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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, May 29, 2019 at 06:19:55PM +0200, Peter Zijlstra wrote: > On Wed, May 29, 2019 at 03:35:10PM +0100, Will Deacon wrote: > > On Wed, May 29, 2019 at 03:25:15PM +0200, Peter Zijlstra wrote: > > > On Wed, May 29, 2019 at 02:05:21PM +0100, Will Deacon wrote: > > > > On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote: > > > > > > > > if (user_mode(regs)) { > > > > > > > > Hmm, so it just occurred to me that Mark's observation is that the regs > > > > can be junk in some cases. In which case, should we be checking for > > > > kthreads first? > > > > > > task_pt_regs() can return garbage, but @regs is the exception (or > > > perf_arch_fetch_caller_regs()) regs, and for those user_mode() had > > > better be correct. > > > > So what should we report for the idle task? > > If an interrupt hits the idle task, @regs would be !user_mode(regs), > we'll find current->flags & PF_KTHREAD (idle not having passed through > exec()) and therefore we'll take ABI_NONE for the user regs. > > Or am I not getting it? Sorry, I'm not trying to catch you out! Just trying to understand what the semantics are supposed to be. I do find the concept of user_mode(regs) bizarre for the idle task. By the above, we definitely have a bug on arm64 (user_mode(regs) tends to be true for the idle task), and I couldn't figure out how you avoided it on x86. I guess it happens to work because the stack is zero-initialised or something? Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel