Documentation/process/botching-up-ioctls.rst was orignally written as a blog post for DRM driver writers, so it it misses some points while going into a lot of detail on others.
Try to provide a replacement that addresses typical issues across a wider range of subsystems, and follows the style of the core-api documentation better.
Signed-off-by: Arnd Bergmann arnd@arndb.de --- Documentation/core-api/index.rst | 1 + Documentation/core-api/ioctl.rst | 250 +++++++++++++++++++++++++++++++ 2 files changed, 251 insertions(+) create mode 100644 Documentation/core-api/ioctl.rst
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index ab0eae1c153a..3f28b2f668be 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -39,6 +39,7 @@ Core utilities ../RCU/index gcc-plugins symbol-namespaces + ioctl
Interfaces for kernel debugging diff --git a/Documentation/core-api/ioctl.rst b/Documentation/core-api/ioctl.rst new file mode 100644 index 000000000000..cb2c86ae63e7 --- /dev/null +++ b/Documentation/core-api/ioctl.rst @@ -0,0 +1,250 @@ +====================== +ioctl based interfaces +====================== + +:c:func:`ioctl` is the most common way for applications to interface +with device drivers. It is flexible and easily extended by adding new +commands and can be passed through character devices, block devices as +well as sockets and other special file descriptors. + +However, it is also very easy to get ioctl command definitions wrong, +and hard to fix them later without breaking existing applications, +so this documentation tries to help developers get it right. + +Command number definitions +========================== + +The command number, or request number, is the second argument passed to +the ioctl system call. While this can be any 32-bit number that uniquely +identifies an action for a particular driver, there are a number of +conventions around defining them. + +``include/uapi/asm-generic/ioctl.h`` provides four macros for defining +ioctl commands that follow modern conventions: ``_IOC``, ``_IOR``, +``_IOW``, and ``_IORW``. These should be used for all new commands, +with the correct parameters: + +_IO/_IOR/_IOW/_IOWR + The macro name determines whether the argument is used for passing + data into kernel (_IOW), from the kernel (_IOR), both (_IOWR) or is + not a pointer (_IOC). It is possible but not recommended to pass an + integer value instead of a pointer with _IOC. + +type + An 8-bit number, often a character literal, specific to a subsystem + or driver, and listed in :doc:`../ioctl/ioctl-number` + +nr + An 8-bit number identifying the specific command, unique for a give + value of 'type' + +size + The name of the data type pointed to by the argument, the command + number encodes the ``sizeof(size)`` value in a 13-bit or 14-bit integer, + leading to a limit of 8191 bytes for the maximum size of the argument. + Note: do not pass sizeof(type) type into _IOR/IOW, as that will lead + to encoding sizeof(sizeof(type)), i.e. sizeof(size_t). + + +Interface versions +================== + +Some subsystems use version numbers in data structures to overload +commands with different interpretations of the argument. + +This is generally a bad idea, since changes to existing commands tend +to break existing applications. + +A better approach is to add a new ioctl command with a new number. The +old command still needs to be implemented in the kernel for compatibility, +but this can be a wrapper around the new implementation. + +Return code +=========== + +ioctl commands can return negative error codes as documented in errno(3), +these get turned into errno values in user space. On success, the return +code should be zero. It is also possible but not recommended to return +a positive 'long' value. + +When the ioctl callback is called with an unknown command number, the +handler returns either -ENOTTY or -ENOIOCTLCMD, which also results in +-ENOTTY being returned from the system call. Some subsystems return +-ENOSYS or -EINVAL here for historic reasons, but this is wrong. + +Prior to Linux-5.5, compat_ioctl handlers were required to return +-ENOIOCTLCMD in order to use the fallback conversion into native +commands. As all subsystems are now responsible for handling compat +mode themselves, this is no longer needed, but it may be important to +consider when backporting bug fixes to older kernels. + +Timestamps +========== + +Traditionally, timestamps and timeout values are passed as ``struct +timespec`` or ``struct timeval``, but these are problematic because of +incompatible definitions of these structures in user space after the +move to 64-bit time_t. + +The __kernel_timespec type can be used instead to be embedded in other +data structures when separate second/nanosecond values are desired, +or passed to user space directly. This is still not ideal though, +as the structure matches neither the kernel's timespec64 nor the user +space timespec exactly. The get_timespec64() and put_timespec64() helper +functions canbe used to ensure that the layout remains compatible with +user space and the padding is treated correctly. + +As it is cheap to convert seconds to nanoseconds, but the opposite +requires an expensive 64-bit division, a simple __u64 nanosecond value +can be simpler and more efficient. + +Timeout values and timestamps should ideally use CLOCK_MONOTONIC time, +as returned by ``ktime_get_ns()`` or ``ktime_get_ts64()``. Unlike +CLOCK_REALTIME, this makes the timestamps immune from jumping backwards +or forwards due to leap second adjustments and clock_settime() calls. + +``ktime_get_real_ns()`` can be used for CLOCK_REALTIME timestamps that +may be required for timestamps that need to be persistent across a reboot +or between multiple machines. + +32-bit compat mode +================== + +In order to support 32-bit user space running on a 64-bit machine, each +subsystem or driver that implements an ioctl callback handler must also +implement the corresponding compat_ioctl handler. + +As long as all the rules for data structures are followed, this is as +easy as setting the .compat_ioctl pointer to a helper function such as +``compat_ptr_ioctl()`` or ``blkdev_compat_ptr_ioctl``. + +compat_ptr() +------------ + +On the s/390 architecture, 31-bit user space has ambiguous representations +for data pointers, with the upper bit being ignored. When running such +a process in compat mode, the ``compat_ptr()`` helper must be used to +clear the upper bit of a compat_uptr_t and turn it into a valid 64-bit +pointer. On other architectures, this macro only performs a cast to a +``void __user *`` pointer. + +In an compat_ioctl() callback, the last argument is an unsigned long, +which can be interpreted as either a pointer or a scalar depending on +the command. If it is a scalar, then compat_ptr() must not be used, to +ensure that the 64-bit kernel behaves the same way as a 32-bit kernel +for arguments with the upper bit set. + +The compat_ptr_ioctl() helper can be used in place of a custom +compat_ioctl file operation for drivers that only take arguments that +are pointers to compatible data structures. + +Structure layout +---------------- + +Compatible data structures have the same layout on all architectures, +avoiding all problematic members: + +* ``long`` and ``unsigned long`` are the size of a register, so + they can be either 32 bit or 64 bit wide and cannot be used in portable + data structures. Fixed-length replacements are ``__s32``, ``__u32``, + ``__s64`` and ``__u64``. + +* Pointers have the same problem, in addition to requiring the + use of ``compat_ptr()``. The best workaround is to use ``__u64`` + in place of pointers, which requires a cast to ``uintptr_t`` in user + space, and the use of ``u64_to_user_ptr()`` in the kernel to convert + it back into a user pointer. + +* On the x86-32 (i386) architecture, the alignment of 64-bit variables + is only 32 bit, but they are naturally aligned on most other + architectures including x86-64. This means a structure like + + :: + + struct foo { + __u32 a; + __u64 b; + __u32 c; + }; + + has four bytes of padding between a and b on x86-64, plus another four + bytes of padding at the end, but no padding on i386, and it needs a + compat_ioctl conversion handler to translate between the two formats. + + To avoid this problem, all structures should have their members + naturally aligned, or explicit reserved fields added in place of the + implicit padding. + +* On ARM OABI user space, 16-bit member variables have 32-bit + alignment, making them incompatible with modern EABI kernels. + Conversely, on the m68k architecture, all struct members have at most + 16-bit alignment. These rarely cause problems as neither ARM-OABI nor + m68k are supported by any compat mode, but for consistency, it is best + to completely avoid 16-bit member variables. + + +* Bitfields and enums generally work as one would expect them to, + but some properties of them are implementation-defined, so it is better + to avoid them completely in ioctl interfaces. + +* ``char`` members can be either signed or unsigned, depending on + the architecture, so the __u8 and __s8 types should be used for 8-bit + integer values, though char arrays are clearer for fixed-length strings. + +Information leaks +================= + +Uninitialized data must not be copied back to user space, as this can +cause an information leak, which can be used to defeat kernel address +space layout randomization (KASLR), helping in an attack. + +As explained for the compat mode, it is best to not avoid any padding in +data structures, but if there is already padding in existing structures, +the kernel driver must be careful to zero out the padding using +``memset()`` or similar before copying it to user space. + +Subsystem abstractions +====================== + +While some device drivers implement their own ioctl function, most +subsystems implement the same command for multiple drivers. Ideally the +subsystem has an .ioctl() handler that copies the arguments from and +to user space, passing them into subsystem specific callback functions +through normal kernel pointers. + +This helps in various ways: + +* Applications written for one driver are more likely to work for + another one in the same subsystem if there are no subtle differences + in the user space ABI. + +* The complexity of user space access and data structure layout at done + in one place, reducing the potential for implementation bugs. + +* It is more likely to be reviewed by experienced developers + that can spot problems in the interface when the ioctl is shared + between multiple drivers than when it is only used in a single driver. + +Alternatives to ioctl +===================== + +There are many cases in which ioctl is not the best solution for a +problem. Alternatives include + +* System calls are a better choice for a system-wide feature that + is not tied to a physical device or constrained by the file system + permissions of a character device node + +* netlink is the preferred way of configuring any network related + objects through sockets. + +* debugfs is used for ad-hoc interfaces for debugging functionality + that does not need to be exposed as a stable interface to applications. + +* sysfs is a good way to expose the state of an in-kernel object + that is not tied to a file descriptor. + +* configfs can be used for more complex configuration than sysfs + +* A custom file system can provide extra flexibility with a simple + user interface but add a lot of complexity in the implementation.