On Mon, Jul 15, 2024 at 9:32 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote: > > > Hi Daniel, Manu > I was able to reproduce this issue on KVM and found the root cause for > this hang! The other issue that we fixed is unrelated to this hang and > doesn't occur on self hosted github runners as they use 48-bit VAs. > > The userspace test code has: > > #define STACK_SIZE (1024 * 1024) > static char child_stack[STACK_SIZE]; > > cpid = clone(do_sleep, child_stack + STACK_SIZE, CLONE_FILES | SIGCHLD, fexit_skel); > > arm64 requires the stack pointer to be 16 byte aligned otherwise > SPAlignmentFault occurs, this appears as Bus error in the userspace. > > The stack provided to the clone system call is not guaranteed to be > aligned properly in this selftest. > > The test hangs on the following line: > while (READ_ONCE(fexit_skel->bss->fentry_cnt) != 2); > > Because the child process is killed due to SPAlignmentFault, the > fentry_cnt remains at 0! > > Reading the man page of clone system call, the correct way to allocate > stack for this call is using mmap like this: > > stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0); > > This fixes the issue, I will send a patch to use this and once again > remove this test from DENYLIST and I hope this time it fixes it for good. Wow. Great find. Good to know. prog_tests/ns_current_pid_tgid.c has the same issue probably.
