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

[Florian Weimer]

* 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.

Thanks,
Florian