254 lines
10 KiB
ReStructuredText
254 lines
10 KiB
ReStructuredText
======================
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ioctl based interfaces
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======================
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ioctl() is the most common way for applications to interface
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with device drivers. It is flexible and easily extended by adding new
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commands and can be passed through character devices, block devices as
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well as sockets and other special file descriptors.
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However, it is also very easy to get ioctl command definitions wrong,
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and hard to fix them later without breaking existing applications,
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so this documentation tries to help developers get it right.
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Command number definitions
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==========================
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The command number, or request number, is the second argument passed to
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the ioctl system call. While this can be any 32-bit number that uniquely
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identifies an action for a particular driver, there are a number of
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conventions around defining them.
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``include/uapi/asm-generic/ioctl.h`` provides four macros for defining
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ioctl commands that follow modern conventions: ``_IO``, ``_IOR``,
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``_IOW``, and ``_IOWR``. These should be used for all new commands,
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with the correct parameters:
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_IO/_IOR/_IOW/_IOWR
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The macro name specifies how the argument will be used. It may be a
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pointer to data to be passed into the kernel (_IOW), out of the kernel
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(_IOR), or both (_IOWR). _IO can indicate either commands with no
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argument or those passing an integer value instead of a pointer.
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It is recommended to only use _IO for commands without arguments,
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and use pointers for passing data.
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type
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An 8-bit number, often a character literal, specific to a subsystem
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or driver, and listed in Documentation/userspace-api/ioctl/ioctl-number.rst
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nr
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An 8-bit number identifying the specific command, unique for a give
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value of 'type'
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data_type
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The name of the data type pointed to by the argument, the command number
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encodes the ``sizeof(data_type)`` value in a 13-bit or 14-bit integer,
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leading to a limit of 8191 bytes for the maximum size of the argument.
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Note: do not pass sizeof(data_type) type into _IOR/_IOW/IOWR, as that
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will lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t).
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_IO does not have a data_type parameter.
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Interface versions
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==================
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Some subsystems use version numbers in data structures to overload
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commands with different interpretations of the argument.
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This is generally a bad idea, since changes to existing commands tend
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to break existing applications.
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A better approach is to add a new ioctl command with a new number. The
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old command still needs to be implemented in the kernel for compatibility,
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but this can be a wrapper around the new implementation.
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Return code
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===========
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ioctl commands can return negative error codes as documented in errno(3);
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these get turned into errno values in user space. On success, the return
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code should be zero. It is also possible but not recommended to return
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a positive 'long' value.
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When the ioctl callback is called with an unknown command number, the
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handler returns either -ENOTTY or -ENOIOCTLCMD, which also results in
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-ENOTTY being returned from the system call. Some subsystems return
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-ENOSYS or -EINVAL here for historic reasons, but this is wrong.
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Prior to Linux 5.5, compat_ioctl handlers were required to return
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-ENOIOCTLCMD in order to use the fallback conversion into native
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commands. As all subsystems are now responsible for handling compat
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mode themselves, this is no longer needed, but it may be important to
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consider when backporting bug fixes to older kernels.
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Timestamps
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==========
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Traditionally, timestamps and timeout values are passed as ``struct
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timespec`` or ``struct timeval``, but these are problematic because of
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incompatible definitions of these structures in user space after the
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move to 64-bit time_t.
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The ``struct __kernel_timespec`` type can be used instead to be embedded
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in other data structures when separate second/nanosecond values are
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desired, or passed to user space directly. This is still not ideal though,
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as the structure matches neither the kernel's timespec64 nor the user
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space timespec exactly. The get_timespec64() and put_timespec64() helper
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functions can be used to ensure that the layout remains compatible with
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user space and the padding is treated correctly.
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As it is cheap to convert seconds to nanoseconds, but the opposite
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requires an expensive 64-bit division, a simple __u64 nanosecond value
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can be simpler and more efficient.
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Timeout values and timestamps should ideally use CLOCK_MONOTONIC time,
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as returned by ktime_get_ns() or ktime_get_ts64(). Unlike
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CLOCK_REALTIME, this makes the timestamps immune from jumping backwards
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or forwards due to leap second adjustments and clock_settime() calls.
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ktime_get_real_ns() can be used for CLOCK_REALTIME timestamps that
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need to be persistent across a reboot or between multiple machines.
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32-bit compat mode
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==================
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In order to support 32-bit user space running on a 64-bit machine, each
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subsystem or driver that implements an ioctl callback handler must also
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implement the corresponding compat_ioctl handler.
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As long as all the rules for data structures are followed, this is as
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easy as setting the .compat_ioctl pointer to a helper function such as
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compat_ptr_ioctl() or blkdev_compat_ptr_ioctl().
