[PATCH v3 1/2] x86_64: Use __seg_fs qualifiers in NPTL accessors
H.J. Lu
hjl.tools@gmail.com
Fri Aug 22 13:35:06 GMT 2025
On Thu, Aug 21, 2025 at 10:33 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> Use __seg_fs named address space qualifiers to cast NPTL accessors
> to %fs: prefixed addresses. Use volatile access only where
> strictly necessary.
>
> Use existing assembly RSEQ_* accessors for x32 to
> work around the GCC bug:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121613
>
> because negative value in __rseq_offset is used
> as an offset from %fs.
>
> Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
> Cc: Florian Weimer <fweimer@redhat.com>
> Cc: Carlos O'Donell <carlos@redhat.com>
LGTM.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Thanks.
> ---
> v3: Move x32 RSEQ_* accessors to sysdeps/x86_64/x32/nptl.
> ---
> sysdeps/unix/sysv/linux/x86_64/64/Implies | 1 +
> sysdeps/x86_64/64/nptl/rseq-access.h | 61 ++++++++++
> sysdeps/x86_64/nptl/tcb-access.h | 127 +++++---------------
> sysdeps/x86_64/{ => x32}/nptl/rseq-access.h | 12 +-
> 4 files changed, 105 insertions(+), 96 deletions(-)
> create mode 100644 sysdeps/unix/sysv/linux/x86_64/64/Implies
> create mode 100644 sysdeps/x86_64/64/nptl/rseq-access.h
> rename sysdeps/x86_64/{ => x32}/nptl/rseq-access.h (92%)
>
> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/Implies
b/sysdeps/unix/sysv/linux/x86_64/64/Implies
> new file mode 100644
> index 0000000000..f379d95141
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86_64/64/Implies
> @@ -0,0 +1 @@
> +x86_64/64/nptl
> diff --git a/sysdeps/x86_64/64/nptl/rseq-access.h
b/sysdeps/x86_64/64/nptl/rseq-access.h
> new file mode 100644
> index 0000000000..214cb96797
> --- /dev/null
> +++ b/sysdeps/x86_64/64/nptl/rseq-access.h
> @@ -0,0 +1,61 @@
> +/* RSEQ_* accessors. x86_64 version.
> + Copyright (C) 2002-2025 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +/* Read member of the RSEQ area directly. */
> +#define RSEQ_GETMEM(member) \
> + ({
\
> + _Static_assert (sizeof (RSEQ_SELF()->member) == 1
\
> + || sizeof (RSEQ_SELF()->member) == 4
\
> + || sizeof (RSEQ_SELF()->member) == 8,
\
> + "size of rseq data");
\
> + (*(__typeof (RSEQ_SELF()->member) __seg_fs *)
\
> + (__rseq_offset + offsetof (struct rseq_area, member)));
\
> + })
> +
> +/* Read member of the RSEQ area directly, with single-copy atomicity
semantics. */
> +#define RSEQ_GETMEM_ONCE(member) \
> + ({
\
> + _Static_assert (sizeof (RSEQ_SELF()->member) == 1
\
> + || sizeof (RSEQ_SELF()->member) == 4
\
> + || sizeof (RSEQ_SELF()->member) == 8,
\
> + "size of rseq data");
\
> + (*(volatile __typeof (RSEQ_SELF()->member) __seg_fs *)
\
> + (__rseq_offset + offsetof (struct rseq_area, member)));
\
> + })
> +
> +/* Set member of the RSEQ area directly. */
> +#define RSEQ_SETMEM(member, value) \
> + ({
\
> + _Static_assert (sizeof (RSEQ_SELF()->member) == 1
\
> + || sizeof (RSEQ_SELF()->member) == 4
\
> + || sizeof (RSEQ_SELF()->member) == 8,
\
> + "size of rseq data");
\
> + (*(__typeof (RSEQ_SELF()->member) __seg_fs *)
\
> + (__rseq_offset + offsetof (struct rseq_area, member)) = (value));
\
> + })
> +
> +/* Set member of the RSEQ area directly, with single-copy atomicity
semantics. */
> +#define RSEQ_SETMEM_ONCE(member, value) \
> + ({
\
> + _Static_assert (sizeof (RSEQ_SELF()->member) == 1
\
> + || sizeof (RSEQ_SELF()->member) == 4
\
