On 2019-09-05, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:+/** + * copy_struct_to_user: copy a struct to user space + * @dst: Destination address, in user space. + * @usize: Size of @dst struct. + * @src: Source address, in kernel space. + * @ksize: Size of @src struct. + * + * Copies a struct from kernel space to user space, in a way that guarantees + * backwards-compatibility for struct syscall arguments (as long as future + * struct extensions are made such that all new fields are *appended* to the + * old struct, and zeroed-out new fields have the same meaning as the old + * struct). + * + * @ksize is just sizeof(*dst), and @usize should've been passed by user space. + * The recommended usage is something like the following: + * + * SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize) + * { + * int err; + * struct foo karg = {}; + * + * // do something with karg + * + * err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg)); + * if (err) + * return err; + * + * // ... + * } + * + * There are three cases to consider: + * * If @usize == @ksize, then it's copied verbatim. + * * If @usize < @ksize, then kernel space is "returning" a newer struct to an + * older user space. In order to avoid user space getting incomplete + * information (new fields might be important), all trailing bytes in @src + * (@ksize - @usize) must be zeroreds/zerored/zero/, right?
It should've been "zeroed".
, otherwise -EFBIG is returned.'Funny' that, copy_struct_from_user() below seems to use E2BIG.
This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for a "too big" struct passed to the kernel, and EFBIG for a "too big" struct passed to user-space. I would personally have preferred EMSGSIZE instead of EFBIG, but felt using the existing error codes would be less confusing.
+ * * If @usize > @ksize, then the kernel is "returning" an older struct to a + * newer user space. The trailing bytes in @dst (@usize - @ksize) will be + * zero-filled. + * + * Returns (in all cases, some data may have been copied): + * * -EFBIG: (@usize < @ksize) and there are non-zero trailing bytes in @src. + * * -EFAULT: access to user space failed. + */ +int copy_struct_to_user(void __user *dst, size_t usize, + const void *src, size_t ksize) +{ + size_t size = min(ksize, usize); + size_t rest = abs(ksize - usize); + + if (unlikely(usize > PAGE_SIZE)) + return -EFAULT;Not documented above. Implementation consistent with *from*, but see below.
Will update the kernel-doc.
+ if (unlikely(!access_ok(dst, usize))) + return -EFAULT; + + /* Deal with trailing bytes. */ + if (usize < ksize) { + if (memchr_inv(src + size, 0, rest)) + return -EFBIG; + } else if (usize > ksize) { + if (__memzero_user(dst + size, rest)) + return -EFAULT; + } + /* Copy the interoperable parts of the struct. */ + if (__copy_to_user(dst, src, size)) + return -EFAULT; + return 0; +} +EXPORT_SYMBOL(copy_struct_to_user); + +/** + * copy_struct_from_user: copy a struct from user space + * @dst: Destination address, in kernel space. This buffer must be @ksize + * bytes long. + * @ksize: Size of @dst struct. + * @src: Source address, in user space. + * @usize: (Alleged) size of @src struct. + * + * Copies a struct from user space to kernel space, in a way that guarantees + * backwards-compatibility for struct syscall arguments (as long as future + * struct extensions are made such that all new fields are *appended* to the + * old struct, and zeroed-out new fields have the same meaning as the old + * struct). + * + * @ksize is just sizeof(*dst), and @usize should've been passed by user space. + * The recommended usage is something like the following: + * + * SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize) + * { + * int err; + * struct foo karg = {}; + * + * err = copy_struct_from_user(&karg, sizeof(karg), uarg, size); + * if (err) + * return err; + * + * // ... + * } + * + * There are three cases to consider: + * * If @usize == @ksize, then it's copied verbatim. + * * If @usize < @ksize, then the user space has passed an old struct to a + * newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize) + * are to be zero-filled. + * * If @usize > @ksize, then the user space has passed a new struct to an + * older kernel. The trailing bytes unknown to the kernel (@usize - @ksize) + * are checked to ensure they are zeroed, otherwise -E2BIG is returned. + * + * Returns (in all cases, some data may have been copied): + * * -E2BIG: (@usize > @ksize) and there are non-zero trailing bytes in @src. + * * -E2BIG: @usize is "too big" (at time of writing, >PAGE_SIZE). + * * -EFAULT: access to user space failed. + */ +int copy_struct_from_user(void *dst, size_t ksize, + const void __user *src, size_t usize) +{ + size_t size = min(ksize, usize); + size_t rest = abs(ksize - usize); + + if (unlikely(usize > PAGE_SIZE)) + return -EFAULT;Documented above as returning -E2BIG.
I will switch this (and to) back to -E2BIG -- I must've had a brain-fart when doing some refactoring.
+ if (unlikely(!access_ok(src, usize))) + return -EFAULT; + + /* Deal with trailing bytes. */ + if (usize < ksize) + memset(dst + size, 0, rest); + else if (usize > ksize) { + const void __user *addr = src + size; + char buffer[BUFFER_SIZE] = {};Isn't that too big for on-stack?
Is a 64-byte buffer too big? I picked the number "at random" to be the size of a cache line, but I could shrink it down to 32 bytes if the size is an issue (I wanted to avoid needless allocations -- hence it being on-stack).
+ + while (rest > 0) { + size_t bufsize = min(rest, sizeof(buffer)); + + if (__copy_from_user(buffer, addr, bufsize)) + return -EFAULT; + if (memchr_inv(buffer, 0, bufsize)) + return -E2BIG; + + addr += bufsize; + rest -= bufsize; + }The perf implementation uses get_user(); but if that is too slow, surely we can do something with uaccess_try() here?
Is there a non-x86-specific way to do that (unless I'm mistaken only x86 has uaccess_try() or the other *_try() wrappers)? The main "performance improvement" (if you can even call it that) is that we use memchr_inv() which finds non-matching characters more efficiently than just doing a loop.
+ } + /* Copy the interoperable parts of the struct. */ + if (__copy_from_user(dst, src, size)) + return -EFAULT; + return 0; +} +EXPORT_SYMBOL(copy_struct_from_user);And personally I'm not a big fan of EXPORT_SYMBOL().
I don't have much of an opinion (after all, it only really makes sense a lot of sense for syscalls) -- though out-of-tree modules that define ioctl()s wouldn't be able to make use of them. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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