[PATCH v5 3/4] riscv: Add ifunc-compatible hwprobe function

[Evan Green <evan@rivosinc.com>]

ifunc selector functions are called very early, and are not able to call
external functions that require relocation. This presents a problem for
applications and libraries outside of glibc that may want to use
__riscv_hwprobe() in their ifunc selectors.

Add a new function, __riscv_hwprobe_early(), that is compiled into
libc_nonshared.a and is safe to use in application and library ifunc
selectors. The _early() variant checks a weak alias of __riscv_hwprobe,
__riscv_hwprobe_weak, to see if the symbol is available, and simply
calls that if so. However if it's called before the symbol is available,
it makes the system call directly. The early call has some caveats,
primarily that it does not take advantage of the cached vDSO data, and
does not set errno (another external symbol) upon failing.

The original dynamic __riscv_hwprobe() function is still available for
users that don't need the overhead of handling early init cases.

Signed-off-by: Evan Green <evan@rivosinc.com>

---

Changes in v5:
 - Introduced __riscv_hwprobe_early()

 sysdeps/unix/sysv/linux/riscv/Makefile        |  3 +-
 sysdeps/unix/sysv/linux/riscv/Versions        |  1 +
 sysdeps/unix/sysv/linux/riscv/hwprobe.c       |  2 ++
 .../unix/sysv/linux/riscv/hwprobe_static.c    | 36 +++++++++++++++++++
 .../unix/sysv/linux/riscv/rv32/libc.abilist   |  1 +
 .../unix/sysv/linux/riscv/rv64/libc.abilist   |  1 +
 sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h   | 21 +++++++++++
 7 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100644 sysdeps/unix/sysv/linux/riscv/hwprobe_static.c

diff --git a/sysdeps/unix/sysv/linux/riscv/Makefile b/sysdeps/unix/sysv/linux/riscv/Makefile
index 45cc29e40d..fd8fd48ade 100644
--- a/sysdeps/unix/sysv/linux/riscv/Makefile
+++ b/sysdeps/unix/sysv/linux/riscv/Makefile
@@ -1,6 +1,7 @@
 ifeq ($(subdir),misc)
 sysdep_headers += sys/cachectl.h sys/hwprobe.h
-sysdep_routines += flush-icache hwprobe
+sysdep_routines += flush-icache hwprobe hwprobe_static
+static-only-routines += hwprobe_static
 endif
 
 ifeq ($(subdir),stdlib)
diff --git a/sysdeps/unix/sysv/linux/riscv/Versions b/sysdeps/unix/sysv/linux/riscv/Versions
index 0c4016382d..571d4c2046 100644
--- a/sysdeps/unix/sysv/linux/riscv/Versions
+++ b/sysdeps/unix/sysv/linux/riscv/Versions
@@ -10,5 +10,6 @@ libc {
   }
   GLIBC_2.38 {
     __riscv_hwprobe;
+    __riscv_hwprobe_weak;
   }
 }
diff --git a/sysdeps/unix/sysv/linux/riscv/hwprobe.c b/sysdeps/unix/sysv/linux/riscv/hwprobe.c
index 14f7136998..a8bd63fca4 100644
--- a/sysdeps/unix/sysv/linux/riscv/hwprobe.c
+++ b/sysdeps/unix/sysv/linux/riscv/hwprobe.c
@@ -29,3 +29,5 @@ int __riscv_hwprobe (struct riscv_hwprobe *pairs, size_t pair_count,
  /* The vDSO may be able to provide the answer without a syscall. */
   return INLINE_VSYSCALL(riscv_hwprobe, 5, pairs, pair_count, cpu_count, cpus, flags);
 }
+
+weak_alias(__riscv_hwprobe, __riscv_hwprobe_weak)
diff --git a/sysdeps/unix/sysv/linux/riscv/hwprobe_static.c b/sysdeps/unix/sysv/linux/riscv/hwprobe_static.c
new file mode 100644
index 0000000000..b1c0f4089f
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/riscv/hwprobe_static.c
@@ -0,0 +1,36 @@
+/* RISC-V hardware feature probing support on Linux, ifunc-compatible
+   Copyright (C) 2023 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/>.  */
+
+#include <sys/syscall.h>
+#include <sys/hwprobe.h>
+#include <sysdep.h>
+#include <sysdep-vdso.h>
+
+attribute_hidden
+int __riscv_hwprobe_early (struct riscv_hwprobe *pairs, size_t pair_count,
+			   size_t cpu_count, unsigned long int *cpus,
+			   unsigned int flags)
+{
+  if (!__riscv_hwprobe_weak) {
+    /* Use INTERNAL_SYSCALL since errno is not available. */
+    return INTERNAL_SYSCALL(riscv_hwprobe, 5, pairs, pair_count, cpu_count, cpus, flags);
+  }
+
+  return __riscv_hwprobe_weak(pairs, pair_count, cpu_count, cpus, flags);
+}
diff --git a/sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist b/sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist
index 8fab4a606f..942e397be0 100644
--- a/sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist
@@ -2437,3 +2437,4 @@ GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
 GLIBC_2.38 __riscv_hwprobe F
+GLIBC_2.38 __riscv_hwprobe_weak F
diff --git a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
index 1ebb91deed..bc140fa7df 100644
--- a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
@@ -2637,3 +2637,4 @@ GLIBC_2.38 strlcpy F
 GLIBC_2.38 wcslcat F
 GLIBC_2.38 wcslcpy F
 GLIBC_2.38 __riscv_hwprobe F
+GLIBC_2.38 __riscv_hwprobe_weak F
diff --git a/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h b/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
index b27af5cb07..d739371806 100644
--- a/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
+++ b/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
@@ -67,6 +67,27 @@ extern int __riscv_hwprobe (struct riscv_hwprobe *pairs, size_t pair_count,
      __fortified_attr_access (__read_write__, 1, 2)
      __fortified_attr_access (__read_only__, 4, 3);
 
+extern int __riscv_hwprobe_weak (struct riscv_hwprobe *pairs, size_t pair_count,
+				 size_t cpu_count, unsigned long int *cpus,
+				 unsigned int flags)
+     __THROW __nonnull ((1)) __wur
+     __fortified_attr_access (__read_write__, 1, 2)
+     __fortified_attr_access (__read_only__, 4, 3);
+
+# ifdef weak_extern
+weak_extern (__riscv_hwprobe_weak)
+# else
+#  pragma weak __riscv_hwprobe_weak
+# endif
+
+extern int __riscv_hwprobe_early (struct riscv_hwprobe *pairs, size_t pair_count,
+				  size_t cpu_count, unsigned long int *cpus,
+				  unsigned int flags)
+     __THROW __nonnull ((1)) __wur
+     __fortified_attr_access (__read_write__, 1, 2)
+     __fortified_attr_access (__read_only__, 4, 3)
+     __attribute__ ((__visibility__ ("hidden")));
+
 __END_DECLS
 
 #endif /* sys/hwprobe.h */
-- 
2.34.1