Re: git: bb5f10e51bd5 - main - linux: implement sched_getattr()

On 10/6/26 10:42, Enji Cooper (yaneurabeya) wrote:
> 
> ...
> 
>>> + attr.size = MIN(args->size, sizeof(attr));
> 
> …
> 
>>> + return (copyout(&attr, args->attr, args->size));
>>
>> So, this lets the caller decide how much stack data they want to copy
>> out, independent of sizeof(attr).  The only constraint is the
>> `args->size > PAGE_SIZE` check at the beginning of the function.
> 
> I eliminated a duplicate calculation in the copyout which contradicted the [Linux] documented interface [1].

.. and introduced a kernel stack disclosure (the kind of thing we normally
have to ship an SA for).  Using attr.size for the length here is the
correct solution.

&attr is on the stack, and it is only sizeof(attr) bytes in size.  If
you copy out _more_ bytes than sizeof(attr), you don't magically copy
out zeroes, you copy out _RANDOM KERNEL STACK DATA_.

> The interface documented by Linux allows the user to specify the amount of data to copy out (https://www.man7.org/linux/man-pages/man2/sched_getattr.2.html):
> 
>>   The fields of the sched_attr structure are as follows:
>>
>> size This field should be set to the size of the structure in
> bytes, as in sizeof(struct sched_attr). If the provided
> structure is smaller than the kernel structure, any
> additional fields are assumed to be '0'. If the provided
> structure is larger than the kernel structure, the kernel
> verifies that all additional fields are 0; if they are not,
> sched_setattr() fails with the error E2BIG and updates size
> to contain the size of the kernel structure.

Right, so the extra bytes in the case that args->size is
> sizeof(attr) are supposed to be all zeroes already (and you
are supposed to check that in the syscall if you aren't already)
so you don't need to copy them back out again, only the
sizeof(attr) bytes including the updated size.

This is not all that weird, and is similar to how we handle
cpuset masks which also can be differently sized than the
kernel's representation.

-- 
John Baldwin