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compat_ptr()
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------------
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On the s390 architecture, 31-bit user space has ambiguous representations
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for data pointers, with the upper bit being ignored. When running such
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a process in compat mode, the compat_ptr() helper must be used to
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clear the upper bit of a compat_uptr_t and turn it into a valid 64-bit
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pointer. On other architectures, this macro only performs a cast to a
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``void __user *`` pointer.
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In an compat_ioctl() callback, the last argument is an unsigned long,
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which can be interpreted as either a pointer or a scalar depending on
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the command. If it is a scalar, then compat_ptr() must not be used, to
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ensure that the 64-bit kernel behaves the same way as a 32-bit kernel
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for arguments with the upper bit set.
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The compat_ptr_ioctl() helper can be used in place of a custom
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compat_ioctl file operation for drivers that only take arguments that
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are pointers to compatible data structures.
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Structure layout
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----------------
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Compatible data structures have the same layout on all architectures,
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avoiding all problematic members:
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* ``long`` and ``unsigned long`` are the size of a register, so
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they can be either 32-bit or 64-bit wide and cannot be used in portable
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data structures. Fixed-length replacements are ``__s32``, ``__u32``,
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``__s64`` and ``__u64``.
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* Pointers have the same problem, in addition to requiring the
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use of compat_ptr(). The best workaround is to use ``__u64``
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in place of pointers, which requires a cast to ``uintptr_t`` in user
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space, and the use of u64_to_user_ptr() in the kernel to convert
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it back into a user pointer.
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* On the x86-32 (i386) architecture, the alignment of 64-bit variables
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is only 32-bit, but they are naturally aligned on most other
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architectures including x86-64. This means a structure like::
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struct foo {
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__u32 a;
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__u64 b;
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__u32 c;
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};
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has four bytes of padding between a and b on x86-64, plus another four
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bytes of padding at the end, but no padding on i386, and it needs a
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compat_ioctl conversion handler to translate between the two formats.
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To avoid this problem, all structures should have their members
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naturally aligned, or explicit reserved fields added in place of the
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implicit padding. The ``pahole`` tool can be used for checking the
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alignment.
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* On ARM OABI user space, structures are padded to multiples of 32-bit,
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making some structs incompatible with modern EABI kernels if they
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do not end on a 32-bit boundary.
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* On the m68k architecture, struct members are not guaranteed to have an
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alignment greater than 16-bit, which is a problem when relying on
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implicit padding.
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* Bitfields and enums generally work as one would expect them to,
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but some properties of them are implementation-defined, so it is better
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to avoid them completely in ioctl interfaces.
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* ``char`` members can be either signed or unsigned, depending on
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the architecture, so the __u8 and __s8 types should be used for 8-bit
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integer values, though char arrays are clearer for fixed-length strings.
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Information leaks
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=================
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Uninitialized data must not be copied back to user space, as this can
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cause an information leak, which can be used to defeat kernel address
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space layout randomization (KASLR), helping in an attack.
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For this reason (and for compat support) it is best to avoid any
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implicit padding in data structures. Where there is implicit padding
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in an existing structure, kernel drivers must be careful to fully
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initialize an instance of the structure before copying it to user
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space. This is usually done by calling memset() before assigning to
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individual members.
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Subsystem abstractions
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======================
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While some device drivers implement their own ioctl function, most
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subsystems implement the same command for multiple drivers. Ideally the
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subsystem has an .ioctl() handler that copies the arguments from and
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to user space, passing them into subsystem specific callback functions
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through normal kernel pointers.
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This helps in various ways:
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* Applications written for one driver are more likely to work for
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another one in the same subsystem if there are no subtle differences
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in the user space ABI.
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* The complexity of user space access and data structure layout is done
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in one place, reducing the potential for implementation bugs.
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* It is more likely to be reviewed by experienced developers
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that can spot problems in the interface when the ioctl is shared
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between multiple drivers than when it is only used in a single driver.
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Alternatives to ioctl
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=====================
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There are many cases in which ioctl is not the best solution for a
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problem. Alternatives include:
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* System calls are a better choice for a system-wide feature that
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is not tied to a physical device or constrained by the file system
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permissions of a character device node
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* netlink is the preferred way of configuring any network related
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objects through sockets.
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* debugfs is used for ad-hoc interfaces for debugging functionality
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that does not need to be exposed as a stable interface to applications.
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* sysfs is a good way to expose the state of an in-kernel object
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that is not tied to a file descriptor.
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* configfs can be used for more complex configuration than sysfs
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* A custom file system can provide extra flexibility with a simple
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user interface but adds a lot of complexity to the implementation.
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