Re: [PATCH] Document that PF_KTHREAD _is_ ABI
From: Andy Lutomirski <luto@kernel.org>
Date: 2021-03-20 17:24:19
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On Mar 20, 2021, at 9:31 AM, Alexey Dobriyan [off-list ref] wrote:
PF_KTHREAD value is visible via field number 9 of /proc/*/stat
$ sudo cat /proc/2/stat
2 (kthreadd) S 0 0 0 0 -1 2129984 0 ...
^^^^^^^
It is used by at least systemd to check for kernel-threadness:
https://github.com/systemd/systemd/blob/main/src/basic/process-util.c#L354
src/basic/process-util.c:is_kernel_thread()Eww. Could we fix it differently and more permanently by modifying the proc code to display the values systemd expects?
quoted hunk ↗ jump to hunk
It means that the value can't be changed despite perceived notion that task_struct flags are internal to kernel and can be shuffled at whim. Formally, _all_ struct task_struct::flags PF_* values are kernel ABI which is a disaster. I hope we can mask everything but few flags and hope for the best :^) Note for beginner Linux programmers: every other way you find on the interwebs and Stack Overflow for checking if pid belongs to a kernel thread is broken one way or another. Signed-off-by: Alexey Dobriyan <redacted> --- include/linux/sched.h | 3 +++ 1 file changed, 3 insertions(+)--- a/include/linux/sched.h +++ b/include/linux/sched.h@@ -1566,6 +1566,9 @@ extern struct pid *cad_pid;#define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */ #define PF_LOCAL_THROTTLE 0x00100000 /* Throttle writes only against the bdi I write to, * I am cleaning dirty pages from some other bdi. */ +/* + * PF_KTHREAD is part of kernel ABI, visible via value #9 in /proc/$pid/stat + */ #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ #define PF_RANDOMIZE 0x00400000 /* Randomize virtual address space */ #define PF_SWAPWRITE 0x00800000 /* Allowed to write to swap */