> + || sizeof (RSEQ_SELF()->member) == 8,
\
> + "size of rseq data");
\
> + (*(volatile __typeof (RSEQ_SELF()->member) __seg_fs *)
\
> + (__rseq_offset + offsetof (struct rseq_area, member)) = (value));
\
> + })
> diff --git a/sysdeps/x86_64/nptl/tcb-access.h
b/sysdeps/x86_64/nptl/tcb-access.h
> index defd76f54f..445f328ac2 100644
> --- a/sysdeps/x86_64/nptl/tcb-access.h
> +++ b/sysdeps/x86_64/nptl/tcb-access.h
> @@ -17,116 +17,55 @@
> <https://www.gnu.org/licenses/>. */
>
> /* Read member of the thread descriptor directly. */
> -# define THREAD_GETMEM(descr, member) \
> - ({ __typeof (descr->member) __value;
\
> - _Static_assert (sizeof (__value) == 1
\
> - || sizeof (__value) == 4
\
> - || sizeof (__value) == 8,
\
> +#define THREAD_GETMEM(descr, member) \
> + ({
\
> + _Static_assert (sizeof (descr->member) == 1
\
> + || sizeof (descr->member) == 4
\
> + || sizeof (descr->member) == 8,
\
> "size of per-thread data");
\
> - if (sizeof (__value) == 1)
\
> - asm volatile ("movb %%fs:%P2,%b0"
\
> - : "=q" (__value)
\
> - : "0" (0), "i" (offsetof (struct pthread, member)));
\
> - else if (sizeof (__value) == 4)
\
> - asm volatile ("movl %%fs:%P1,%0"
\
> - : "=r" (__value)
\
> - : "i" (offsetof (struct pthread, member)));
\
> - else /* 8 */
\
> - {
\
> - asm volatile ("movq %%fs:%P1,%q0"
\
> - : "=r" (__value)
\
> - : "i" (offsetof (struct pthread, member)));
\
> - }
\
> - __value; })
> + (*(__typeof (descr->member) __seg_fs *)
\
> + offsetof (struct pthread, member));
\
> + })
>
> -/* THREAD_GETMEM already forces a read. */
> -#define THREAD_GETMEM_VOLATILE(descr, member) THREAD_GETMEM (descr,
member)
> +#define THREAD_GETMEM_VOLATILE(descr, member) \
> + ({
\
> + _Static_assert (sizeof (descr->member) == 1
\
> + || sizeof (descr->member) == 4
\
> + || sizeof (descr->member) == 8,
\
> + "size of per-thread data");
\
> + (*(volatile __typeof (descr->member) __seg_fs *)
\
> + offsetof (struct pthread, member));
\
> + })
>
> /* Same as THREAD_GETMEM, but the member offset can be non-constant. */
> -# define THREAD_GETMEM_NC(descr, member, idx) \
> - ({ __typeof (descr->member[0]) __value;
\
> - _Static_assert (sizeof (__value) == 1
\
> - || sizeof (__value) == 4
\
> - || sizeof (__value) == 8,
\
> +#define THREAD_GETMEM_NC(descr, member, idx) \
> + ({
\
> + _Static_assert (sizeof (descr->member[0]) == 1
\
> + || sizeof (descr->member[0]) == 4
\
> + || sizeof (descr->member[0]) == 8,
\
> "size of per-thread data");
\
> - if (sizeof (__value) == 1)
\
> - asm volatile ("movb %%fs:%P2(%q3),%b0"
\
> - : "=q" (__value)
\
> - : "0" (0), "i" (offsetof (struct pthread,
member[0])), \
> - "r" (idx));
\
> - else if (sizeof (__value) == 4)
\
> - asm volatile ("movl %%fs:%P1(,%q2,4),%0"
\
> - : "=r" (__value)
\
> - : "i" (offsetof (struct pthread, member[0])), "r"
(idx));\
> - else /* 8 */
\
> - {
\
> - asm volatile ("movq %%fs:%P1(,%q2,8),%q0"
\
> - : "=r" (__value)
\
> - : "i" (offsetof (struct pthread, member[0])),
\
> - "r" (idx));
\
> - }
\
> - __value; })
> -
> -
> -/* Loading addresses of objects on x86-64 needs to be treated special
> - when generating PIC code. */
> -#ifdef __pic__
> -# define IMM_MODE "nr"
> -#else
> -# define IMM_MODE "ir"
> -#endif
> -
> + (*(__typeof (descr->member[0]) __seg_fs *)
\
> + offsetof (struct pthread, member[idx]));
\
> + })
>
> /* Set member of the thread descriptor directly. */
> -# define THREAD_SETMEM(descr, member, value) \
