[PATCH v3 0/3] Support userspace-selected fds

[PATCH v3 0/3] Support userspace-selected fds

[Josh Triplett]

(Note: numbering this updated version v3, to avoid confusion with Jens'
v2 that built on my v1. Jens, if you like this approach, please feel
free to stack your additional patches from the io_uring-fd-select branch
atop this series. 5.8 material, not intended for the current merge window.)

This new version has an API to atomically increase the minimum fd and
return the previous minimum, rather than just getting and setting the
minimum; this makes it easier to allocate a range. (A library that might
initialize after the program has already opened other file descriptors may need
to check for existing open fds in the range after reserving it, and
reserve more fds if needed; this can be done entirely in userspace, and
we can't really do anything simpler in the kernel due to limitations on
file-descriptor semantics, so this patch series avoids introducing any
extra complexity in the kernel.)

This new version also supports a __get_specific_unused_fd_flags call
which accepts the limit for RLIMIT_NOFILE as an argument, analogous to
__get_unused_fd_flags, since io_uring needs that to correctly handle
RLIMIT_NOFILE.


Inspired by the X protocol's handling of XIDs, allow userspace to select
the file descriptor opened by a call like openat2, so that it can use
the resulting file descriptor in subsequent system calls without waiting
for the response the initial openat2 syscall.

The first patch is independent of the other two; it allows reserving
file descriptors below a certain minimum for userspace-selected fd
allocation only.

The second patch implements userspace-selected fd allocation for
openat2, introducing a new O_SPECIFIC_FD flag and an fd field in struct
open_how. 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.

The third patch adds userspace-selected fd allocation to pipe2 as well.
I did this partly as a demonstration of how simple it is to wire up
O_SPECIFIC_FD support for a new fd-allocating system call, and partly in
the hopes that this may make it more useful to wire up io_uring support
for pipe2 in the future.

Josh Triplett (3):
  fs: Support setting a minimum fd for "lowest available fd" allocation
  fs: openat2: Extend open_how to allow userspace-selected fds
  fs: pipe2: Support O_SPECIFIC_FD

 fs/fcntl.c                       |  2 +-
 fs/file.c                        | 62 ++++++++++++++++++++++++++++----
 fs/io_uring.c                    |  3 +-
 fs/open.c                        |  6 ++--
 fs/pipe.c                        | 16 ++++++---
 include/linux/fcntl.h            |  5 +--
 include/linux/fdtable.h          |  1 +
 include/linux/file.h             |  4 +++
 include/uapi/asm-generic/fcntl.h |  4 +++
 include/uapi/linux/openat2.h     |  2 ++
 include/uapi/linux/prctl.h       |  3 ++
 kernel/sys.c                     |  5 +++
 12 files changed, 97 insertions(+), 16 deletions(-)

-- 
2.26.0

[PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation

[Josh Triplett]

Some applications want to prevent the usual "lowest available fd"
allocation from allocating certain file descriptors. For instance, they
may want to prevent allocation of a closed fd 0, 1, or 2 other than via
dup2/dup3, or reserve some low file descriptors for other purposes.

Add a prctl to increase the minimum fd and return the previous minimum.

System calls that allocate a specific file descriptor, such as
dup2/dup3, ignore this minimum.

exec resets the minimum fd, to prevent one program from interfering with
another program's expectations about fd allocation.

Test program:

    #include <err.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <sys/prctl.h>

    int main(int argc, char *argv[])
    {
        if (prctl(PR_INCREASE_MIN_FD, 100, 0, 0, 0) < 0)
            err(1, "prctl");
        int fd = open("/dev/null", O_RDONLY);
        if (fd < 0)
            err(1, "open");
        printf("%d\n", fd); // prints 100
        return 0;
    }

