On 2020-04-08, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> On 2020-04-07, Josh Triplett <josh@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> 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.
Ah, I missed that the pipe2 patch allows you to choose which fd is being
specifically chosen by setting it to -1. So this check is fine but my
comment for openat2 still applies.
> > +
> > + 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/>
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
Attachment:
signature.asc
Description: PGP signature
