On Thu, 06 Apr 2023 11:17:12 +0900 Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote: > >> + if (len / PAGE_SIZE > totalram_pages()) > >> + return ERR_PTR(-ENOMEM); > > > >We're catering for a buggy caller here, aren't we? Are such large > >requests ever reasonable? > > > >How about we decide what's the largest reasonable size and do a > >WARN_ON(larger-than-that), so the buggy caller gets fixed? > > Yes we're considering a buggy caller. I thought even totalram_pages() / 2 in > the old ion system is also unreasonable. To avoid the /2, I changed it to > totalram_pages() though. > > Because userspace can request that size repeately, I think WARN_ON() may be > called to too often, so that it would fill the kernel log buffer. Oh geeze. I trust that userspace needs elevated privileges of some form? If so, then spamming dmesg isn't an issue - root can do much worse than that. > Even we think WARN_ON_ONCE rather than WARN_ON, the buggy point is not kernel > layer. Unlike page fault mechanism, this dma-buf system heap gets the size from > userspace, and it is allowing unlimited size. I think we can't fix the buggy > user space with the kernel warning log. So I think warning is not enough, > and we need a safeguard in kernel layer. I really dislike that ram/2 thing - it's so arbitrary, hence is surely wrong for all cases. Is there something more thoughtful we can do? I mean, top priority here is to inform userspace that it's buggy so that it gets fixed (assuming this requires elevated privileges). And userspace which requests (totalram_pages()/2 - 1) bytes is still buggy, but we did nothing to get the bug fixed.