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 fs/file.c                  | 23 +++++++++++++++++------
 include/linux/fdtable.h    |  1 +
 include/linux/file.h       |  1 +
 include/uapi/linux/prctl.h |  3 +++
 kernel/sys.c               |  5 +++++
 5 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index c8a4e4c86e55..ba06140d89af 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 	spin_lock_init(&newf->file_lock);
 	newf->resize_in_progress = false;
 	init_waitqueue_head(&newf->resize_wait);
-	newf->next_fd = 0;
 	new_fdt = &newf->fdtab;
 	new_fdt->max_fds = NR_OPEN_DEFAULT;
 	new_fdt->close_on_exec = newf->close_on_exec_init;
@@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 	new_fdt->fd = &newf->fd_array[0];
 
 	spin_lock(&oldf->file_lock);
+	newf->next_fd = newf->min_fd = oldf->min_fd;
 	old_fdt = files_fdtable(oldf);
 	open_files = count_open_files(old_fdt);
 
@@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files,
 	spin_lock(&files->file_lock);
 repeat:
 	fdt = files_fdtable(files);
-	fd = start;
-	if (fd < files->next_fd)
-		fd = files->next_fd;
+	fd = max3(start, files->min_fd, files->next_fd);
 
 	if (fd < fdt->max_fds)
 		fd = find_next_fd(fdt, fd);
@@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files,
 		goto repeat;
 
 	if (start <= files->next_fd)
-		files->next_fd = fd + 1;
+		files->next_fd = max(fd + 1, files->min_fd);
 
 	__set_open_fd(fd, fdt);
 	if (flags & O_CLOEXEC)
@@ -555,7 +553,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
 {
 	struct fdtable *fdt = files_fdtable(files);
 	__clear_open_fd(fd, fdt);
-	if (fd < files->next_fd)
+	if (fd < files->next_fd && fd >= files->min_fd)
 		files->next_fd = fd;
 }
 
@@ -684,6 +682,7 @@ void do_close_on_exec(struct files_struct *files)
 
 	/* exec unshares first */
 	spin_lock(&files->file_lock);
+	files->min_fd = 0;
 	for (i = 0; ; i++) {
 		unsigned long set;
 		unsigned fd = i * BITS_PER_LONG;
@@ -865,6 +864,18 @@ bool get_close_on_exec(unsigned int fd)
 	return res;
 }
 
+unsigned int increase_min_fd(unsigned int num)
+{
+	struct files_struct *files = current->files;
+	unsigned int old_min_fd;
+
+	spin_lock(&files->file_lock);
+	old_min_fd = files->min_fd;
+	files->min_fd += num;
+	spin_unlock(&files->file_lock);
+	return old_min_fd;
+}
+
 static int do_dup2(struct files_struct *files,
 	struct file *file, unsigned fd, unsigned flags)
 __releases(&files->file_lock)
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..d1980443d8b3 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -60,6 +60,7 @@ struct files_struct {
    */
 	spinlock_t file_lock ____cacheline_aligned_in_smp;
 	unsigned int next_fd;
+	unsigned int min_fd; /* min for "lowest available fd" allocation */
 	unsigned long close_on_exec_init[1];
 	unsigned long open_fds_init[1];
 	unsigned long full_fds_bits_init[1];
diff --git a/include/linux/file.h b/include/linux/file.h
index 142d102f285e..b67986f818d2 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -88,6 +88,7 @@ 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 void put_unused_fd(unsigned int fd);
+extern unsigned int increase_min_fd(unsigned int num);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..916327272d21 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,7 @@ struct prctl_mm_map {
 #define PR_SET_IO_FLUSHER		57
 #define PR_GET_IO_FLUSHER		58
 
+/* Increase minimum file descriptor for "lowest available fd" allocation */
+#define PR_INCREASE_MIN_FD		59
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..daa0ce43cecc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2514,6 +2514,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 
 		error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
 		break;
+	case PR_INCREASE_MIN_FD:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = increase_min_fd((unsigned int)arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.26.0

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

[Josh Triplett]

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>
---
 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);
+
+	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;
+	}
+	__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,
 		.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;
 
 	/* 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
+
 /* 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 */
 };
 
 /* how->resolve flags for openat2(2). */
-- 
2.26.0

[PATCH v3 3/3] fs: pipe2: Support O_SPECIFIC_FD

[Josh Triplett]

This allows the caller of pipe2 to specify one or both file descriptors
rather than having them automatically use the lowest available file
descriptor. The caller can specify either file descriptor as -1 to
allow that file descriptor to use the lowest available.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 fs/pipe.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 16fb72e9abf7..4681a0d1d587 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -936,19 +936,19 @@ static int __do_pipe_flags(int *fd, struct file **files, int flags)
 	int error;
 	int fdw, fdr;
 
