On Sun, Aug 14, 2016 at 5:55 AM, Brian Gerst <[email protected]> wrote: > On Sun, Aug 14, 2016 at 3:26 AM, Andy Lutomirski <[email protected]> wrote: >> On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <[email protected]> wrote: >>> On x86_32, when an interrupt happens from kernel space, SS and SP aren't >>> pushed and the existing stack is used. So pt_regs is effectively two >>> words shorter, and the previous stack pointer is normally the memory >>> after the shortened pt_regs, aka '®s->sp'. >>> >>> But in the rare case where the interrupt hits right after the stack >>> pointer has been changed to point to an empty stack, like for example >>> when call_on_stack() is used, the address immediately after the >>> shortened pt_regs is no longer on the stack. In that case, instead of >>> '®s->sp', the previous stack pointer should be retrieved from the >>> beginning of the current stack page. >>> >>> kernel_stack_pointer() wants to do that, but it forgets to dereference >>> the pointer. So instead of returning a pointer to the previous stack, >>> it returns a pointer to the beginning of the current stack. >>> >>> Fixes: 0788aa6a23cb ("x86: Prepare removal of previous_esp from i386 >>> thread_info structure") >>> Signed-off-by: Josh Poimboeuf <[email protected]> >> >> This seems like a valid fix, but I'm not sure I agree with the intent >> of the code. ®s->sp really is the previous stack pointer in the >> sense that the stack pointer was ®s->sp when the entry happened. >> From an unwinder's perspective, how is: >> >> movl [whatever], $esp >> <-- interrupt >> >> any different from: >> >> movl [whatever], $esp >> pushl [something] >> <-- interrupt >> >> Also, does x86_32 do this type of stack switching at all? AFAICS >> 32-bit kernels don't use IRQ stacks in the first place. Do they? Am >> I just missing the code that does it? > > 32-bit uses a software-based stack switch to run on the IRQ stack. > See execute_on_irq_stack() in irq_32.c. >
Indeed, thanks. I'm still not convinced that kernel_stack_pojnter() needs to handle this.
