On Sat, Mar 20, 2021 at 10:23:12AM -0700, Andy Lutomirski wrote: > > On Mar 20, 2021, at 9:31 AM, Alexey Dobriyan <adobriyan@xxxxxxxxx> 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. > > 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 :^) > > +/* > > + * 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.
