[PATCH] uml: fix TT mode by reverting "use fork instead of clone"

With Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

Revert the following patch, because of miscompilation problems in different
environments leading to UML not working *at all* in TT mode; it was merged
lately in 2.6 development cycle, a little after being written, and has
caused problems to lots of people; I know it's a bit too long, but it
shouldn't have been merged in first place, so I still apply for inclusion
in the -stable tree.  Anyone using this feature currently is either using
some older kernel (some reports even used 2.6.12-rc4-mm2) or using this
patch, as included in my -bs patchset.

For now there's not yet a fix for this patch, so for now the best thing is
to drop it (which was widely reported to give a working kernel, and as such
was even merged in -stable tree).

"Convert the boot-time host ptrace testing from clone to fork.  They were
essentially doing fork anyway.  This cleans up the code a bit, and makes
valgrind a bit happier about grinding it."

URL:
http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=98fdffccea6cc3fe9dba32c0fcc310bcb5d71529

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/kernel/process.c

@@ -131,7 +131,7 @@ int start_fork_tramp(void *thread_arg, unsigned long temp_stack,
 	return(arg.pid);
 }
 
-static int ptrace_child(void)
+static int ptrace_child(void *arg)
 {
 	int ret;
 	int pid = os_getpid(), ppid = getppid();
@@ -160,16 +160,20 @@ static int ptrace_child(void)
 	_exit(ret);
 }
 
-static int start_ptraced_child(void)
+static int start_ptraced_child(void **stack_out)
 {
+	void *stack;
+	unsigned long sp;
 	int pid, n, status;
 	
-	pid = fork();
-	if(pid == 0)
-		ptrace_child();
-
+	stack = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
+		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if(stack == MAP_FAILED)
+		panic("check_ptrace : mmap failed, errno = %d", errno);
+	sp = (unsigned long) stack + PAGE_SIZE - sizeof(void *);
+	pid = clone(ptrace_child, (void *) sp, SIGCHLD, NULL);
 	if(pid < 0)
-		panic("check_ptrace : fork failed, errno = %d", errno);
+		panic("check_ptrace : clone failed, errno = %d", errno);
 	CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 	if(n < 0)
 		panic("check_ptrace : wait failed, errno = %d", errno);
@@ -177,6 +181,7 @@ static int start_ptraced_child(void)
 		panic("check_ptrace : expected SIGSTOP, got status = %d",
 		      status);
 
+	*stack_out = stack;
 	return(pid);
 }
 
@@ -184,12 +189,12 @@ static int start_ptraced_child(void)
  * just avoid using sysemu, not panic, but only if SYSEMU features are broken.
  * So only for SYSEMU features we test mustpanic, while normal host features
  * must work anyway!*/
-static int stop_ptraced_child(int pid, int exitcode, int mustexit)
+static int stop_ptraced_child(int pid, void *stack, int exitcode, int mustpanic)
 {
 	int status, n, ret = 0;
 
 	if(ptrace(PTRACE_CONT, pid, 0, 0) < 0)
-		panic("stop_ptraced_child : ptrace failed, errno = %d", errno);
+		panic("check_ptrace : ptrace failed, errno = %d", errno);
 	CATCH_EINTR(n = waitpid(pid, &status, 0));
 	if(!WIFEXITED(status) || (WEXITSTATUS(status) != exitcode)) {
 		int exit_with = WEXITSTATUS(status);
@@ -200,13 +205,15 @@ static int stop_ptraced_child(int pid, int exitcode, int mustexit)
 		printk("check_ptrace : child exited with exitcode %d, while "
 		      "expecting %d; status 0x%x", exit_with,
 		      exitcode, status);
-		if (mustexit)
+		if (mustpanic)
 			panic("\n");
 		else
 			printk("\n");
 		ret = -1;
 	}
 
+	if(munmap(stack, PAGE_SIZE) < 0)
+		panic("check_ptrace : munmap failed, errno = %d", errno);
 	return ret;
 }
 
@@ -242,11 +249,12 @@ __uml_setup("nosysemu", nosysemu_cmd_param,
 
 static void __init check_sysemu(void)
 {
+	void *stack;
 	int pid, syscall, n, status, count=0;
 
 	printk("Checking syscall emulation patch for ptrace...");
 	sysemu_supported = 0;
-	pid = start_ptraced_child();
+	pid = start_ptraced_child(&stack);
 
 	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) < 0)
 		goto fail;
@@ -264,7 +272,7 @@ static void __init check_sysemu(void)
 		panic("check_sysemu : failed to modify system "
 		      "call return, errno = %d", errno);
 
-	if (stop_ptraced_child(pid, 0, 0) < 0)
+	if (stop_ptraced_child(pid, stack, 0, 0) < 0)
 		goto fail_stopped;
 
 	sysemu_supported = 1;
@@ -272,7 +280,7 @@ static void __init check_sysemu(void)
 	set_using_sysemu(!force_sysemu_disabled);
 
 	printk("Checking advanced syscall emulation patch for ptrace...");
-	pid = start_ptraced_child();
+	pid = start_ptraced_child(&stack);
 	while(1){
 		count++;
 		if(ptrace(PTRACE_SYSEMU_SINGLESTEP, pid, 0, 0) < 0)
@@ -297,7 +305,7 @@ static void __init check_sysemu(void)
 			break;
 		}
 	}
-	if (stop_ptraced_child(pid, 0, 0) < 0)
+	if (stop_ptraced_child(pid, stack, 0, 0) < 0)
 		goto fail_stopped;
 
 	sysemu_supported = 2;
@@ -308,17 +316,18 @@ static void __init check_sysemu(void)
 	return;
 
 fail:
-	stop_ptraced_child(pid, 1, 0);
+	stop_ptraced_child(pid, stack, 1, 0);
 fail_stopped:
 	printk("missing\n");
 }
 
 void __init check_ptrace(void)
 {
+	void *stack;
 	int pid, syscall, n, status;
 
 	printk("Checking that ptrace can change system call numbers...");
-	pid = start_ptraced_child();
+	pid = start_ptraced_child(&stack);
 
 	if (ptrace(PTRACE_OLDSETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD) < 0)
 		panic("check_ptrace: PTRACE_SETOPTIONS failed, errno = %d", errno);
@@ -345,7 +354,7 @@ void __init check_ptrace(void)
 			break;
 		}
 	}
-	stop_ptraced_child(pid, 0, 1);
+	stop_ptraced_child(pid, stack, 0, 1);
 	printk("OK\n");
 	check_sysemu();
 }
@@ -380,10 +389,11 @@ extern void *__syscall_stub_start, __syscall_stub_end;
 static inline void check_skas3_ptrace_support(void)
 {
 	struct ptrace_faultinfo fi;
+	void *stack;
 	int pid, n;
 
 	printf("Checking for the skas3 patch in the host...");
-	pid = start_ptraced_child();
+	pid = start_ptraced_child(&stack);
 
 	n = ptrace(PTRACE_FAULTINFO, pid, 0, &fi);
 	if (n < 0) {
@@ -402,7 +412,7 @@ static inline void check_skas3_ptrace_support(void)
 	}
 
 	init_registers(pid);
-	stop_ptraced_child(pid, 1, 1);
+	stop_ptraced_child(pid, stack, 1, 1);
 }
 
 int can_do_skas(void)