[PATCH] fuse: Add ioctl flag for x32 compat ioctl

[Ian Abbott]

Currently, a CUSE server running on a 64-bit kernel can tell when an
ioctl request comes from a process running a 32-bit ABI, but cannot tell
whether the requesting process is using legacy IA32 emulation or x32
ABI.  In particular, the server does not know the size of the client
process's `time_t` type.

For 64-bit kernels, the `FUSE_IOCTL_COMPAT` and `FUSE_IOCTL_32BIT` flags
are currently set in the ioctl input request (`struct fuse_ioctl_in`
member `flags`) for a 32-bit requesting process.  This patch defines a
new flag `FUSE_IOCTL_COMPAT_X32` and sets it if the 32-bit requesting
process is using the x32 ABI.  This allows the server process to
distinguish between requests from requesting client processes using IA32
emulation or the x32 ABI and so infer the size of the client process's
`time_t` type and any other IA32/x32 differences.

Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
---
This is based on an earlier patch titled "fuse: Add ioctl flag for
compat ioctl with 64-bit time_t" that focussed specifically on `time_t`
compatibility.  At the insistence of Miklos, I have changed it to focus
on x32 compatibility. -- Ian Abbott
---
 fs/fuse/file.c            | 7 ++++++-
 include/uapi/linux/fuse.h | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 06096b60f1df..44d52c54dd75 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2576,8 +2576,13 @@ long fuse_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg,
 #if BITS_PER_LONG == 32
 	inarg.flags |= FUSE_IOCTL_32BIT;
 #else
-	if (flags & FUSE_IOCTL_COMPAT)
+	if (flags & FUSE_IOCTL_COMPAT) {
 		inarg.flags |= FUSE_IOCTL_32BIT;
+#ifdef CONFIG_X86_X32
+		if (in_x32_syscall())
+			inarg.flags |= FUSE_IOCTL_COMPAT_X32;
+#endif
+	}
 #endif
 
 	/* assume all the iovs returned by client always fits in a page */
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 2ac598614a8f..c8ba8230639e 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -335,6 +335,7 @@ struct fuse_file_lock {
  * FUSE_IOCTL_RETRY: retry with new iovecs
  * FUSE_IOCTL_32BIT: 32bit ioctl
  * FUSE_IOCTL_DIR: is a directory
+ * FUSE_IOCTL_COMPAT_X32: x32 compat ioctl on 64bit machine (64bit time_t)
  *
  * FUSE_IOCTL_MAX_IOV: maximum of in_iovecs + out_iovecs
  */
@@ -343,6 +344,7 @@ struct fuse_file_lock {
 #define FUSE_IOCTL_RETRY	(1 << 2)
 #define FUSE_IOCTL_32BIT	(1 << 3)
 #define FUSE_IOCTL_DIR		(1 << 4)
+#define FUSE_IOCTL_COMPAT_X32	(1 << 5)
 
 #define FUSE_IOCTL_MAX_IOV	256
 
-- 
2.20.1