On Fri, Apr 29, 2016 at 02:38:02PM -0700, Andy Lutomirski wrote: > On Fri, Apr 29, 2016 at 1:50 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > 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? > > > > Look at collect_syscall in lib/syscall.c I don't see how collect_syscall() can be called for the per-cpu idle "swapper" tasks (which is what the above code affects). They don't have pids or /proc entries so you can't do /proc/<pid>/syscall on them. -- 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
