task? How could that cause a problem? Since the idle task never returns from a system call, I'd assume that memory never gets accessed? -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
