Re: [PATCH v3 2/3] fs: openat2: Extend open_how to allow userspace-selected fds

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-04-07, Josh Triplett <josh@joshtriplett.org> wrote:
> Inspired by the X protocol's handling of XIDs, allow userspace to select
> the file descriptor opened by openat2, so that it can use the resulting
> file descriptor in subsequent system calls without waiting for the
> response to openat2.
> 
> In io_uring, this allows sequences like openat2/read/close without
> waiting for the openat2 to complete. Multiple such sequences can
> overlap, as long as each uses a distinct file descriptor.
> 
> Add a new O_SPECIFIC_FD open flag to enable this behavior, only accepted
> by openat2 for now (ignored by open/openat like all unknown flags). Add
> an fd field to struct open_how (along with appropriate padding, and
> verify that the padding is 0 to allow replacing the padding with a field
> in the future).
> 
> The file table has a corresponding new function
> get_specific_unused_fd_flags, which gets the specified file descriptor
> if O_SPECIFIC_FD is set (and the fd isn't -1); otherwise it falls back
> to get_unused_fd_flags, to simplify callers.

> 
> The specified file descriptor must not already be open; if it is,
> get_specific_unused_fd_flags will fail with -EBUSY. This helps catch
> userspace errors.
> 
> When O_SPECIFIC_FD is set, and fd is not -1, openat2 will use the
> specified file descriptor rather than finding the lowest available one.
> 
> Test program:
> 
>     #include <err.h>
>     #include <fcntl.h>
>     #include <stdio.h>
>     #include <unistd.h>
> 
>     int main(void)
>     {
>         struct open_how how = {
> 	    .flags = O_RDONLY | O_SPECIFIC_FD,
> 	    .fd = 42
> 	};
>         int fd = openat2(AT_FDCWD, "/dev/null", &how, sizeof(how));
>         if (fd < 0)
>             err(1, "openat2");
>         printf("fd=%d\n", fd); // prints fd=42
>         return 0;
>     }
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Maybe I'm misunderstanding something, but this Signed-off-by looks
strange -- was it Co-developed-by Jens?

> ---
>  fs/fcntl.c                       |  2 +-
>  fs/file.c                        | 39 ++++++++++++++++++++++++++++++++
>  fs/io_uring.c                    |  3 ++-
>  fs/open.c                        |  6 +++--
>  include/linux/fcntl.h            |  5 ++--
>  include/linux/file.h             |  3 +++
>  include/uapi/asm-generic/fcntl.h |  4 ++++
>  include/uapi/linux/openat2.h     |  2 ++
>  8 files changed, 58 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 2e4c0fa2074b..0357ad667563 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -1033,7 +1033,7 @@ static int __init fcntl_init(void)
>  	 * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
>  	 * is defined as O_NONBLOCK on some platforms and not on others.
>  	 */
> -	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
> +	BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
>  		HWEIGHT32(
>  			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
>  			__FMODE_EXEC | __FMODE_NONOTIFY));
> diff --git a/fs/file.c b/fs/file.c
> index ba06140d89af..0c34206314dc 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -567,6 +567,45 @@ void put_unused_fd(unsigned int fd)
>  
>  EXPORT_SYMBOL(put_unused_fd);
>  
> +int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags,
> +				   unsigned long nofile)
> +{
> +	int ret;
> +	struct fdtable *fdt;
> +	struct files_struct *files = current->files;
> +
> +	if (!(flags & O_SPECIFIC_FD) || fd == -1)
> +		return __get_unused_fd_flags(flags, nofile);

This check should just be (flags & O_SPECIFIC_FD) -- see my comment
below about ->fd being negative.

> +
> +	if (fd >= nofile)
> +		return -EBADF;
> +
> +	spin_lock(&files->file_lock);
> +	ret = expand_files(files, fd);
> +	if (unlikely(ret < 0))
> +		goto out_unlock;
> +	fdt = files_fdtable(files);
> +	if (fdt->fd[fd]) {
> +		ret = -EBUSY;
> +		goto out_unlock;

It would be remiss of me to not mention that this is inconsistent with
the other way of explicitly picking a file descriptor on Unix -- the dup
family closes newfd if it's already used.

But that being said, I do actually prefer this behaviour since it means
that two threads trying to open a file with the same specific file
descriptor won't see the file descriptor change underneath them (leading
to who knows how much head-scratching).

