On Thu, 2025-06-19 at 21:22 +0900, Hajime Tazaki wrote:
>
> On Thu, 19 Jun 2025 19:40:49 +0900,
> Benjamin Berg wrote:
> >
> > On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> > > As userspace on UML/!MMU also need to configure %fs register when
> > > it is
> > > running to correctly access thread structure, host syscalls
> > > implemented
> > > in os-Linux drivers may be puzzled when they are called. Thus it
> > > has to
> > > configure %fs register via arch_prctl(SET_FS) on every host
> > > syscalls.
> >
> > Really, I still think that we should "just" get rid of libc
> > entirely
> > inside UML. That would avoid so many weird/potential issues …
>
> I'm not sure if I understand your point.
>
> Q1) what do you mean by 'get rid of libc entirely' here ?
> do you mean the following code block adds the dependency ?
> + int os_arch_prctl(int pid, int option, unsigned long *arg2)
> + {
> + return syscall(SYS_arch_prctl, option, arg2);
> + }
>
> I guess this can be replaced with inline assembly instead of using
> libc's one. but this is the code under os-Linux, which I thought
> we're
> allowed to use the host code ?
>
> Q2) "That would avoid so many weird/potential issues …"
> I'm new to this; I'm wondering what kind of issues did you see ?
Oh, I am just being annoyed by libc in general in UM. It isn't specific
to this patchset.
An example is that we need to keep malloc() working for libc. Which I
would think is kind of weird. Or we had issues because libc turned on
rseq and that was inherited into userspace, causing random crashes and
such.
> > Doesn't change the fact that FS/GS needs to be restored when doing
> > thread switches and such. Though one might be able to do it
> > entirely
> > within arch_switch_to then.
>
> I believe this is already done in arch_switch_to. This particular
> patch does the control to the host context.
OK, need to look at that again a bit. I haven't really wrapped my mind
around how everything fits together, so I probably got some stuff
wrong.
Benjamin
