| Message ID | 20191217221708.3730997-28-arnd@arndb.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2,01/27] pktcdvd: fix regression on 64-bit architectures | expand |
On Tue, 2019-12-17 at 23:17 +0100, Arnd Bergmann wrote: [...] > --- /dev/null > +++ b/Documentation/core-api/ioctl.rst > @@ -0,0 +1,248 @@ > +====================== > +ioctl based interfaces > +====================== > + > +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: ``_IO``, ``_IOR``, > +``_IOW``, and ``_IORW``. These should be used for all new commands, Typo: "_IORW" should be "_IOWR". > +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 (_IO). It is possible but not recommended to pass an > + integer value instead of a pointer with _IO. I feel the explanation of _IO here could be confusing. I think what you meant to say was that it is possible, but not recommended, to pass integers directly (arg is integer) rather than indirectly (arg is pointer to integer). I suggest the alternate wording: The macro name specifies how the argument will be used. It may be a pointer to data to be passed into the kernel (_IOW), out of the kernel (_IOR), or both (_IOWR). The argument may also be an integer value instead of a pointer (_IO), but this is not recommended. > +type > + An 8-bit number, often a character literal, specific to a subsystem > + or driver, and listed in :doc:`../userspace-api/ioctl/ioctl-number` > + > +nr > + An 8-bit number identifying the specific command, unique for a give > + value of 'type' > + > +data_type > + The name of the data type pointed to by the argument, the command number > + encodes the ``sizeof(data_type)`` 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(data_type) type into _IOR/IOW, as that will > + lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t). You left out _IOWR here. It might also be worth mentioning that _IO doesn't have this parameter. [...] > +Return code > +=========== > + > +ioctl commands can return negative error codes as documented in errno(3), > +these get turned into errno values in user space. Use a semi-colon instead of a comma, or change "these" to "which". > 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 Space instead of hyphen. > +-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 It's not a typedef, so ``struct __kernel_timespec``. [...] > +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 IBM never used the name "S/390" for the 64-bit mainframe architecture, but they have rebranded it several times. Rather than trying to follow what it's called this year, maybe just write "s390" to match what we usually call it? [...] > +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. This should explain how to check that - presumably by running pahole on some sensible architecture. > +* On ARM OABI user space, 16-bit member variables have 32-bit > + alignment, making them incompatible with modern EABI kernels. I thought that OABI required structures as a whole to have alignment of 4, not individual members? Which obviously does affect small structures as members of other structures. [...] > +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 implicit Delete "not". > +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. This sentence is rather too long. Also it can be read as suggesting that one should somehow identify and memset() the padding just before copying to user-space. I suggest an alternate wording: For this reason (and for compat support) it is best to avoid any implicit padding in data structures. Where there is implicit padding in an existing structure, kernel drivers must be careful to fully initialize an instance of the structure before copying it to user space. This is usually done by calling memset() before assigning to individual members. [...] > +Alternatives to ioctl > +===================== [...] > +* A custom file system can provide extra flexibility with a simple > + user interface but add a lot of complexity to the implementation. Typo: "add" should be "adds". Anyway, it's great to have documentation for this all in one place. Ben.
