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

[Benjamin Berg]

Hi,

that probably means the size detection for the FPU state (i.e.
PTRACE_GETREGSET for NT_X86_XSTATE is incorrect on a 32bit host in some
way.

Is there anything special about the qemu setup or it is just a default
qemu-x86?

Benjamin

On Mon, 2024-12-02 at 23:02 -0800, SeongJae Park wrote:
> Hello,
> 
> 
> On Wed, 23 Oct 2024 11:41:20 +0200 Benjamin Berg <benjamin at sipsolutions.net> wrote:
> 
> > From: Benjamin Berg <benjamin.berg at 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 at 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
> 
> [...]
>