On Tue, Jan 26, 2021 at 06:36:02PM +0000, Mark Rutland wrote: > On Tue, Jan 26, 2021 at 11:00:05AM -0600, Bjorn Andersson wrote: > > On Tue 26 Jan 10:21 CST 2021, Mark Rutland wrote: > > > > > On Tue, Jan 26, 2021 at 02:58:33PM +0000, Matthew Wilcox wrote: > > > > On Tue, Jan 26, 2021 at 10:47:34AM +0000, Mark Rutland wrote: > > > > > Hi, > > > > > > > > > > When fuzzing arm64 with Syzkaller, I'm seeing some splats where > > > > > this_cpu_ptr() is used in the bowels of idr_alloc(), by way of > > > > > radix_tree_node_alloc(), in a preemptible context: > > > > > > > > I sent a patch to fix this last June. The maintainer seems to be > > > > under the impression that I care an awful lot more about their > > > > code than I do. > > > > > > > > https://lore.kernel.org/netdev/20200605120037.17427-1-wi...@infradead.org/ > > > > > > Ah; I hadn't spotted the (glaringly obvious) GFP_ATOMIC abuse, thanks > > > for the pointer, and sorry for the noise. > > > > > > > I'm afraid this isn't as obvious to me as it is to you. Are you saying > > that one must not use GFP_ATOMIC in non-atomic contexts? > > > > That said, glancing at the code I'm puzzled to why it would use > > GFP_ATOMIC. > > I'm also not entirely sure about the legitimacy of GFP_ATOMIC outside of > atomic contexts -- I couldn't spot any documentation saying that wasn't > legitimate, but Matthew's commit message implies so, and it sticks out > as odd.
It's actually an assumption in the radix tree code. If you say you can't be preempted by saying GFP_ATOMIC, it takes you at your word and does some things which cannot be done in preemptable context.
