[PATCH 2/2] x86_64: Use __seg_fs qualifiers in NPTL accessors

[H.J. Lu]

On Wed, Aug 20, 2025 at 5:30 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Uros Bizjak:
>
> > Hm, the original approach does this:
> >
> >        asm volatile ("movl %%fs:%P1(%q2),%0"
> >                       : "=r" (__value)
> >                       : "i" (offsetof (struct rseq_area, member)),
> >                         "r" ((long long int) __rseq_offset));
> >
> > Won't this also segfault with negative __rseq_offset?
>
> It works because it's sign-extending into an r register.  Only the
> loaded quantity is 32 bits.  This was changed in:
>
> commit 0b6ad02b33448c0b8b6fdd781dffad329d1f0f7d
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Sat Jan 11 22:57:02 2025 +0800
>
>     x86-64: Cast __rseq_offset to long long int [BZ #32543]
>
>     commit 494d65129ed5ae1154b75cc189bbdde5e9ecf1df
>     Author: Michael Jeanson <mjeanson@efficios.com>
>     Date:   Thu Aug 1 10:35:34 2024 -0400
>
>         nptl: Introduce <rseq-access.h> for RSEQ_* accessors
>
>     added things like
>
>            asm volatile ("movl %%fs:%P1(%q2),%0"                                  \
>                          : "=r" (__value)                                         \
>                          : "i" (offsetof (struct rseq_area, member)),             \
>                            "r" (__rseq_offset));                                  \
>
>     But this doesn't work for x32 when __rseq_offset is negative since the
>     address is computed as
>
>     FS + 32-bit to 64-bit zero extension of __rseq_offset
>     + offsetof (struct rseq_area, member)
>
>     Cast __rseq_offset to long long int
>
>                            "r" ((long long int) __rseq_offset));                  \
>
>     to sign-extend 32-bit __rseq_offset to 64-bit.  This is a no-op for x86-64
>     since x86-64 __rseq_offset is 64-bit.  This fixes BZ #32543.
>
>     Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
>     Reviewed-by: Florian Weimer <fweimer@redhat.com>
>
> You could move this implementation into
> sysdeps/unix/sysv/linux/x86_64/x32/rseq-access.h.  But the other
> architectures should move out …/nptl/… for consistency.
>

Let's do this.   I will see if I can improve x32 later.


-- 
H.J.