[PATCH] linux-user: Emulate CLONE_PIDFD flag in clone()

[Helge Deller]

Add emulation for the CLONE_PIDFD flag of the clone() syscall.
This flag was added in Linux kernel 5.2.

Successfully tested on a x86-64 Linux host with hppa-linux target.
Can be verified by running the testsuite of the qcoro debian package,
which breaks hard and kills the currently logged-in user without this
patch.

Signed-off-by: Helge Deller <del...@gmx.de>

diff --git a/linux-user/strace.c b/linux-user/strace.c
index 9ae5a812cd..8fa5c1ec3d 100644
--- a/linux-user/strace.c
+++ b/linux-user/strace.c
@@ -1097,6 +1097,7 @@ UNUSED static struct flags clone_flags[] = {
     FLAG_GENERIC(CLONE_FS),
     FLAG_GENERIC(CLONE_FILES),
     FLAG_GENERIC(CLONE_SIGHAND),
+    FLAG_GENERIC(CLONE_PIDFD),
     FLAG_GENERIC(CLONE_PTRACE),
     FLAG_GENERIC(CLONE_VFORK),
     FLAG_GENERIC(CLONE_PARENT),
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 24b25759be..0468a1bad7 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -213,7 +213,7 @@ struct file_clone_range {

 /* Flags for fork which we can implement within QEMU itself */
 #define CLONE_OPTIONAL_FORK_FLAGS               \
-    (CLONE_SETTLS | CLONE_PARENT_SETTID |       \
+    (CLONE_SETTLS | CLONE_PARENT_SETTID | CLONE_PIDFD | \
      CLONE_CHILD_CLEARTID | CLONE_CHILD_SETTID)

 /* Flags for thread creation which we can implement within QEMU itself */
@@ -6747,6 +6747,17 @@ static int do_fork(CPUArchState *env, unsigned int 
flags, abi_ulong newsp,
             return -TARGET_EINVAL;
         }

+#if !defined(__NR_pidfd_open) || !defined(TARGET_NR_pidfd_open)
+        if (flags & CLONE_PIDFD) {
+            return -TARGET_EINVAL;
+        }
+#endif
+
+        /* Can not allow CLONE_PIDFD with CLONE_PARENT_SETTID */
+        if ((flags & CLONE_PIDFD) && (flags & CLONE_PARENT_SETTID)) {
+            return -TARGET_EINVAL;
+        }
+
         if (block_signals()) {
             return -QEMU_ERESTARTSYS;
         }
@@ -6774,6 +6785,20 @@ static int do_fork(CPUArchState *env, unsigned int 
flags, abi_ulong newsp,
                 ts->child_tidptr = child_tidptr;
         } else {
             cpu_clone_regs_parent(env, flags);
+            if (flags & CLONE_PIDFD) {
+                int pid_fd = 0;
+#if defined(__NR_pidfd_open) && defined(TARGET_NR_pidfd_open)
+                int pid_child = ret;
+                pid_fd = pidfd_open(pid_child, 0);
+                if (pid_fd >= 0) {
+                        fcntl(pid_fd, F_SETFD, fcntl(pid_fd, F_GETFL)
+                                               | FD_CLOEXEC);
+                } else {
+                        pid_fd = 0;
+                }
+#endif
+                put_user_u32(pid_fd, parent_tidptr);
+                }
             fork_end(0);
         }
     }