Playing around with singlestep on i386, I found this:
1. User sets TF and starts to singlestep through code,
handling SIGTRAPS as they occur.
2. Makes vsyscall; debug trap occurs in kernel mode and TF
is cleared. TIF_SINGLESTEP gets set so kernel will remember
to re-enable TF on return to user. But when user eflags
is saved on the stack, TF has already been cleared.
3. When user gets control back, TF is not re-enabled.
4. Even after user clears TF, TIF_SINGLESTEP remains set
for that thread.
None of this happens when using int80 because the flag
is cleared and re-enabled by the interrupt handler.
TIF_SINGLESTEP never gets set and doesn't need to be.
--
Chuck
"Equations are the Devil's sentences." --Stephen Colbert
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