On 23 August 2021 22:09:08 BST, Alyssa Rosenzweig <alyssa@xxxxxxxxxxxxx> 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
