Re: [RFC PATCH glibc 11/12] hurd, htl: Add some x86_64-specific code

[Sergey Bugaev]

On Sun, Feb 12, 2023 at 7:11 PM Samuel Thibault <samuel.thibault@gnu.org> wrote:
>
> Sergey Bugaev, le dim. 12 févr. 2023 14:10:42 +0300, a ecrit:
> > We should not need a getter routine, because one can simply inspect the 
> > target
> > thread's state (unless, again, I misunderstand things horribly).
>
> For 16bit fs/gs values we could read them from userland yes. But for
> fs/gs base, the FSGSBASE instruction is not available on all 64bit
> processors. And ATM in THREAD_TCB we want to be able to get the base of
> another thread.

What I've meant is:

__thread_get_state (whatever_thread, &state);
uintptr_t its_fs_base = state->fs_base;

You can't really do the same to *write* [fg]s_base, because doing
thread_set_state on your own thread is bound to end badly.

> > diff --git a/sysdeps/mach/hurd/x86_64/static-start.S 
> > b/sysdeps/mach/hurd/x86_64/static-start.S
> > new file mode 100644
> > index 00000000..982d3d52
> > --- /dev/null
> > +++ b/sysdeps/mach/hurd/x86_64/static-start.S
> > @@ -0,0 +1,27 @@
> > +/* Type of the TCB.  */
> > +typedef struct
> > +{
> > +  void *tcb;                 /* Points to this structure.  */
> > +  dtv_t *dtv;                        /* Vector of pointers to TLS data.  */
> > +  thread_t self;             /* This thread's control port.  */
> > +  int __glibc_padding1;
> > +  int multiple_threads;
> > +  int gscope_flag;
> > +  uintptr_t sysinfo;
> > +  uintptr_t stack_guard;
> > +  uintptr_t pointer_guard;
> > +  long __glibc_padding2[2];
> > +  int private_futex;
>
> ? Isn't that rather feature_1 ?

sysdeps/mach/hurd/i386/tls.h has 'int private_futex;', which is where
I stole this from. A quick grep confirms that it's never used, so we
might rename both to feature_1, or maybe another instance of
__glibc_padding.

> > +/* GCC generates %fs:0x28 to access the stack guard.  */
> > +_Static_assert (offsetof (tcbhead_t, stack_guard) == 0x28,
> > +                "stack guard offset");
> > +/* libgcc uses %fs:0x70 to access the split stack pointer.  */
> > +_Static_assert (offsetof (tcbhead_t, __private_ss) == 0x70,
> > +                "split stack pointer offset");
>
> Indeed. Could you perhaps also add them to the i386 tls.h?

> > +/* Install new dtv for current thread.  */
> > +# define INSTALL_NEW_DTV(dtvp) THREAD_SETMEM (THREAD_SELF, dtv, dtvp)
> > +/* Return the address of the dtv for the current thread.  */
> > +# define THREAD_DTV() THREAD_GETMEM (THREAD_SELF, dtv)
>
> While at it, try to make the i386 version use that too?

Yeah, I have not ported the improvements back to the 32-bit version;
maybe I should. Another cool one is doing fs/gs-relative access using
GCC's __seg_fs/__seg_gs when supported.

Sergey