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15858da535
Add a test which checks that the kstkesp field in /proc/pid/stat can be read for all threads of a coredumping process. For full details including the motivation for this test and how it works, see the README file added by this commit. Reviewed-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Nam Cao <namcao@linutronix.de> Link: https://lore.kernel.org/r/50e737b6576208566d14efcf1934fe840de6b1f4.1735805772.git.namcao@linutronix.de Signed-off-by: Christian Brauner <brauner@kernel.org>
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coredump selftest
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=================
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Background context
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------------------
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`coredump` is a feature which dumps a process's memory space when the process terminates
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unexpectedly (e.g. due to segmentation fault), which can be useful for debugging. By default,
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`coredump` dumps the memory to the file named `core`, but this behavior can be changed by writing a
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different file name to `/proc/sys/kernel/core_pattern`. Furthermore, `coredump` can be piped to a
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user-space program by writing the pipe symbol (`|`) followed by the command to be executed to
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`/proc/sys/kernel/core_pattern`. For the full description, see `man 5 core`.
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The piped user program may be interested in reading the stack pointers of the crashed process. The
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crashed process's stack pointers can be read from `procfs`: it is the `kstkesp` field in
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`/proc/$PID/stat`. See `man 5 proc` for all the details.
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The problem
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-----------
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While a thread is active, the stack pointer is unsafe to read and therefore the `kstkesp` field
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reads zero. But when the thread is dead (e.g. during a coredump), this field should have valid
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value.
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However, this was broken in the past and `kstkesp` was zero even during coredump:
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* commit 0a1eb2d474ed ("fs/proc: Stop reporting eip and esp in /proc/PID/stat") changed kstkesp to
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always be zero
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* commit fd7d56270b52 ("fs/proc: Report eip/esp in /prod/PID/stat for coredumping") fixed it for the
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coredumping thread. However, other threads in a coredumping process still had the problem.
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* commit cb8f381f1613 ("fs/proc/array.c: allow reporting eip/esp for all coredumping threads") fixed
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for all threads in a coredumping process.
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* commit 92307383082d ("coredump: Don't perform any cleanups before dumping core") broke it again
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for the other threads in a coredumping process.
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The problem has been fixed now, but considering the history, it may appear again in the future.
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The goal of this test
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---------------------
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This test detects problem with reading `kstkesp` during coredump by doing the following:
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#. Tell the kernel to execute the "stackdump" script when a coredump happens. This script
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reads the stack pointers of all threads of crashed processes.
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#. Spawn a child process who creates some threads and then crashes.
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#. Read the output from the "stackdump" script, and make sure all stack pointer values are
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non-zero.
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