1 files changed, 30 insertions, 1 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 31c9be2c2bdd..819d68656673 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -108,7 +108,7 @@ static void get_skas_faultinfo(int pid, struct faultinfo *fi)
         wait_stub_done(pid);
 
         /*
-         * faultinfo is prepared by the stub-segv-handler at start of
+         * faultinfo is prepared by the stub_segv_handler at start of
          * the stub stack page. We just have to copy it.
          */
         memcpy(fi, (void *)current_stub_stack(), sizeof(*fi));
@@ -175,6 +175,21 @@ static void handle_trap(int pid, struct uml_pt_regs *regs,
 
 extern char __syscall_stub_start[];
 
+/**
+ * userspace_tramp() - userspace trampoline
+ * @stack:      pointer to the new userspace stack page, can be NULL, if? FIXME:
+ *
+ * The userspace trampoline is used to setup a new userspace process in start_userspace() after it was clone()'ed.
+ * This function will run on a temporary stack page.
+ * It ptrace()'es itself, then
+ * Two pages are mapped into the userspace address space:
+ * - STUB_CODE (with EXEC), which contains the skas stub code
+ * - STUB_DATA (with R/W), which contains a data page that is used to transfer certain data between the UML userspace process and the UML kernel.
+ * Also for the userspace process a SIGSEGV handler is installed to catch pagefaults in the userspace process.
+ * And last the process stops itself to give control to the UML kernel for this userspace process.
+ *
+ * Return: Always zero, otherwise the current userspace process is ended with non null exit() call
+ */
 static int userspace_tramp(void *stack)
 {
         void *addr;
@@ -236,12 +251,24 @@ static int userspace_tramp(void *stack)
 
 int userspace_pid[NR_CPUS];
 
+/**
+ * start_userspace() - prepare a new userspace process
+ * @stub_stack: pointer to the stub stack. Can be NULL, if? FIXME:
+ *
+ * Setups a new temporary stack page that is used while userspace_tramp() runs
+ * Clones the kernel process into a new userspace process, with FDs only.
+ *
+ * Return: When positive: the process id of the new userspace process,
+ *         when negative: an error number.
+ * FIXME: can PIDs become negative?!
+ */
 int start_userspace(unsigned long stub_stack)
 {
         void *stack;
         unsigned long sp;
         int pid, status, n, flags, err;
 
+        /* setup a temporary stack page */
         stack = mmap(NULL, UM_KERN_PAGE_SIZE,
                      PROT_READ | PROT_WRITE | PROT_EXEC,
                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
@@ -252,10 +279,12 @@ int start_userspace(unsigned long stub_stack)
                 return err;
         }
 
+        /* set stack pointer to the end of the stack page, so it can grow downwards */
         sp = (unsigned long) stack + UM_KERN_PAGE_SIZE - sizeof(void *);
 
         flags = CLONE_FILES | SIGCHLD;
 
+        /* clone into new userspace process */
         pid = clone(userspace_tramp, (void *) sp, flags, (void *) stub_stack);
         if (pid < 0) {
                 err = -errno;