[PATCH v4] fcntl: Add 32bit filesystem mode

[Linus Walleij]

It was brought to my attention that this bug from 2018 was
still unresolved: 32 bit emulators like QEMU were given
64 bit hashes when running 32 bit emulation on 64 bit systems.

This adds a flag to the fcntl() F_GETFD and F_SETFD operations
to set the underlying filesystem into 32bit mode even if the
file handle was opened using 64bit mode without the compat
syscalls.

Programs that need the 32 bit file system behavior need to
issue a fcntl() system call such as in this example:

  #define FD_32BIT_MODE 2

  int main(int argc, char** argv) {
    DIR* dir;
    int err;
    int mode;
    int fd;

    dir = opendir("/boot");
    fd = dirfd(dir);
    mode = fcntl(fd, F_GETFD);
    mode |= FD_32BIT_MODE;
    err = fcntl(fd, F_SETFD, mode);
    if (err) {
      printf("fcntl() failed! err=%d\n", err);
      return 1;
    }
    printf("dir=%p\n", dir);
    printf("readdir(dir)=%p\n", readdir(dir));
    printf("errno=%d: %s\n", errno, strerror(errno));
    return 0;
  }

This can be pretty hard to test since C libraries and linux
userspace security extensions aggressively filter the parameters
that are passed down and allowed to commit into actual system
calls.

Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Eric Blake <eblake@redhat.com>
Reported-by: 罗勇刚(Yonggang Luo) <luoyonggang@gmail.com>
Suggested-by: Theodore Ts'o <tytso@mit.edu>
Link: https://bugs.launchpad.net/qemu/+bug/1805913
Link: https://lore.kernel.org/lkml/87bm56vqg4.fsf@mid.deneb.enyo.de/
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=205957
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v3 RESEND 1-> v4:
- Update the example in the commit message to a read/modify/write
  version.
- Notice that Yonggang Luo sees the sema problem on i386 binaries
  as we see on ARM 32bit binaries.
ChangeLog v3->v3 RESEND 1:
- Resending during the v5.10 merge window to get attention.
ChangeLog v2->v3:
- Realized that I also have to clear the flag correspondingly
  if someone ask for !FD_32BIT_MODE after setting it the
  first time.
ChangeLog v1->v2:
- Use a new flag FD_32BIT_MODE to F_GETFD and F_SETFD
  instead of a new fcntl operation, there is already a fcntl
  operation to set random flags.
- Sorry for taking forever to respin this patch :(
---
 fs/fcntl.c                       | 7 +++++++
 include/uapi/asm-generic/fcntl.h | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 19ac5baad50f..6c32edc4099a 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -335,10 +335,17 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned 
long arg,
                break;
        case F_GETFD:
                err = get_close_on_exec(fd) ? FD_CLOEXEC : 0;
+               /* Report 32bit file system mode */
+               if (filp->f_mode & FMODE_32BITHASH)
+                       err |= FD_32BIT_MODE;
                break;
        case F_SETFD:
                err = 0;
                set_close_on_exec(fd, arg & FD_CLOEXEC);
+               if (arg & FD_32BIT_MODE)
+                       filp->f_mode |= FMODE_32BITHASH;
+               else
+                       filp->f_mode &= ~FMODE_32BITHASH;
                break;
        case F_GETFL:
                err = filp->f_flags;
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 9dc0bf0c5a6e..edd3573cb7ef 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -160,6 +160,14 @@ struct f_owner_ex {
 
 /* for F_[GET|SET]FL */
 #define FD_CLOEXEC     1       /* actually anything with low bit set goes */
+/*
+ * This instructs the kernel to provide 32bit semantics (such as hashes) from
+ * the file system layer, when running a userland that depend on 32bit
+ * semantics on a kernel that supports 64bit userland, but does not use the
+ * compat ioctl() for e.g. open(), so that the kernel would otherwise assume
+ * that the userland process is capable of dealing with 64bit semantics.
+ */
+#define FD_32BIT_MODE  2
 
 /* for posix fcntl() and lockf() */
 #ifndef F_RDLCK
-- 
2.26.2