Re: [PATCH] x86: fix typo(?) in step.c

[Roland McGrath]

> my interpretation of the bug would be that we fail to mask out the 
> block-step MSR bit [because we mask out bit 25 instead of bit 1], and 
> hence the bug would cause that MSR bit to stay enabled in other tasks 
> too.

The wrong bit is in calls to write_debugctlmsr, only used when setting up a
thread to step.  It does not affect context switch, so it would never have
an effect on other tasks as you suggest here.

> So in theory the bug should manifest itself as block-step mode never 
> clearing itself, once activated. 

That doesn't happen in the trivial sense of "never", because in the normal
case an actual block-step exception happens and that makes the hardware
clear BTF from the MSR (as well as TF from eflags).  So it would only come
up in a more obscure case.  That is, you set up for block-step but didn't
actually finish the user-mode instruction block.  e.g. interrupted by a
signal or faulting instruction.  The child stops again but not by SIGTRAP,
and next time you don't block-step it.  Then, the BTF bit stays set in
thread.debugctlmsr and gets switched back in when the child runs again.
If you then resume with single-step instead, it will block-step because
BTF is set, but you wanted instruction-step.  Like I said, I didn't
produce a case that behaved that way.  I may be overlooking something.
But that's the scenario I imagine.

> (but this would never leak into other tasks because we've got the
> thread.debugctlmsr abstraction that protects them)

Correct.


Thanks,
Roland
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