Re: [PATCH v5] um: switch to regset API and depend on XSTATE

[SeongJae Park <sj@kernel.org>]

Hello,


On Wed, 23 Oct 2024 11:41:20 +0200 Benjamin Berg <benjamin@sipsolutions.net> wrote:

> From: Benjamin Berg <benjamin.berg@intel.com>
> 
> The PTRACE_GETREGSET API has now existed since Linux 2.6.33. The XSAVE
> CPU feature should also be sufficiently common to be able to rely on it.
> 
> With this, define our internal FP state to be the hosts XSAVE data. Add
> discovery for the hosts XSAVE size and place the FP registers at the end
> of task_struct so that we can adjust the size at runtime.
> 
> Next we can implement the regset API on top and update the signal
> handling as well as ptrace APIs to use them. Also switch coredump
> creation to use the regset API and finally set HAVE_ARCH_TRACEHOOK.
> 
> This considerably improves the signal frames. Previously they might not
> have contained all the registers (i386) and also did not have the
> sizes and magic values set to the correct values to permit userspace to
> decode the frame.
> 
> As a side effect, this will permit UML to run on hosts with newer CPU
> extensions (such as AMX) that need even more register state.

I just found kunit starts failing from the mainline commit of this patch on my
qemu-x86 Debian system, as below.

    $ git checkout 3f17fed2149192c7d3b76a45a6a87b4ff22cd586
    $ ./tools/testing/kunit/kunit.py run --kunitconfig mm/damon/tests/ --build_dir ../kunit.out/
    [...]
    [22:48:27] Configuring KUnit Kernel ...
    Regenerating .config ...
    Populating config with:
    $ make ARCH=um O=../kunit.out/ olddefconfig
    [22:48:30] Building KUnit Kernel ...
    Populating config with:
    $ make ARCH=um O=../kunit.out/ olddefconfig
    Building with:
    $ make all compile_commands.json ARCH=um O=../kunit.out/ --jobs=40
    [...]
    [22:48:46] Starting KUnit Kernel (1/1)...
    [22:48:46] ============================================================
    Running tests with:
    $ ../kunit.out/linux kunit.enable=1 mem=1G console=tty kunit_shutdown=halt
    [22:48:46] [ERROR] Test: <missing>: Could not find any KTAP output. Did any KUnit tests run?
    [22:48:46] ============================================================
    [22:48:46] Testing complete. Ran 0 tests: errors: 1
    [22:48:46] Elapsed time: 19.285s total, 3.805s configuring, 15.475s building, 0.006s running

> 
> Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
[...]
> -void arch_init_registers(int pid)
> -{
> -	struct user_fpxregs_struct fpx_regs;
> -	int err;
> -
> -	err = ptrace(PTRACE_GETFPXREGS, pid, 0, &fpx_regs);
> -	if (!err)
> -		return;
> -
> -	if (errno != EIO)
> -		panic("check_ptrace : PTRACE_GETFPXREGS failed, errno = %d",
> -		      errno);
> -
> -	have_fpx_regs = 0;
> -}
> -#else
> -
> -int get_fp_registers(int pid, unsigned long *regs)
> +int arch_init_registers(int pid)
>  {
> -	return save_fp_registers(pid, regs);
> +	struct iovec iov = {
> +		/* Just use plenty of space, it does not cost us anything */
> +		.iov_len = 2 * 1024 * 1024,
> +	};
> +	int ret;
> +
> +	iov.iov_base = mmap(NULL, iov.iov_len, PROT_WRITE | PROT_READ,
> +			    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> +	if (iov.iov_base == MAP_FAILED)
> +		return -ENOMEM;
> +
> +	/* GDB has x86_xsave_length, which uses x86_cpuid_count */
> +	ret = ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
> +	if (ret)
> +		ret = -errno;
> +	munmap(iov.iov_base, 2 * 1024 * 1024);
> +
> +	host_fp_size = iov.iov_len;
> +
> +	return ret;
>  }

And seems it fails from the registers initialization step:

    $ ../kunit.out/linux
    Core dump limits :
            soft - 0
            hard - NONE
    Checking that ptrace can change system call numbers...OK
    Checking syscall emulation for ptrace...OK
    Checking environment variables for a tempdir...none found
    Checking if /dev/shm is on tmpfs...OK
    Checking PROT_EXEC mmap in /dev/shm...OK
    Failed to initialize default registers

I'm not familiar with uml code, so reporting this issue first.  Any thought, please?


Thanks,
SJ

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