> +#define THREAD_SETMEM(descr, member, value) \
> ({
\
> _Static_assert (sizeof (descr->member) == 1
\
> || sizeof (descr->member) == 4
\
> || sizeof (descr->member) == 8,
\
> "size of per-thread data");
\
> - if (sizeof (descr->member) == 1)
\
> - asm volatile ("movb %b0,%%fs:%P1" :
\
> - : "iq" (value),
\
> - "i" (offsetof (struct pthread, member)));
\
> - else if (sizeof (descr->member) == 4)
\
> - asm volatile ("movl %0,%%fs:%P1" :
\
> - : IMM_MODE (value),
\
> - "i" (offsetof (struct pthread, member)));
\
> - else /* 8 */
\
> - {
\
> - /* Since movq takes a signed 32-bit immediate or a register
source \
> - operand, use "er" constraint for 32-bit signed integer
constant \
> - or register. */
\
> - asm volatile ("movq %q0,%%fs:%P1" :
\
> - : "er" ((uint64_t) cast_to_integer (value)),
\
> - "i" (offsetof (struct pthread, member)));
\
> - }})
> -
> + (*(__typeof (descr->member) __seg_fs *)
\
> + offsetof (struct pthread, member) = (value));
\
> + })
>
> /* Same as THREAD_SETMEM, but the member offset can be non-constant. */
> -# define THREAD_SETMEM_NC(descr, member, idx, value) \
> +#define THREAD_SETMEM_NC(descr, member, idx, value) \
> ({
\
> _Static_assert (sizeof (descr->member[0]) == 1
\
> || sizeof (descr->member[0]) == 4
\
> || sizeof (descr->member[0]) == 8,
\
> "size of per-thread data");
\
> - if (sizeof (descr->member[0]) == 1)
\
> - asm volatile ("movb %b0,%%fs:%P1(%q2)" :
\
> - : "iq" (value),
\
> - "i" (offsetof (struct pthread, member[0])),
\
> - "r" (idx));
\
> - else if (sizeof (descr->member[0]) == 4)
\
> - asm volatile ("movl %0,%%fs:%P1(,%q2,4)" :
\
> - : IMM_MODE (value),
\
> - "i" (offsetof (struct pthread, member[0])),
\
> - "r" (idx));
\
> - else /* 8 */
\
> - {
\
> - /* Since movq takes a signed 32-bit immediate or a register
source \
> - operand, use "er" constraint for 32-bit signed integer
constant \
> - or register. */
\
> - asm volatile ("movq %q0,%%fs:%P1(,%q2,8)" :
\
> - : "er" ((uint64_t) cast_to_integer (value)),
\
> - "i" (offsetof (struct pthread, member[0])),
\
> - "r" (idx));
\
> - }})
> + (*(__typeof (descr->member[0]) __seg_fs *)
\
> + offsetof (struct pthread, member[idx]) = (value));
\
> + })
> diff --git a/sysdeps/x86_64/nptl/rseq-access.h
b/sysdeps/x86_64/x32/nptl/rseq-access.h
> similarity index 92%
> rename from sysdeps/x86_64/nptl/rseq-access.h
> rename to sysdeps/x86_64/x32/nptl/rseq-access.h
> index bc966b2972..8386ebd4bf 100644
> --- a/sysdeps/x86_64/nptl/rseq-access.h
> +++ b/sysdeps/x86_64/x32/nptl/rseq-access.h
> @@ -1,5 +1,5 @@
> -/* RSEQ_* accessors. x86_64 version.
> - Copyright (C) 2002-2025 Free Software Foundation, Inc.
> +/* RSEQ_* accessors. x32 version.
> + Copyright (C) 2025 Free Software Foundation, Inc.
> This file is part of the GNU C Library.
>
> The GNU C Library is free software; you can redistribute it and/or
> @@ -45,6 +45,14 @@
> /* Read member of the RSEQ area directly. */
> #define RSEQ_GETMEM(member) RSEQ_GETMEM_ONCE(member)
>
> +/* Loading addresses of objects on x86-64 needs to be treated special
> + when generating PIC code. */
> +#ifdef __pic__
> +# define IMM_MODE "nr"
> +#else
> +# define IMM_MODE "ir"
> +#endif
> +
> /* Set member of the RSEQ area directly, with single-copy atomicity
semantics. */
> #define RSEQ_SETMEM_ONCE(member, value) \
> ({
\
> --
> 2.50.1
>
--
H.J.
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