Messages in this thread | | | Date | Tue, 17 Mar 2020 09:41:54 +0100 | From | Christian Brauner <> | Subject | Re: clone3: allow creation of time namespace with offset |
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On Tue, Mar 17, 2020 at 09:30:40AM +0100, Adrian Reber wrote: > This is an attempt to add time namespace support to clone3(). I am not > really sure which way clone3() should handle time namespaces. The time > namespace through /proc cannot be used with clone3() because the offsets > for the time namespace need to be written before a process has been > created in that time namespace. This means it is necessary to somehow > tell clone3() the offsets for the clocks. > > The time namespace offers the possibility to set offsets for > CLOCK_MONOTONIC and CLOCK_BOOTTIME. My first approach was to extend > 'struct clone_args` with '__aligned_u64 monotonic_offset' and > '__aligned_u64 boottime_offset'. The problem with this approach was that > it was not possible to set nanoseconds for the clocks in the time > namespace. > > One of the motivations for clone3() with CLONE_NEWTIME was to enable > CRIU to restore a process in a time namespace with the corresponding > offsets. And although the nanosecond value can probably never be > restored to the same value it had during checkpointing, because the > clock keeps on running between CRIU pausing all processes and CRIU > actually reading the value of the clocks, the nanosecond value is still > necessary for CRIU to not restore a process where the clock jumps back > due to CRIU restoring it with a nanonsecond value that is too small. > > Requiring nanoseconds as well as seconds for two clocks during clone3() > means that it would require 4 additional members to 'struct clone_args': > > __aligned_u64 tls; > __aligned_u64 set_tid; > __aligned_u64 set_tid_size; > + __aligned_u64 boottime_offset_seconds; > + __aligned_u64 boottime_offset_nanoseconds; > + __aligned_u64 monotonic_offset_seconds; > + __aligned_u64 monotonic_offset_nanoseconds; > }; > > To avoid four additional members to 'struct clone_args' this patchset > uses another approach: > > __aligned_u64 tls; > __aligned_u64 set_tid; > __aligned_u64 set_tid_size; > + __aligned_u64 timens_offset; > + __aligned_u64 timens_offset_size;
Hm, so for set_tid we did set_tid and set_tid_size which makes sense because set_tid wasn't actually a struct. But I'm not a fan of establishing a pattern whereby we always have to grow two member, the object and it's size; at least when we're adding a struct. So at a first glance here are two possible ideas: - Don't add a size argument and assume that struct timens_offset won't grow. I'm not sure how likely it is it will grow. - Make the size the first member of struct timens_offset the size of the struct. (See examples for this pattern in the sched syscalls.)
Christian
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