> +	}
> +	__set_open_fd(fd, fdt);
> +	if (flags & O_CLOEXEC)
> +		__set_close_on_exec(fd, fdt);
> +	else
> +		__clear_close_on_exec(fd, fdt);
> +	ret = fd;
> +
> +out_unlock:
> +	spin_unlock(&files->file_lock);
> +	return ret;
> +}
> +
> +int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags)
> +{
> +	return __get_specific_unused_fd_flags(fd, flags, rlimit(RLIMIT_NOFILE));
> +}
> +
>  /*
>   * Install a file pointer in the fd array.
>   *
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 358f97be9c7b..4a69e1daf3fe 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2997,7 +2997,8 @@ static int io_openat2(struct io_kiocb *req, bool force_nonblock)
>  	if (ret)
>  		goto err;
>  
> -	ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile);
> +	ret = __get_specific_unused_fd_flags(req->open.how.fd,
> +			req->open.how.flags, req->open.nofile);
>  	if (ret < 0)
>  		goto err;
>  
> diff --git a/fs/open.c b/fs/open.c
> index 719b320ede52..d4225e6f9d4c 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -958,7 +958,7 @@ EXPORT_SYMBOL(open_with_fake_path);
>  inline struct open_how build_open_how(int flags, umode_t mode)
>  {
>  	struct open_how how = {
> -		.flags = flags & VALID_OPEN_FLAGS,
> +		.flags = flags & VALID_OPEN_FLAGS & ~O_SPECIFIC_FD,

This is getting a little ugly, maybe filter out O_SPECIFIC_FD later on
in build_open_how() -- where we handle O_PATH.

>  		.mode = mode & S_IALLUGO,
>  	};
>  
> @@ -1143,7 +1143,7 @@ static long do_sys_openat2(int dfd, const char __user *filename,
>  	if (IS_ERR(tmp))
>  		return PTR_ERR(tmp);
>  
> -	fd = get_unused_fd_flags(how->flags);
> +	fd = get_specific_unused_fd_flags(how->fd, how->flags);
>  	if (fd >= 0) {
>  		struct file *f = do_filp_open(dfd, tmp, &op);
>  		if (IS_ERR(f)) {
> @@ -1193,6 +1193,8 @@ SYSCALL_DEFINE4(openat2, int, dfd, const char __user *, filename,
>  	err = copy_struct_from_user(&tmp, sizeof(tmp), how, usize);
>  	if (err)
>  		return err;
> +	if (tmp.pad != 0)
> +		return -EINVAL;

This check should be done in build_open_flags(), where the other sanity
checks are done. In addition, there must be an additional check like

  if (!(flags & O_SPECIFIC_FD) && how->fd != 0)
    return -EINVAL;

Since we must not allow garbage values to be passed and ignored by us in
openat2().

>  	/* O_LARGEFILE is only allowed for non-O_PATH. */
>  	if (!(tmp.flags & O_PATH) && force_o_largefile())
> diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> index 7bcdcf4f6ab2..728849bcd8fa 100644
> --- a/include/linux/fcntl.h
> +++ b/include/linux/fcntl.h
> @@ -10,7 +10,7 @@
>  	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
>  	 O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
>  	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
> -	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
> +	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_SPECIFIC_FD)
>  
>  /* List of all valid flags for the how->upgrade_mask argument: */
>  #define VALID_UPGRADE_FLAGS \
> @@ -23,7 +23,8 @@
>  
>  /* List of all open_how "versions". */
>  #define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
> -#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
> +#define OPEN_HOW_SIZE_VER1	32 /* added fd and pad */
> +#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER1
>  
>  #ifndef force_o_largefile
>  #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
> diff --git a/include/linux/file.h b/include/linux/file.h
> index b67986f818d2..a63301864a36 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -87,6 +87,9 @@ extern void set_close_on_exec(unsigned int fd, int flag);
>  extern bool get_close_on_exec(unsigned int fd);
>  extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile);
>  extern int get_unused_fd_flags(unsigned flags);
> +extern int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags,
> +	unsigned long nofile);
> +extern int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags);
>  extern void put_unused_fd(unsigned int fd);
>  extern unsigned int increase_min_fd(unsigned int num);
>  
> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 9dc0bf0c5a6e..d3de5b8b3955 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -89,6 +89,10 @@
>  #define __O_TMPFILE	020000000
>  #endif
>  
> +#ifndef O_SPECIFIC_FD
> +#define O_SPECIFIC_FD	01000000000	/* open as specified fd */
> +#endif

Maybe you've already done this (since you skipped several bits in the
O_* flag space), but I would double-check that there is no conflict on
other architectures. I faintly recall that FMODE_NOTIFY has a different
value on sparc, and there was some oddness on alpha too... But as long
as fcntl.c builds on all the arches then it's fine.

> +
>  /* a horrid kludge trying to make sure that this will fail on old kernels */
>  #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
>  #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)      
> diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
> index 58b1eb711360..50d1206b64c2 100644
> --- a/include/uapi/linux/openat2.h
> +++ b/include/uapi/linux/openat2.h
> @@ -20,6 +20,8 @@ struct open_how {
>  	__u64 flags;
>  	__u64 mode;
>  	__u64 resolve;
> +	__s32 fd;
> +	__u32 pad; /* Must be 0 in the current version */

I'm not sure I see why the new field is an s32 -- there should be no
situation where a user specifies O_SPECIFIC_FD and a negative file
descriptor (if we keep it as an s32, you should get an -EINVAL in that
case). If you don't want O_SPECIFIC_FD, don't specify it.

But I think this should be a u32. I'm tempted to argue this should
actually be a u64, but nothing supports 64-bit file descriptor numbers
(including the return type of openat2).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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