On 23 August 2021 22:09:08 BST, Alyssa Rosenzweig alyssa@collabora.com wrote:
In lock_region, simplify the calculation of the region_width parameter. This field is the size, but encoded as log2(ceil(size)) - 1. log2(ceil(size)) may be computed directly as fls(size - 1). However, we want to use the 64-bit versions as the amount to lock can exceed 32-bits.
This avoids undefined behaviour when locking all memory (size ~0), caught by UBSAN.
It might have been useful to mention what it is that UBSAN specifically picked up (it took me a while to spot) - but anyway I think there's a bigger issue with it being completely wrong when size == ~0 (see below).
Indeed. I've updated the commit message in v2 to explain what goes wrong (your analysis was spot on, but a mailing list message is more ephermal than a commit message). I'll send out v2 tomorrow assuming nobody objects to v1 in the mean time.
Thanks for the review.
There is potentially a third bug which kbase only recently attempted to fix. The lock address is effectively rounded down by the hardware (the bottom bits are ignored). So if you have mask=(1<<region_width)-1 but (iova & mask) != ((iova + size) & mask) then you are potentially failing to lock the end of the intended region. kbase has added some code to handle this:
/* Round up if some memory pages spill into the next region. */ region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT); region_frame_number_end = (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT);
if (region_frame_number_start < region_frame_number_end) lockaddr_size_log2 += 1;
I guess we should too?
Oh, I missed this one. Guess we have 4 bugs with this code instead of just 3, yikes. How could such a short function be so deeply and horribly broken? ����
Should I add a fourth patch to the series to fix this?
Yes please!
Thanks, Steve