coredump selftest
=================
Background context
------------------
`coredump` is a feature which dumps a process's memory space when the process terminates
unexpectedly (e.g. due to segmentation fault), which can be useful for debugging. By default,
`coredump` dumps the memory to the file named `core`, but this behavior can be changed by writing a
different file name to `/proc/sys/kernel/core_pattern`. Furthermore, `coredump` can be piped to a
user-space program by writing the pipe symbol (`|`) followed by the command to be executed to
`/proc/sys/kernel/core_pattern`. For the full description, see `man 5 core`.
The piped user program may be interested in reading the stack pointers of the crashed process. The
crashed process's stack pointers can be read from `procfs`: it is the `kstkesp` field in
`/proc/$PID/stat`. See `man 5 proc` for all the details.
The problem
-----------
While a thread is active, the stack pointer is unsafe to read and therefore the `kstkesp` field
reads zero. But when the thread is dead (e.g. during a coredump), this field should have valid
value.
However, this was broken in the past and `kstkesp` was zero even during coredump:
* commit 0a1eb2d474ed ("fs/proc: Stop reporting eip and esp in /proc/PID/stat") changed kstkesp to
always be zero
* commit fd7d56270b52 ("fs/proc: Report eip/esp in /prod/PID/stat for coredumping") fixed it for the
coredumping thread. However, other threads in a coredumping process still had the problem.
* commit cb8f381f1613 ("fs/proc/array.c: allow reporting eip/esp for all coredumping threads") fixed
for all threads in a coredumping process.
* commit 92307383082d ("coredump: Don't perform any cleanups before dumping core") broke it again
for the other threads in a coredumping process.
The problem has been fixed now, but considering the history, it may appear again in the future.
The goal of this test
---------------------
This test detects problem with reading `kstkesp` during coredump by doing the following:
#. Tell the kernel to execute the "stackdump" script when a coredump happens. This script
reads the stack pointers of all threads of crashed processes.
#. Spawn a child process who creates some threads and then crashes.
#. Read the output from the "stackdump" script, and make sure all stack pointer values are
non-zero.
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