selftests/powerpc/ptrace: Update ptrace-perf watchpoint selftest

Now that ptrace and perf are no longer exclusive, update the
test to exercise interesting interactions.

An assembly file is used for the children to allow precise instruction
choice and addresses, while avoiding any compiler quirks.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230801011744.153973-7-bgray@linux.ibm.com

selftests/powerpc/ptrace: Fix typo in pid_max search error

pid_max_addr() searches for the 'pid_max' symbol in /proc/kallsyms, and
prints an error if it cannot find it. The error message has a typo,
calling it pix_max.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230725005841.28854-4-bgray@linux.ibm.com

selftests/powerpc/ptrace: Explain why tests are skipped

Many tests require specific hardware features/configurations that a
typical machine might not have. As a result, it's common to see a test
is skipped. But it is tedious to find out why a test is skipped
when all it gives is the file location of the skip macro.

Convert SKIP_IF() to SKIP_IF_MSG(), with appropriate descriptions of why
the test is being skipped. This gives a general idea of why a test is
skipped, which can be looked into further if it doesn't make sense.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230725005841.28854-3-bgray@linux.ibm.com

powerpc/selftests: Add selftest to test concurrent perf/ptrace events

ptrace and perf watchpoints can't co-exists if their address range
overlaps. See commit 29da4f91c0c1 ("powerpc/watchpoint: Don't allow
concurrent perf and ptrace events") for more detail. Add selftest
for the same.

Sample o/p:
# ./ptrace-perf-hwbreak
test: ptrace-perf-hwbreak
tags: git_version:powerpc-5.8-7-118-g937fa174a15d-dirty
perf cpu event -> ptrace thread event (Overlapping): Ok
perf cpu event -> ptrace thread event (Non-overlapping): Ok
perf thread event -> ptrace same thread event (Overlapping): Ok
perf thread event -> ptrace same thread event (Non-overlapping): Ok
perf thread event -> ptrace other thread event: Ok
ptrace thread event -> perf kernel event: Ok
ptrace thread event -> perf same thread event (Overlapping): Ok
ptrace thread event -> perf same thread event (Non-overlapping): Ok
ptrace thread event -> perf other thread event: Ok
ptrace thread event -> perf cpu event (Overlapping): Ok
ptrace thread event -> perf cpu event (Non-overlapping): Ok
ptrace thread event -> perf same thread & cpu event (Overlapping): Ok
ptrace thread event -> perf same thread & cpu event (Non-overlapping): Ok
ptrace thread event -> perf other thread & cpu event: Ok
success: ptrace-perf-hwbreak

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412112218.128183-5-ravi.bangoria@linux.ibm.com