-	if (flags & ~(O_CLOEXEC | O_NONBLOCK | O_DIRECT))
+	if (flags & ~(O_CLOEXEC | O_NONBLOCK | O_DIRECT | O_SPECIFIC_FD))
 		return -EINVAL;
 
 	error = create_pipe_files(files, flags);
 	if (error)
 		return error;
 
-	error = get_unused_fd_flags(flags);
+	error = get_specific_unused_fd_flags(fd[0], flags);
 	if (error < 0)
 		goto err_read_pipe;
 	fdr = error;
 
-	error = get_unused_fd_flags(flags);
+	error = get_specific_unused_fd_flags(fd[1], flags);
 	if (error < 0)
 		goto err_fdr;
 	fdw = error;
@@ -969,7 +969,11 @@ static int __do_pipe_flags(int *fd, struct file **files, int flags)
 int do_pipe_flags(int *fd, int flags)
 {
 	struct file *files[2];
-	int error = __do_pipe_flags(fd, files, flags);
+	int error;
+
+	if (flags & O_SPECIFIC_FD)
+		return -EINVAL;
+	error = __do_pipe_flags(fd, files, flags);
 	if (!error) {
 		fd_install(fd[0], files[0]);
 		fd_install(fd[1], files[1]);
@@ -987,6 +991,10 @@ static int do_pipe2(int __user *fildes, int flags)
 	int fd[2];
 	int error;
 
+	if (flags & O_SPECIFIC_FD)
+		if (copy_from_user(fd, fildes, sizeof(fd)))
+			return -EFAULT;
+
 	error = __do_pipe_flags(fd, files, flags);
 	if (!error) {
 		if (unlikely(copy_to_user(fildes, fd, sizeof(fd)))) {
-- 
2.26.0

Re: [PATCH v3 0/3] Support userspace-selected fds

[Jens Axboe]

On 4/3/20 10:57 PM, Josh Triplett wrote:
> (Note: numbering this updated version v3, to avoid confusion with Jens'
> v2 that built on my v1. Jens, if you like this approach, please feel
> free to stack your additional patches from the io_uring-fd-select branch
> atop this series. 5.8 material, not intended for the current merge window.)
> 
> This new version has an API to atomically increase the minimum fd and
> return the previous minimum, rather than just getting and setting the
> minimum; this makes it easier to allocate a range. (A library that might
> initialize after the program has already opened other file descriptors may need
> to check for existing open fds in the range after reserving it, and
> reserve more fds if needed; this can be done entirely in userspace, and
> we can't really do anything simpler in the kernel due to limitations on
> file-descriptor semantics, so this patch series avoids introducing any
> extra complexity in the kernel.)
> 
> This new version also supports a __get_specific_unused_fd_flags call
> which accepts the limit for RLIMIT_NOFILE as an argument, analogous to
> __get_unused_fd_flags, since io_uring needs that to correctly handle
> RLIMIT_NOFILE.
> 
> 
> Inspired by the X protocol's handling of XIDs, allow userspace to select
> the file descriptor opened by a call like openat2, so that it can use
> the resulting file descriptor in subsequent system calls without waiting
> for the response the initial openat2 syscall.
> 
> The first patch is independent of the other two; it allows reserving
> file descriptors below a certain minimum for userspace-selected fd
> allocation only.
> 
> The second patch implements userspace-selected fd allocation for
> openat2, introducing a new O_SPECIFIC_FD flag and an fd field in struct
> open_how. 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.
> 
> The third patch adds userspace-selected fd allocation to pipe2 as well.
> I did this partly as a demonstration of how simple it is to wire up
> O_SPECIFIC_FD support for a new fd-allocating system call, and partly in
> the hopes that this may make it more useful to wire up io_uring support
> for pipe2 in the future.

This looks pretty clean and flexible to me, and it'll work fine for
io_uring.

Care to post this a bit wider (and CC Al)?

