Re: [PATCH bpf] selftests/bpf: DENYLIST.aarch64: Remove fexit_sleep

[Alexei Starovoitov]

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.