syscall: add CgroupFD support for ForkExec on Linux

Implement CLONE_INTO_CGROUP feature, allowing to put a child in a
specified cgroup in a clean and simple way. Note that the feature only
works for cgroup v2, and requires Linux kernel 5.7 or newer.

Using the feature requires a new syscall, clone3. Currently this is the
only reason to use clone3, but the code is structured in a way so that
other cases may be easily added in the future.

Add a test case.

While at it, try to simplify the syscall calling code in
forkAndExecInChild1, which became complicated over time because:

1. It was using either rawVforkSyscall or RawSyscall6 depending on
   whether CLONE_NEWUSER was set.

2. On Linux/s390, the first two arguments to clone(2) system call are
   swapped (which deserved a mention in Linux ABI hall of shame). It
   was worked around in rawVforkSyscall on s390, but had to be
   implemented via a switch/case when using RawSyscall6, making the code
   less clear.

Let's

 - modify rawVforkSyscall to have two arguments (which is also required
   for clone3);

 - remove the arguments workaround from s390 asm, instead implementing
   arguments swap in the caller (which still looks ugly but at least
   it's done once and is clearly documented now);

 - use rawVforkSyscall for all cases (since it is essentially similar to
   RawSyscall6, except for having less parameters, not returning r2, and
   saving/restoring the return address before/after syscall on 386 and
   amd64).

Updates #51246.

Change-Id: Ifcd418ebead9257177338ffbcccd0bdecb94474e
Reviewed-on: https://go-review.googlesource.com/c/go/+/417695
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/src/syscall/asm_linux_mipsx.s b/src/syscall/asm_linux_mipsx.s
index b8cae96b1a..3e5e8b1139 100644
--- a/src/syscall/asm_linux_mipsx.s
+++ b/src/syscall/asm_linux_mipsx.s
@@ -44,21 +44,21 @@ ok9:
 	JAL	runtime·exitsyscall(SB)
 	RET
 
-// func rawVforkSyscall(trap, a1 uintptr) (r1, err uintptr)
-TEXT ·rawVforkSyscall(SB),NOSPLIT|NOFRAME,$0-16
+// func rawVforkSyscall(trap, a1, a2 uintptr) (r1, err uintptr)
+TEXT ·rawVforkSyscall(SB),NOSPLIT|NOFRAME,$0-20
 	MOVW	a1+4(FP), R4
-	MOVW	R0, R5
+	MOVW	a2+8(FP), R5
 	MOVW	R0, R6
 	MOVW	trap+0(FP), R2	// syscall entry
 	SYSCALL
 	BEQ	R7, ok
 	MOVW	$-1, R1
-	MOVW	R1, r1+8(FP)	// r1
-	MOVW	R2, err+12(FP)	// errno
+	MOVW	R1, r1+12(FP)	// r1
+	MOVW	R2, err+16(FP)	// errno
 	RET
 ok:
-	MOVW	R2, r1+8(FP)	// r1
-	MOVW	R0, err+12(FP)	// errno
+	MOVW	R2, r1+12(FP)	// r1
+	MOVW	R0, err+16(FP)	// errno
 	RET
 
 TEXT ·rawSyscallNoError(SB),NOSPLIT,$20-24