[07/10] hurd: Fix x86_64 _hurd_tls_fork
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Commit Message
It is illegal to call thread_get_state () on mach_thread_self (), so
this codepath cannot be used as-is to fork the calling thread's TLS.
Fortunately we can use THREAD_SELF (aka %fs:0x0) to find out the value
of our fs_base without calling into the kernel.
Fixes: f6cf701efc61c9ad910372bda14b9a235db310a8
"hurd: Implement TLS for x86_64"
Checked on x86_64-gnu: fork () now works!
Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
It would be nice if GNU Mach allowed
thread_get_state (mach_thread_self (), i386_FSGS_BASE_STATE)
like it already allows thread_set_state for this case. But even if
gnumach adds suppport for this, doing it in userspace without an extra
RPC is still better.
sysdeps/mach/hurd/x86_64/tls.h | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
Comments
Applied, thanks!
Sergey Bugaev via Libc-alpha, le mer. 17 mai 2023 22:14:33 +0300, a ecrit:
> It is illegal to call thread_get_state () on mach_thread_self (), so
> this codepath cannot be used as-is to fork the calling thread's TLS.
> Fortunately we can use THREAD_SELF (aka %fs:0x0) to find out the value
> of our fs_base without calling into the kernel.
>
> Fixes: f6cf701efc61c9ad910372bda14b9a235db310a8
> "hurd: Implement TLS for x86_64"
>
> Checked on x86_64-gnu: fork () now works!
>
> Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
> ---
> It would be nice if GNU Mach allowed
>
> thread_get_state (mach_thread_self (), i386_FSGS_BASE_STATE)
>
> like it already allows thread_set_state for this case. But even if
> gnumach adds suppport for this, doing it in userspace without an extra
> RPC is still better.
>
> sysdeps/mach/hurd/x86_64/tls.h | 25 +++++++++++++++++++------
> 1 file changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/sysdeps/mach/hurd/x86_64/tls.h b/sysdeps/mach/hurd/x86_64/tls.h
> index 35dcef44..6487ed35 100644
> --- a/sysdeps/mach/hurd/x86_64/tls.h
> +++ b/sysdeps/mach/hurd/x86_64/tls.h
> @@ -140,12 +140,25 @@ _hurd_tls_fork (thread_t child, thread_t orig,
> error_t err;
> struct i386_fsgs_base_state state;
> mach_msg_type_number_t state_count = i386_FSGS_BASE_STATE_COUNT;
> - err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
> - (thread_state_t) &state,
> - &state_count);
> - if (err)
> - return err;
> - assert (state_count == i386_FSGS_BASE_STATE_COUNT);
> +
> + extern thread_t hurd_thread_self (void);
> + if (orig != hurd_thread_self ())
> + {
> + err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
> + (thread_state_t) &state,
> + &state_count);
> + if (err)
> + return err;
> + assert (state_count == i386_FSGS_BASE_STATE_COUNT);
> + }
> + else
> + {
> + /* It is illegal to call thread_get_state () on mach_thread_self ().
> + But we're only interested in the value of fs_base, and since we're
> + this thread, we know it points to our TCB. */
> + state.fs_base = (unsigned long) THREAD_SELF;
> + state.gs_base = 0;
> + }
>
> return __thread_set_state (child, i386_FSGS_BASE_STATE,
> (thread_state_t) &state,
> --
> 2.40.1
>
@@ -140,12 +140,25 @@ _hurd_tls_fork (thread_t child, thread_t orig,
error_t err;
struct i386_fsgs_base_state state;
mach_msg_type_number_t state_count = i386_FSGS_BASE_STATE_COUNT;
- err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
- (thread_state_t) &state,
- &state_count);
- if (err)
- return err;
- assert (state_count == i386_FSGS_BASE_STATE_COUNT);
+
+ extern thread_t hurd_thread_self (void);
+ if (orig != hurd_thread_self ())
+ {
+ err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
+ (thread_state_t) &state,
+ &state_count);
+ if (err)
+ return err;
+ assert (state_count == i386_FSGS_BASE_STATE_COUNT);
+ }
+ else
+ {
+ /* It is illegal to call thread_get_state () on mach_thread_self ().
+ But we're only interested in the value of fs_base, and since we're
+ this thread, we know it points to our TCB. */
+ state.fs_base = (unsigned long) THREAD_SELF;
+ state.gs_base = 0;
+ }
return __thread_set_state (child, i386_FSGS_BASE_STATE,
(thread_state_t) &state,