On Wed, Dec 18, 2019 at 11:45 PM Ben Hutchings <ben.hutchings@codethink.co.uk> wrote: > On Tue, 2019-12-17 at 23:17 +0100, Arnd Bergmann wrote: > > --- /dev/null > > +++ b/Documentation/core-api/ioctl.rst > > +``include/uapi/asm-generic/ioctl.h`` provides four macros for defining > > +ioctl commands that follow modern conventions: ``_IO``, ``_IOR``, > > +``_IOW``, and ``_IORW``. These should be used for all new commands, > > Typo: "_IORW" should be "_IOWR". Fixed now > > +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 (_IO). It is possible but not recommended to pass an > > + integer value instead of a pointer with _IO. > > I feel the explanation of _IO here could be confusing. I think what > you meant to say was that it is possible, but not recommended, to pass > integers directly (arg is integer) rather than indirectly (arg is > pointer to integer). I suggest the alternate wording: > > The macro name specifies how the argument will be used. It may be a > pointer to data to be passed into the kernel (_IOW), out of the kernel > (_IOR), or both (_IOWR). The argument may also be an integer value > instead of a pointer (_IO), but this is not recommended. That's probably better than my version, but I find that misleading as well: it sounds like _IO() is not recommended, but having no argument with _IO() is actually fine. This is what I have now: The macro name specifies how the argument will be used. It may be a pointer to data to be passed into the kernel (_IOW), out of the kernel (_IOR), or both (_IOWR). _IO can indicate either commands with no argument or those passing an integer value instead of a pointer. It is recommended to only use _IO for commands without arguments, and use pointers for passing data. > > +data_type > > + The name of the data type pointed to by the argument, the command number > > + encodes the ``sizeof(data_type)`` 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(data_type) type into _IOR/IOW, as that will > > + lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t). > > You left out _IOWR here. It might also be worth mentioning that _IO > doesn't have this parameter. Changed now. > [...] > > +Return code > > +=========== > > + > > +ioctl commands can return negative error codes as documented in errno(3), > > +these get turned into errno values in user space. > > Use a semi-colon instead of a comma, or change "these" to "which". done > > 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 > > Space instead of hyphen. done > > +-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 > > It's not a typedef, so ``struct __kernel_timespec``. done > [...] > > +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 > > IBM never used the name "S/390" for the 64-bit mainframe architecture, > but they have rebranded it several times. Rather than trying to follow > what it's called this year, maybe just write "s390" to match what we > usually call it? ok, done > > + > > + 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. > > This should explain how to check that - presumably by running pahole on > some sensible architecture. Ok, added "The ``pahole`` tool can be used for checking the alignment.". > > +* On ARM OABI user space, 16-bit member variables have 32-bit > > + alignment, making them incompatible with modern EABI kernels. > > I thought that OABI required structures as a whole to have alignment of > 4, not individual members? Which obviously does affect small > structures as members of other structures. You are right, I clearly misunderstood that. Changed the paragraph now to * On ARM OABI user space, structures are padded to multiples of 32-bit, making some structs incompatible with modern EABI kernels if they do not end on a 32-bit boundary. * On the m68k architecture, struct members are not guaranteed to have an alignment greater than 16-bit, which is a problem when relying on implicit padding. > [...] > > +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 implicit > > Delete "not". Done. > +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. > > This sentence is rather too long. Also it can be read as suggesting > that one should somehow identify and memset() the padding just before > copying to user-space. I suggest an alternate wording: > > For this reason (and for compat support) it is best to avoid any > implicit padding in data structures. Where there is implicit padding > in an existing structure, kernel drivers must be careful to fully > initialize an instance of the structure before copying it to user > space. This is usually done by calling memset() before assigning to > individual members. Sounds good, I've taken that paragraph now. > [...] > > +Alternatives to ioctl > > +===================== > [...] > > +* A custom file system can provide extra flexibility with a simple > > + user interface but add a lot of complexity to the implementation. > > Typo: "add" should be "adds". Fixed Thanks for all the good suggestions! Arnd
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..99892f96c5cb --- /dev/null +++ b/Documentation/core-api/ioctl.rst @@ -0,0 +1,248 @@ +====================== +ioctl based interfaces +====================== + +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: ``_IO``, ``_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 (_IO). It is possible but not recommended to pass an + integer value instead of a pointer with _IO. + +type + An 8-bit number, often a character literal, specific to a subsystem + or driver, and listed in :doc:`../userspace-api/ioctl/ioctl-number` + +nr + An 8-bit number identifying the specific command, unique for a give + value of 'type' + +data_type + The name of the data type pointed to by the argument, the command number + encodes the ``sizeof(data_type)`` 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(data_type) type into _IOR/IOW, as that will + lead to encoding sizeof(sizeof(data_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 can be 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 +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, struct members are not + guaranteed to have an alignment greater than 16-bit. + 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 implicit +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 is 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 to the implementation.
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 | 248 +++++++++++++++++++++++++++++++ 2 files changed, 249 insertions(+) create mode 100644 Documentation/core-api/ioctl.rst