On 2019-09-05, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
On Thu, Sep 05, 2019 at 11:43:05AM +0200, Peter Zijlstra wrote:On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote: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".That reads wrong to me; that way it reads like this function must take that action and zero out the 'rest'; which is just wrong. This function must verify those bytes are zero, not make them zero., 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.Sadly a recent commit: 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code") Made the situation even 'worse'.And thinking more about things; I'm not convinced the above patch is actually right. Do we really want to simply truncate all the attributes of the task? And should we not at least set sched_flags when there are non-default clamp values applied? See; that is I think the primary bug that had chrt failing; we tried to publish the default clamp values as !0.
I just saw this patch in -rc8 -- should I even attempt to port sched_getattr(2) to copy_struct_to_user()? I agree that publishing a default non-zero value is a mistake -- once you do that, old user space will either get confused or lose information. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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