Re: semtimedop, powerpc, time64 and older kernels

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 30/09/2020 15:29, Matheus Castanho wrote:
> Also, looks like my email client messed up the diff *sigh*. I'm sending
> a proper patch attached this time.
> 
> --
> Matheus Castanho
> 

> From 1c0a497a3f986bc6980581c9eab482ccf7bb190f Mon Sep 17 00:00:00 2001
> From: Matheus Castanho <msc@linux.ibm.com>
> Date: Wed, 30 Sep 2020 14:22:18 -0300
> Subject: [PATCH] sysvipc: Fix semtimedop for Linux < 5.1
> 
> Kernels older than 5.1 will fail with ENOSYS when calling
> semtimedop_time64 syscall in __semtimedop_time64. Just like for
> !__ASSUME_TIME64_SYSCALLS, we should fallback to using the old mechanism
> in such cases.
> ---
>  sysdeps/unix/sysv/linux/semtimedop.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/semtimedop.c b/sysdeps/unix/sysv/linux/semtimedop.c
> index a9ad922ee2..510fea1852 100644
> --- a/sysdeps/unix/sysv/linux/semtimedop.c
> +++ b/sysdeps/unix/sysv/linux/semtimedop.c
> @@ -32,7 +32,7 @@ __semtimedop64 (int semid, struct sembuf *sops, size_t nsops,
>    int r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops,
>  			       timeout);
>  
> -#ifndef __ASSUME_TIME64_SYSCALLS
> +#if !(defined __ASSUME_TIME64_SYSCALLS) || __LINUX_KERNEL_VERSION < 0x050100
>    if (r == 0 || errno != ENOSYS)
>      return r;
>  
> -- 
> 2.26.2

Thanks for catching it and although it fixes the regression, we have 
kernel-features.h exactly to avoid using __LINUX_KERNEL_VERSION through the
implementations. Also this is sub-optimal since it forces semtimeopd issues
__NR_semtimeop and then __NR_ipc on powerpc64 and we have 
__ASSUME_DIRECT_SYSVIPC_SYSCALLS exactly to avoid this strategy of handling 
ENOSYS for newer syscalls and thus slowing it down the implementation on 
older kernels (--enable-kernel exists exactly to get rid of this older kernel
support).

I forgot that powerpc64 and s390x used the older multiplexed __NR_ipc and
kernel v5.1 decided to add proper __NR_semtimedop (and it was in fact handled
by 720e9541f5d919).  I think a better fix is the one below, since it:

  1. Issues __NR_semtimeop_time64 iff it is defined (32-bit architectures).
  2. Issues __NR_semtimeop otherwise iff glibc is configured for a kernel that
     supports it (for powerpc64 it will only for --enable-kernel=5.1). 
     Otherwise it will use only 3.
  3. Issues __NR_ipc with IPCOP_semtimedop.

For powerpc64 it will issue either __NR_ipc (default) or __NR_semtimeop
(--enable-kernel=5.1), while for powerpc it will use either
__NR_semtimeop_time64 and fallback to __NR_ipc or just issue
__NR_semtimeop_time64.

I am running some regressions before commit it.

---

diff --git a/sysdeps/unix/sysv/linux/semtimedop.c b/sysdeps/unix/sysv/linux/semtimedop.c
index a9ad922ee2..29647f8ccd 100644
--- a/sysdeps/unix/sysv/linux/semtimedop.c
+++ b/sysdeps/unix/sysv/linux/semtimedop.c
@@ -26,11 +26,15 @@ int
 __semtimedop64 (int semid, struct sembuf *sops, size_t nsops,
                const struct __timespec64 *timeout)
 {
-#ifndef __NR_semtimedop_time64
-# define __NR_semtimedop_time64 __NR_semtimedop
+  int r;
+#if defined __NR_semtimedop_time64
+  r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops, timeout);
+#elif defined __ASSUME_DIRECT_SYSVIPC_SYSCALLS && defined __NR_semtimedop
+  r = INLINE_SYSCALL_CALL (semtimedop, semid, sops, nsops, timeout);
+#else
+  r = INLINE_SYSCALL_CALL (ipc, IPCOP_semtimedop, semid,
+                          SEMTIMEDOP_IPC_ARGS (nsops, sops, timeout));
 #endif
-  int r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops,
-                              timeout);
 
 #ifndef __ASSUME_TIME64_SYSCALLS
   if (r == 0 || errno != ENOSYS)