ptrace and perf watchpoints were considered incompatible in
commit 29da4f91 ("powerpc/watchpoint: Don't allow concurrent perf
and ptrace events"), but the logic in that commit doesn't really apply.

Ptrace doesn't automatically single step; the ptracer must request this
explicitly. And the ptracer can do so regardless of whether a
ptrace/perf watchpoint triggered or not: it could single step every
instruction if it wanted to. Whatever stopped the ptracee before
executing the instruction that would trigger the perf watchpoint is no
longer relevant by this point.

To get correct behaviour when perf and ptrace are watching the same
data we must ignore the perf watchpoint. After all, ptrace has
before-execute semantics, and perf is after-execute, so perf doesn't
actually care about the watchpoint trigger at this point in time.
Pausing before execution does not mean we will actually end up executing
the instruction.

Importantly though, we don't remove the perf watchpoint yet. This is
key.

The ptracer is free to do whatever it likes right now. E.g., it can
continue the process, single step. or even set the child PC somewhere
completely different.

If it does try to execute the instruction though, without reinserting
the watchpoint (in which case we go back to the start of this example),
the perf watchpoint would immediately trigger. This time there is no
ptrace watchpoint, so we can safely perform a single step and increment
the perf counter. Upon receiving the single step exception, the existing
code already handles propagating or consuming it based on whether
another subsystem (e.g. ptrace) requested a single step. Again, this is
needed with or without perf/ptrace exclusion, because ptrace could be
single stepping this instruction regardless of if a watchpoint is
involved.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230801011744.153973-6-bgray@linux.ibm.com