linuxkpi: Fix __sg_alloc_table_from_pages loop

[ashafer_badland.io]

Summary

Commit 3e0856b63fe0e375a0951e05c2ef98bb2ebd9421 updated
__sg_alloc_table_from_pages to use the same API as linux, but modified
the loop condition when going over the pages in a sg list. Part of the
change included moving the sg_next call out of the for loop and into the
body, which causes an off by one error when traversing the list. Since
sg_next is called before the loop body it will skip the first element
and read one past the last element.

This caused panics when running PRIME with nvidia-drm as the off-by-one
issue causes a NULL dereference.

[bz]

Good catch and likely something which may have been bugging me as well on other work.

sys/compat/linuxkpi/common/include/linux/scatterlist.h

382

I wonder at the same time why we switched from using for_each_sg() in first place as that would have avoided the problem, but I am probably too tired to see that...

[• hselasky]

Looks good!

[ashafer_badland.io]

sys/compat/linuxkpi/common/include/linux/scatterlist.h

382

I think the intention was to add a check of the value of s, but I thought it looked doable to do that while still using for_each_sg(). I'm not 100% sure either.