On 2019-09-05, Peter Zijlstra 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 zerored > > s/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