On Wed, Apr 26, 2023 at 8:30 AM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> writes: > > On Tue, Apr 25, 2023 at 6:58 AM Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> wrote: > ... > > > > Out of curiosity for PPC folks, why cannot 64-bit PPC use per-task > > canary? Michael, is this an optimization? Adding Christophe as well > > since it came in a few years ago via the following commit: > > I think Christophe also answered these in his reply. > > We do use a per-task canary, but because we don't have "current" in a > register, we can't use the value in current for GCC. > > In one of my replies I said a possible solution would be to keep current > in a register on 64-bit, but we'd need to do that in addition to the > paca, so that would consume another GPR which we'd need to think hard > about. Makes sense. I'd think it is not worth allocating a separate GPR just for this, and may cause similar register optimization issues as well. > There's another reason to have it in the paca, which is that the paca is > always accessible, even when the MMU is off, whereas current isn't (in > some situations). > > In general we don't want to use stack protector in code that runs with > the MMU off, but if the canary wasn't in the paca then we'd have a hard > requirement to not use stack protector in that code. How could you control which code paths don't have the stack protector? Just curious. thanks, - Joel