-- 
Jens Axboe

Re: [PATCH v3 0/3] Support userspace-selected fds

[Josh Triplett]

On Tue, Apr 07, 2020 at 03:11:50PM -0700, Jens Axboe wrote:
> This looks pretty clean and flexible to me, and it'll work fine for
> io_uring.
> 
> Care to post this a bit wider (and CC Al)?

Will do.

Re: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation

[Josh Triplett]

On Wed, Apr 08, 2020 at 10:00:40PM +1000, Aleksa Sarai wrote:
> On 2020-04-07, Josh Triplett <josh@joshtriplett.org> wrote:
> > Some applications want to prevent the usual "lowest available fd"
> > allocation from allocating certain file descriptors. For instance, they
> > may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> > dup2/dup3, or reserve some low file descriptors for other purposes.
> > 
> > Add a prctl to increase the minimum fd and return the previous minimum.
> > 
> > System calls that allocate a specific file descriptor, such as
> > dup2/dup3, ignore this minimum.
> > 
> > exec resets the minimum fd, to prevent one program from interfering with
> > another program's expectations about fd allocation.
> 
> Why is it implemented as an "increase the value" interface? It feels
> like this is meant to avoid some kind of security trap (with a library
> reducing the value) but it means that if you want to temporarily raise
> the minimum fd number it's not possible (without re-exec()ing yourself,
> which is hardly a fun thing to do).
> 
> Then again, this might've been discussed before and I missed it...

It was: the previous version was a "get" and "set" interface. That
interface didn't allow for the possibility that something else in the
process had already set a minimum. This new atomic increase interface
(which also serves as a "get" interface if you pass 0) makes it possible
for a userspace library to reserve a range. (You have no guarantee about
previously allocated descriptors in that range, but you know that no
*new* automatically allocated descriptors will appear in that range,
which suffices; userspace can do the rest.)

- Josh Triplett

Re: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 6506 bytes --]

On 2020-04-07, Josh Triplett <josh@joshtriplett.org> wrote:
> Some applications want to prevent the usual "lowest available fd"
> allocation from allocating certain file descriptors. For instance, they
> may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> dup2/dup3, or reserve some low file descriptors for other purposes.
> 
> Add a prctl to increase the minimum fd and return the previous minimum.
> 
> System calls that allocate a specific file descriptor, such as
> dup2/dup3, ignore this minimum.
> 
> exec resets the minimum fd, to prevent one program from interfering with
> another program's expectations about fd allocation.

Why is it implemented as an "increase the value" interface? It feels
like this is meant to avoid some kind of security trap (with a library
reducing the value) but it means that if you want to temporarily raise
the minimum fd number it's not possible (without re-exec()ing yourself,
which is hardly a fun thing to do).

Then again, this might've been discussed before and I missed it...

