Merge tag 'v4.13-rc1' into fixes

The fixes branch is based off a random pre-rc1 commit, because we had
some fixes that needed to go in before rc1 was released.

However we now need to fix some code that went in after that point, but
before rc1, so merge rc1 to get that code into fixes so we can fix it!

1 files changed, 38 insertions, 3 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 03b3c4cc7735..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;
@@ -323,11 +352,17 @@ void userspace(struct uml_pt_regs *regs)
 		 * fail.  In this case, there is nothing to do but
 		 * just kill the process.
 		 */
-		if (ptrace(PTRACE_SETREGS, pid, 0, regs->gp))
+		if (ptrace(PTRACE_SETREGS, pid, 0, regs->gp)) {
+			printk(UM_KERN_ERR "userspace - ptrace set regs "
+			       "failed, errno = %d\n", errno);
 			fatal_sigsegv();
+		}
 
-		if (put_fp_registers(pid, regs->fp))
+		if (put_fp_registers(pid, regs->fp)) {
+			printk(UM_KERN_ERR "userspace - ptrace set fp regs "
+			       "failed, errno = %d\n", errno);
 			fatal_sigsegv();
+		}
 
 		/* Now we set local_using_sysemu to be used for one loop */
 		local_using_sysemu = get_using_sysemu();