On Tue, Oct 13, 2020 at 12:06:20AM +0200, Linus Walleij wrote:
> 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 fd;
>
> dir = opendir("/boot");
> fd = dirfd(dir);
> err = fcntl(fd, F_SETFD, FD_32BIT_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@xxxxxxxxxxxxx>
> Cc: Peter Maydell <peter.maydell@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Suggested-by: Theodore Ts'o <tytso@xxxxxxx>
> Link: https://bugs.launchpad.net/qemu/+bug/1805913
> Link: https://lore.kernel.org/lkml/87bm56vqg4.fsf@xxxxxxxxxxxxxxxxx/
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=205957
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---
> 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;
This seems inconsistent? F_SETFD is for setting flags on a file
descriptor. Won't setting a flag on filp here instead cause the
behaviour to change for all file descriptors across the system that are
open on this struct file? Compare set_close_on_exec().
I don't see any discussion on whether this should be an F_SETFL or an
F_SETFD, though I see F_SETFD was Ted's suggestion originally.
[...]
Cheers
---Dave
