| From | Andy Lutomirski <> | Date | Sun, 14 Aug 2016 00:26:29 -0700 | Subject | Re: [PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack_pointer() previous stack access |
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On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <jpoimboe@redhat.com> 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 <jpoimboe@redhat.com>
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?
--Andy
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