Re: [PATCH] Document that PF_KTHREAD _is_ ABI
From: Alexey Dobriyan <hidden>
Date: 2021-03-20 17:40:08
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On Sat, Mar 20, 2021 at 10:23:12AM -0700, Andy Lutomirski wrote:
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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?
Right now there is no need to fix anything because 4 bits are available. I put a comment so that PF_KTHREAD won't be moved accidently definitely breaking systemd.
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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 :^)
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+/* + * PF_KTHREAD is part of kernel ABI, visible via value #9 in /proc/$pid/stat + */ #define PF_KTHREAD 0x00200000 /* I am a kernel thread */
I think everything should be masked except PF_KTHREAD and maybe few flags for which known users exist and then don't touch them. Some flags are clearly internal like PF_MEMALLOC and PF_IDLE. Some aren't -- PF_FORKNOEXEC. However it is silly for userspace to query it because programs knows if it forked but didn't exec without external help.