diff mbox series

[v11,4/7] riscv: Add __riscv_hwprobe pointer to ifunc calls

Message ID 20240110205140.754849-5-evan@rivosinc.com
State Superseded
Headers
Series RISC-V: ifunced memcpy using new kernel hwprobe interface |

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Commit Message

Evan Green Jan. 10, 2024, 8:51 p.m. UTC 
  The new __riscv_hwprobe() function is designed to be used by ifunc
selector functions. This presents a challenge for applications and
libraries, as ifunc selectors are invoked before all relocations have
been performed, so an external call to __riscv_hwprobe() from an ifunc
selector won't work. To address this, pass a pointer to the
__riscv_hwprobe() function into ifunc selectors as the second
argument (alongside dl_hwcap, which was already being passed).

Include a typedef as well for convenience, so that ifunc users don't
have to go through contortions to call this routine. Users will need to
remember to check the second argument for NULL, to account for older
glibcs that don't pass the function.

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

---

(no changes since v10)

Changes in v10:
 - Hand in pointer to __riscv_hwprobe(), not vDSO function

Changes in v7:
 - Remove __THROW from function pointer type, as it creates warnings
   together with __fortified_attr_access.

 sysdeps/riscv/dl-irel.h                     |  9 +++++----
 sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h | 10 ++++++++++
 2 files changed, 15 insertions(+), 4 deletions(-)

Comments

Jessica Clarke Jan. 15, 2024, 10:21 p.m. UTC | #1 
O Wed, Jan 10, 2024 at 12:51:37PM -0800, Evan Green wrote:
> The new __riscv_hwprobe() function is designed to be used by ifunc
> selector functions. This presents a challenge for applications and
> libraries, as ifunc selectors are invoked before all relocations have
> been performed, so an external call to __riscv_hwprobe() from an ifunc
> selector won't work. To address this, pass a pointer to the
> __riscv_hwprobe() function into ifunc selectors as the second
> argument (alongside dl_hwcap, which was already being passed).
> 
> Include a typedef as well for convenience, so that ifunc users don't
> have to go through contortions to call this routine. Users will need to
> remember to check the second argument for NULL, to account for older
> glibcs that don't pass the function.
> 
> Signed-off-by: Evan Green <evan@rivosinc.com>

I still don't understand why you need to do this rather than just alter
relocation processing such that ifunc resolvers are called after other
relocations in the object have been processed, like FreeBSD, which would
obviate the need for the function pointer and allow any non-ifunced
function (that itself does not transitively call an ifunc not guaranteed
to have been resolved already), including __riscv_hwprobe itself, to be
called from the resolver. This avoids tying ifunc resolvers to a
specific interface for querying extensions, instead giving a general
solution that seems like a useful thing to have even outside of RISC-V.

I'll note that telling other OSes they should just pass in a dummy +1
value is a non-solution; other OSes don't want to have to add a
meaningless parameter for a Linux-specific interface.

So, please, I urge you to reconsider this interface before it forever
gets baked ino the ABI and the parameter can never be used for anything
else. It may fit your needs, but I do not think it is a good idea for
the wider ecosystem.

Jess
diff mbox series

Patch

diff --git a/sysdeps/riscv/dl-irel.h b/sysdeps/riscv/dl-irel.h
index e6ab51ccd4..61b3511c96 100644
--- a/sysdeps/riscv/dl-irel.h
+++ b/sysdeps/riscv/dl-irel.h
@@ -24,6 +24,7 @@ 
 #include <unistd.h>
 #include <ldsodefs.h>
 #include <sysdep.h>
+#include <sys/hwprobe.h>
 
 #define ELF_MACHINE_IRELA	1
 
@@ -31,10 +32,10 @@  static inline ElfW(Addr)
 __attribute ((always_inline))
 elf_ifunc_invoke (ElfW(Addr) addr)
 {
-  /* The second argument is a void pointer to preserve the extension
-     fexibility.  */
-  return ((ElfW(Addr) (*) (uint64_t, void *)) (addr))
-	 (GLRO(dl_hwcap), NULL);
+  /* The third argument is a void pointer to preserve the extension
+     flexibility.  */
+  return ((ElfW(Addr) (*) (uint64_t, void *, void *)) (addr))
+	 (GLRO(dl_hwcap), __riscv_hwprobe, NULL);
 }
 
 static inline void
diff --git a/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h b/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
index 5592b9e100..34a2e3dbc2 100644
--- a/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
+++ b/sysdeps/unix/sysv/linux/riscv/sys/hwprobe.h
@@ -69,6 +69,16 @@  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);
 
+/* A pointer to the __riscv_hwprobe vDSO function is passed as the second
+   argument to ifunc selector routines. Include a function pointer type for
+   convenience in calling the function in those settings. */
+typedef int (*__riscv_hwprobe_t) (struct riscv_hwprobe *__pairs, size_t __pair_count,
+				  size_t __cpu_count, unsigned long int *__cpus,
+				  unsigned int __flags)
+     __nonnull ((1)) __wur
+     __fortified_attr_access (__read_write__, 1, 2)
+     __fortified_attr_access (__read_only__, 4, 3);
+
 __END_DECLS
 
 #endif /* sys/hwprobe.h */