> Test program:
> 
>     #include <err.h>
>     #include <fcntl.h>
>     #include <stdio.h>
>     #include <sys/prctl.h>
> 
>     int main(int argc, char *argv[])
>     {
>         if (prctl(PR_INCREASE_MIN_FD, 100, 0, 0, 0) < 0)
>             err(1, "prctl");
>         int fd = open("/dev/null", O_RDONLY);
>         if (fd < 0)
>             err(1, "open");
>         printf("%d\n", fd); // prints 100
>         return 0;
>     }
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> ---
>  fs/file.c                  | 23 +++++++++++++++++------
>  include/linux/fdtable.h    |  1 +
>  include/linux/file.h       |  1 +
>  include/uapi/linux/prctl.h |  3 +++
>  kernel/sys.c               |  5 +++++
>  5 files changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index c8a4e4c86e55..ba06140d89af 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
>  	spin_lock_init(&newf->file_lock);
>  	newf->resize_in_progress = false;
>  	init_waitqueue_head(&newf->resize_wait);
> -	newf->next_fd = 0;
>  	new_fdt = &newf->fdtab;
>  	new_fdt->max_fds = NR_OPEN_DEFAULT;
>  	new_fdt->close_on_exec = newf->close_on_exec_init;
> @@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
>  	new_fdt->fd = &newf->fd_array[0];
>  
>  	spin_lock(&oldf->file_lock);
> +	newf->next_fd = newf->min_fd = oldf->min_fd;
>  	old_fdt = files_fdtable(oldf);
>  	open_files = count_open_files(old_fdt);
>  
> @@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files,
>  	spin_lock(&files->file_lock);
>  repeat:
>  	fdt = files_fdtable(files);
> -	fd = start;
> -	if (fd < files->next_fd)
> -		fd = files->next_fd;
> +	fd = max3(start, files->min_fd, files->next_fd);
>  
>  	if (fd < fdt->max_fds)
>  		fd = find_next_fd(fdt, fd);
> @@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files,
>  		goto repeat;
>  
>  	if (start <= files->next_fd)
> -		files->next_fd = fd + 1;
> +		files->next_fd = max(fd + 1, files->min_fd);
>  
>  	__set_open_fd(fd, fdt);
>  	if (flags & O_CLOEXEC)
> @@ -555,7 +553,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
>  {
>  	struct fdtable *fdt = files_fdtable(files);
>  	__clear_open_fd(fd, fdt);
> -	if (fd < files->next_fd)
> +	if (fd < files->next_fd && fd >= files->min_fd)
>  		files->next_fd = fd;
>  }
>  
> @@ -684,6 +682,7 @@ void do_close_on_exec(struct files_struct *files)
>  
>  	/* exec unshares first */
>  	spin_lock(&files->file_lock);
> +	files->min_fd = 0;
>  	for (i = 0; ; i++) {
>  		unsigned long set;
>  		unsigned fd = i * BITS_PER_LONG;
> @@ -865,6 +864,18 @@ bool get_close_on_exec(unsigned int fd)
>  	return res;
>  }
>  
> +unsigned int increase_min_fd(unsigned int num)
> +{
> +	struct files_struct *files = current->files;
> +	unsigned int old_min_fd;
> +
> +	spin_lock(&files->file_lock);
> +	old_min_fd = files->min_fd;
> +	files->min_fd += num;
> +	spin_unlock(&files->file_lock);
> +	return old_min_fd;
> +}
> +
>  static int do_dup2(struct files_struct *files,
>  	struct file *file, unsigned fd, unsigned flags)
>  __releases(&files->file_lock)
> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index f07c55ea0c22..d1980443d8b3 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -60,6 +60,7 @@ struct files_struct {
>     */
>  	spinlock_t file_lock ____cacheline_aligned_in_smp;
>  	unsigned int next_fd;
> +	unsigned int min_fd; /* min for "lowest available fd" allocation */
>  	unsigned long close_on_exec_init[1];
>  	unsigned long open_fds_init[1];
>  	unsigned long full_fds_bits_init[1];
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 142d102f285e..b67986f818d2 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -88,6 +88,7 @@ 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 void put_unused_fd(unsigned int fd);
> +extern unsigned int increase_min_fd(unsigned int num);
>  
>  extern void fd_install(unsigned int fd, struct file *file);
>  
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 07b4f8131e36..916327272d21 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -238,4 +238,7 @@ struct prctl_mm_map {
>  #define PR_SET_IO_FLUSHER		57
>  #define PR_GET_IO_FLUSHER		58
>  
> +/* Increase minimum file descriptor for "lowest available fd" allocation */
> +#define PR_INCREASE_MIN_FD		59
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d325f3ab624a..daa0ce43cecc 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2514,6 +2514,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  
>  		error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
>  		break;
> +	case PR_INCREASE_MIN_FD:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = increase_min_fd((unsigned int)arg2);
> +		break;
>  	default:
>  		error = -EINVAL;
>  		break;
> -- 
> 2.26.0
> 


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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

[PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation

[Josh Triplett]

Some applications want to prevent the usual "lowest available fd"
allocation from allocating certain file descriptors. For instance, they
may want to prevent allocation of a closed fd 0, 1, or 2 other than via
dup2/dup3, or reserve some low file descriptors for other purposes.

Add a prctl to increase the minimum fd and return the previous minimum.

System calls that allocate a specific file descriptor, such as
dup2/dup3, ignore this minimum.

exec resets the minimum fd, to prevent one program from interfering with
another program's expectations about fd allocation.

