libct: unify IOPriority setting

For some reason, io priority is set in different places between runc
start/run and runc exec:

 - for runc start/run, it is done in the middle of (*linuxStandardInit).Init,
   close to the place where we exec runc init.
 - for runc exec, it is done much earlier, in (*setnsProcess) start().

Let's move setIOPriority call for runc exec to (*linuxSetnsInit).Init,
so it is in the same logical place as for runc start/run.

Also, move the function itself to init_linux.go as it's part of init.

Should not have any visible effect, except part of runc init is run with
a different I/O priority.

While at it, rename setIOPriority to setupIOPriority, and make it accept
the whole *configs.Config, for uniformity with other similar functions.

Fixes: bfbd0305 ("Add I/O priority")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/init_linux.go

@@ -678,6 +678,28 @@ func setupScheduler(config *configs.Config) error {
 	return nil
 }
 
+func setupIOPriority(config *configs.Config) error {
+	const ioprioWhoPgrp = 1
+
+	ioprio := config.IOPriority
+	if ioprio == nil {
+		return nil
+	}
+	class, ok := configs.IOPrioClassMapping[ioprio.Class]
+	if !ok {
+		return fmt.Errorf("invalid io priority class: %s", ioprio.Class)
+	}
+
+	// Combine class and priority into a single value
+	// https://github.com/torvalds/linux/blob/v5.18/include/uapi/linux/ioprio.h#L5-L17
+	iop := (class << 13) | ioprio.Priority
+	_, _, errno := unix.RawSyscall(unix.SYS_IOPRIO_SET, ioprioWhoPgrp, 0, uintptr(iop))
+	if errno != 0 {
+		return fmt.Errorf("failed to set io priority: %w", errno)
+	}
+	return nil
+}
+
 func setupPersonality(config *configs.Config) error {
 	return system.SetLinuxPersonality(config.Personality.Domain)
 }

libcontainer/process_linux.go

@@ -166,10 +166,6 @@ type setnsProcess struct {
 func (p *setnsProcess) start() (retErr error) {
 	defer p.comm.closeParent()
 
-	if err := setIOPriority(p.process.IOPriority); err != nil {
-		return err
-	}
-
 	// get the "before" value of oom kill count
 	oom, _ := p.manager.OOMKillCount()
 	err := p.cmd.Start()
@@ -906,24 +902,3 @@ func (p *Process) InitializeIO(rootuid, rootgid int) (i *IO, err error) {
 	}
 	return i, nil
 }
-
-func setIOPriority(ioprio *configs.IOPriority) error {
-	const ioprioWhoPgrp = 1
-
-	if ioprio == nil {
-		return nil
-	}
-	class, ok := configs.IOPrioClassMapping[ioprio.Class]
-	if !ok {
-		return fmt.Errorf("invalid io priority class: %s", ioprio.Class)
-	}
-
-	// Combine class and priority into a single value
-	// https://github.com/torvalds/linux/blob/v5.18/include/uapi/linux/ioprio.h#L5-L17
-	iop := (class << 13) | ioprio.Priority
-	_, _, errno := unix.RawSyscall(unix.SYS_IOPRIO_SET, ioprioWhoPgrp, 0, uintptr(iop))
-	if errno != 0 {
-		return fmt.Errorf("failed to set io priority: %w", errno)
-	}
-	return nil
-}

libcontainer/setns_init_linux.go

@@ -75,6 +75,9 @@ func (l *linuxSetnsInit) Init() error {
 		return err
 	}
 
+	if err := setupIOPriority(l.config.Config); err != nil {
+		return err
+	}
 	// Tell our parent that we're ready to exec. This must be done before the
 	// Seccomp rules have been applied, because we need to be able to read and
 	// write to a socket.

libcontainer/standard_init_linux.go

@@ -159,7 +159,7 @@ func (l *linuxStandardInit) Init() error {
 		return err
 	}
 
-	if err := setIOPriority(l.config.Config.IOPriority); err != nil {
+	if err := setupIOPriority(l.config.Config); err != nil {
 		return err
 	}