[PATCH 1/3] um: separate child and parent errors in clone stub

* [PATCH 1/3] um: separate child and parent errors in clone stub
@ 2021-01-13 21:09 Johannes Berg
  2021-01-13 21:09 ` [PATCH 2/3] um: rework userspace stubs to not hard-code stub location Johannes Berg
  2021-01-13 21:09 ` [PATCH 3/3] um: remove process stub VMA Johannes Berg
  0 siblings, 2 replies; 4+ messages in thread
From: Johannes Berg @ 2021-01-13 21:09 UTC (permalink / raw)
  To: linux-um; +Cc: linux-arch, Johannes Berg

From: Johannes Berg <johannes.berg@intel.com>

If the two are mixed up, then it looks as though the parent
returned an error if the child failed (before) the mmap(),
and then the resulting process never gets killed. Fix this
by splitting the child and parent errors, reporting and
using them appropriately.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 arch/um/include/shared/skas/stub-data.h |  2 +-
 arch/um/kernel/skas/clone.c             | 25 +++++++++++--------------
 arch/um/os-Linux/skas/process.c         | 23 +++++++++++++----------
 arch/x86/um/shared/sysdep/stub_32.h     |  2 +-
 arch/x86/um/shared/sysdep/stub_64.h     |  2 +-
 5 files changed, 27 insertions(+), 27 deletions(-)

diff --git a/arch/um/include/shared/skas/stub-data.h b/arch/um/include/shared/skas/stub-data.h
index 6b01d97a9386..5e3ade3fb38b 100644
--- a/arch/um/include/shared/skas/stub-data.h
+++ b/arch/um/include/shared/skas/stub-data.h
@@ -11,7 +11,7 @@
 struct stub_data {
 	unsigned long offset;
 	int fd;
-	long err;
+	long parent_err, child_err;
 };
 
 #endif
diff --git a/arch/um/kernel/skas/clone.c b/arch/um/kernel/skas/clone.c
index bfb70c456b30..7c592c788cbf 100644
--- a/arch/um/kernel/skas/clone.c
+++ b/arch/um/kernel/skas/clone.c
@@ -24,29 +24,26 @@
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_clone_handler(void)
 {
-	struct stub_data *data = (struct stub_data *) STUB_DATA;
+	int stack;
+	struct stub_data *data = (void *) ((unsigned long)&stack & ~(UM_KERN_PAGE_SIZE - 1));
 	long err;
 
 	err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD,
-			    STUB_DATA + UM_KERN_PAGE_SIZE / 2 - sizeof(void *));
-	if (err != 0)
-		goto out;
+			    (unsigned long)data + UM_KERN_PAGE_SIZE / 2 - sizeof(void *));
+	if (err) {
+		data->parent_err = err;
+		goto done;
+	}
 
 	err = stub_syscall4(__NR_ptrace, PTRACE_TRACEME, 0, 0, 0);
-	if (err)
-		goto out;
+	if (err) {
+		data->child_err = err;
+		goto done;
+	}
 
 	remap_stack(data->fd, data->offset);
 	goto done;
 
- out:
-	/*
-	 * save current result.
-	 * Parent: pid;
-	 * child: retcode of mmap already saved and it jumps around this
-	 * assignment
-	 */
-	data->err = err;
  done:
 	trap_myself();
 }
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index d910e25c273e..623b0aeadf4c 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -545,8 +545,14 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 	 * and child's mmap2 calls
 	 */
 	*data = ((struct stub_data) {
-			.offset	= MMAP_OFFSET(new_offset),
-			.fd     = new_fd
+		.offset	= MMAP_OFFSET(new_offset),
+		.fd     = new_fd,
+		.parent_err = -ESRCH,
+		.child_err = 0,
+	});
+
+	*child_data = ((struct stub_data) {
+		.child_err = -ESRCH,
 	});
 
 	err = ptrace_setregs(pid, thread_regs);
@@ -564,9 +570,6 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 		return err;
 	}
 
-	/* set a well known return code for detection of child write failure */
-	child_data->err = 12345678;
-
 	/*
 	 * Wait, until parent has finished its work: read child's pid from
 	 * parent's stack, and check, if bad result.
@@ -581,7 +584,7 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 
 	wait_stub_done(pid);
 
-	pid = data->err;
+	pid = data->parent_err;
 	if (pid < 0) {
 		printk(UM_KERN_ERR "copy_context_skas0 - stub-parent reports "
 		       "error %d\n", -pid);
@@ -593,10 +596,10 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 	 * child's stack and check it.
 	 */
 	wait_stub_done(pid);
-	if (child_data->err != STUB_DATA) {
-		printk(UM_KERN_ERR "copy_context_skas0 - stub-child reports "
-		       "error %ld\n", child_data->err);
-		err = child_data->err;
+	if (child_data->child_err != STUB_DATA) {
+		printk(UM_KERN_ERR "copy_context_skas0 - stub-child %d reports "
+		       "error %ld\n", pid, data->child_err);
+		err = data->child_err;
 		goto out_kill;
 	}
 
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index 51fd256c75f0..8ea69211e53c 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -86,7 +86,7 @@ static inline void remap_stack(int fd, unsigned long offset)
 			    "d" (PROT_READ | PROT_WRITE),
 			    "S" (MAP_FIXED | MAP_SHARED), "D" (fd),
 			    "a" (offset),
-			    "i" (&((struct stub_data *) STUB_DATA)->err)
+			    "i" (&((struct stub_data *) STUB_DATA)->child_err)
 			  : "memory");
 }
 
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 994df93c5ed3..b7b8b8e4359d 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -92,7 +92,7 @@ static inline void remap_stack(long fd, unsigned long offset)
 			    "d" (PROT_READ | PROT_WRITE),
                             "g" (MAP_FIXED | MAP_SHARED), "g" (fd),
 			    "g" (offset),
-			    "i" (&((struct stub_data *) STUB_DATA)->err)
+			    "i" (&((struct stub_data *) STUB_DATA)->child_err)
 			  : __syscall_clobber, "r10", "r8", "r9" );
 }
 
-- 
2.26.2


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