Test program:

    #include <err.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <sys/prctl.h>

    int main(int argc, char *argv[])
    {
        if (prctl(PR_INCREASE_MIN_FD, 100, 0, 0, 0) < 0)
            err(1, "prctl");
        int fd = open("/dev/null", O_RDONLY);
        if (fd < 0)
            err(1, "open");
        printf("%d\n", fd); // prints 100
        return 0;
    }

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---
 fs/file.c                  | 23 +++++++++++++++++------
 include/linux/fdtable.h    |  1 +
 include/linux/file.h       |  1 +
 include/uapi/linux/prctl.h |  3 +++
 kernel/sys.c               |  5 +++++
 5 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index c8a4e4c86e55..ba06140d89af 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 	spin_lock_init(&newf->file_lock);
 	newf->resize_in_progress = false;
 	init_waitqueue_head(&newf->resize_wait);
-	newf->next_fd = 0;
 	new_fdt = &newf->fdtab;
 	new_fdt->max_fds = NR_OPEN_DEFAULT;
 	new_fdt->close_on_exec = newf->close_on_exec_init;
@@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 	new_fdt->fd = &newf->fd_array[0];
 
 	spin_lock(&oldf->file_lock);
+	newf->next_fd = newf->min_fd = oldf->min_fd;
 	old_fdt = files_fdtable(oldf);
 	open_files = count_open_files(old_fdt);
 
@@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files,
 	spin_lock(&files->file_lock);
 repeat:
 	fdt = files_fdtable(files);
-	fd = start;
-	if (fd < files->next_fd)
-		fd = files->next_fd;
+	fd = max3(start, files->min_fd, files->next_fd);
 
 	if (fd < fdt->max_fds)
 		fd = find_next_fd(fdt, fd);
@@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files,
 		goto repeat;
 
 	if (start <= files->next_fd)
-		files->next_fd = fd + 1;
+		files->next_fd = max(fd + 1, files->min_fd);
 
 	__set_open_fd(fd, fdt);
 	if (flags & O_CLOEXEC)
@@ -555,7 +553,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
 {
 	struct fdtable *fdt = files_fdtable(files);
 	__clear_open_fd(fd, fdt);
-	if (fd < files->next_fd)
+	if (fd < files->next_fd && fd >= files->min_fd)
 		files->next_fd = fd;
 }
 
@@ -684,6 +682,7 @@ void do_close_on_exec(struct files_struct *files)
 
 	/* exec unshares first */
 	spin_lock(&files->file_lock);
+	files->min_fd = 0;
 	for (i = 0; ; i++) {
 		unsigned long set;
 		unsigned fd = i * BITS_PER_LONG;
@@ -865,6 +864,18 @@ bool get_close_on_exec(unsigned int fd)
 	return res;
 }
 
+unsigned int increase_min_fd(unsigned int num)
+{
+	struct files_struct *files = current->files;
+	unsigned int old_min_fd;
+
+	spin_lock(&files->file_lock);
+	old_min_fd = files->min_fd;
+	files->min_fd += num;
+	spin_unlock(&files->file_lock);
+	return old_min_fd;
+}
+
 static int do_dup2(struct files_struct *files,
 	struct file *file, unsigned fd, unsigned flags)
 __releases(&files->file_lock)
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..d1980443d8b3 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -60,6 +60,7 @@ struct files_struct {
    */
 	spinlock_t file_lock ____cacheline_aligned_in_smp;
 	unsigned int next_fd;
+	unsigned int min_fd; /* min for "lowest available fd" allocation */
 	unsigned long close_on_exec_init[1];
 	unsigned long open_fds_init[1];
 	unsigned long full_fds_bits_init[1];
diff --git a/include/linux/file.h b/include/linux/file.h
index 142d102f285e..b67986f818d2 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -88,6 +88,7 @@ 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 void put_unused_fd(unsigned int fd);
+extern unsigned int increase_min_fd(unsigned int num);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..916327272d21 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,7 @@ struct prctl_mm_map {
 #define PR_SET_IO_FLUSHER		57
 #define PR_GET_IO_FLUSHER		58
 
+/* Increase minimum file descriptor for "lowest available fd" allocation */
+#define PR_INCREASE_MIN_FD		59
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..daa0ce43cecc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2514,6 +2514,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 
 		error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
 		break;
+	case PR_INCREASE_MIN_FD:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = increase_min_fd((unsigned int)arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.26.0