um: Memory corruption on startup

The reverse case of this race (you must msync before read) is
well known. This is the not so common one.

It can be triggered only on systems which do a lot of task
switching and only at UML startup. If you are starting 200+ UMLs
~ 0.5% will always die without this fix.

Signed-off-by: Anton Ivanov <antivano@cisco.com>
[rw: minor whitespace fixes]
Signed-off-by: Richard Weinberger <richard@nod.at>

3 files changed, 8 insertions, 0 deletions

diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index 75298d3358e7..08eec0b691b0 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -136,6 +136,7 @@ extern int os_ioctl_generic(int fd, unsigned int cmd, unsigned long arg);
 extern int os_get_ifname(int fd, char *namebuf);
 extern int os_set_slip(int fd);
 extern int os_mode_fd(int fd, int mode);
+extern int os_fsync_file(int fd);
 
 extern int os_seek_file(int fd, unsigned long long offset);
 extern int os_open_file(const char *file, struct openflags flags, int mode);
diff --git a/arch/um/kernel/physmem.c b/arch/um/kernel/physmem.c
index f116db15d402..30fdd5d0067b 100644
--- a/arch/um/kernel/physmem.c
+++ b/arch/um/kernel/physmem.c
@@ -103,6 +103,7 @@ void __init setup_physmem(unsigned long start, unsigned long reserve_end,
          */
         os_seek_file(physmem_fd, __pa(&__syscall_stub_start));
         os_write_file(physmem_fd, &__syscall_stub_start, PAGE_SIZE);
+        os_fsync_file(physmem_fd);
 
         bootmap_size = init_bootmem(pfn, pfn + delta);
         free_bootmem(__pa(reserve_end) + bootmap_size,
diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
index 07a750197bb0..08d90fba952c 100644
--- a/arch/um/os-Linux/file.c
+++ b/arch/um/os-Linux/file.c
@@ -237,6 +237,12 @@ void os_close_file(int fd)
 {
         close(fd);
 }
+int os_fsync_file(int fd)
+{
+        if (fsync(fd) < 0)
+            return -errno;
+        return 0;
+}
 
 int os_seek_file(int fd, unsigned long long offset)
 {