On 2019-09-05, Peter Zijlstra peterz@infradead.org 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@infradead.org 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".
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.