Re: [RFC] [PATCH 2.6.37-rc5-tip 13/20] 13: x86: x86 specific probehandling

[Roland McGrath]

> And reset the hardware back to block step when done, and provide the
> actual break blockstep would have.

Oh, sure, that too.  If you're that ambitious, then the place to start
first is with plain single-step working right.  When TF was already set
(either via user_enable_single_step, so TIF_SINGLESTEP is set, or just from
user mode, so it and TIF_FORCED_TF are not set, but TF is in the user
state's eflags) and you hit a uprobe, then after servicing the uprobe and
stepping over the copied original instruction and restoring the PC to where
it should be, you should let the trap turn into a SIGTRAP as normal rather
than swallowing it.

To support block-step correctly, you have to do something more clever.
If block-step was enabled (TIF_BLOCKSTEP set), then you need to figure
out which of two things is the right one to do.  If the copied original
instruction uprobes just single-stepped over is one that would trigger
block-step, then you should treat it as if plain single-step were
enabled, i.e. let that SIGTRAP go as above.  If not, then you should
swallow the signal, re-enable block-step and set TF (i.e. do the work of
user_enable_block_step) before resuming.  You have to decide which case
it is based on instruction analysis.  If it's a control-flow instruction
(including the syscall instructions), then it would trigger block-step.
IIRC a conditional branch instruction triggers it only if the branch is
taken (check the book), so you have to notice that too.


Thanks,
Roland
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