Re: [PATCH] Cygwin: introduce close_range

[Christian Franke <Christian.Franke@t-online.de>]

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Corinna Vinschen wrote:
> Hi Christian,
>
> On Jan 15 09:56, Christian Franke wrote:
>> Christian Franke wrote:
>>> Jon Turney wrote:
>>>> On 14/01/2024 16:07, Christian Franke wrote:
>>>>> Recently I learned about the existence and usefulness of close_range():
>>>>> https://github.com/smartmontools/smartmontools/issues/235
>>>>>
>>>>> https://man.freebsd.org/cgi/man.cgi?query=close_range&sektion=2
>>>>> https://man7.org/linux/man-pages/man2/close_range.2.html
>>>>>
>>>>> Note that the above Linux man page is not fully correct. The
>>>>> include file "linux/close_range.h" exists, but provides only the
>>>>> defines. It is sufficient to include "unistd.h" as on FreeBSD.
>>>>>
>>>>> The attached patch adds this to Cygwin. It does not implement
>>>>> the Linux-specific CLOSE_RANGE_UNSHARE as I have no idea how to
>>>>> do this :-)
>>>> This API should also be mentioned in the
>>>> "System interfaces compatible with GNU or Linux extensions" section
>>>> of doc/posix.xml
>>>>
>>>>
>>> Thanks for the info. I used the recent "Cygwin: introduce fallocate(2)"
>>> patch as a blueprint for which other files should be changed (fallocate
>>> is also missing in the posix.xml file).
>>>
>>> I will provide a new patch soon which also fixes an unlikely but
>>> possible corner case: Pass a value larger than MAX_INT as lower limit.
>>>
>> Attached. I also decided to simply ignore CLOSE_RANGE_UNSHARE for now.
> After reading up on this issue, I think we should not ignore
> CLOSE_RANGE_UNSHARE, but quite explicitely not implement it as a valid
> flag.
>
> The whole idea behind CLOSE_RANGE_UNSHARE depends on the way the Linux
> kernel creates threads and (forked) processes and the fact that it has a
> wide range of ways to share parts of the execution context between
> parent and child process/thread.
>
> So on Linux, a process/thread can actually decide if they share or not
> share the descriptor table with the created process/thread.  That's
> what the CLONE_FILES flag to clone(2) and unshare(2) manage.
>
> However, just as in FreeBSD, there's no such thing in Cygwin.  Threads
> always share descriptors, processes never share file desriptors.
>
> The bottom line is, I think the decision of the FreeBSD developer not to
> expose the CLOSE_RANGE_UNSHARE flag at all, was the right decision.
>
> We should not claim that we even remotely have a way of doing this
> the Linux way.
>
> Does that make sense?

Yes - new patch attached.


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From 3d538a35732b1e8cd3d7e7921e06dfdffc6d28db Mon Sep 17 00:00:00 2001
From: Christian Franke <christian.franke@t-online.de>
Date: Mon, 15 Jan 2024 12:13:30 +0100
Subject: [PATCH] Cygwin: introduce close_range(2)

This function closes or sets the close-on-exec flag for a specified
range of file descriptors.  It is available on FreeBSD and Linux.

Signed-off-by: Christian Franke <christian.franke@t-online.de>
---
 newlib/libc/include/sys/unistd.h       |  6 ++++
 winsup/cygwin/cygwin.din               |  1 +
 winsup/cygwin/include/cygwin/version.h |  3 +-
 winsup/cygwin/release/3.5.0            |  2 ++
 winsup/cygwin/syscalls.cc              | 42 ++++++++++++++++++++++++++
 winsup/doc/new-features.xml            |  4 +++
 winsup/doc/posix.xml                   |  5 +++
 7 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/newlib/libc/include/sys/unistd.h b/newlib/libc/include/sys/unistd.h
index 25532251c..00901540f 100644
--- a/newlib/libc/include/sys/unistd.h
+++ b/newlib/libc/include/sys/unistd.h
@@ -26,6 +26,12 @@ int     chown (const char *__path, uid_t __owner, gid_t __group);
 int     chroot (const char *__path);
 #endif
 int     close (int __fildes);
+#if defined(__CYGWIN__) && (__BSD_VISIBLE || __GNU_VISIBLE)
+/* Available on FreeBSD (__BSD_VISIBLE) and Linux (__GNU_VISIBLE). */
+int     close_range (unsigned int __firstfd, unsigned int __lastfd, int __flags);
+/*      CLOSE_RANGE_UNSHARE (1 << 1) */ /* Linux-specific, not supported. */
+#define CLOSE_RANGE_CLOEXEC (1 << 2)
+#endif
 #if __POSIX_VISIBLE >= 199209
 size_t	confstr (int __name, char *__buf, size_t __len);
 #endif
diff --git a/winsup/cygwin/cygwin.din b/winsup/cygwin/cygwin.din
index 9b76ce67a..9e354acc6 100644
--- a/winsup/cygwin/cygwin.din
+++ b/winsup/cygwin/cygwin.din
@@ -347,6 +347,7 @@ clog10l NOSIGFE
 clogf NOSIGFE
 clogl NOSIGFE
 close SIGFE
+close_range SIGFE
 closedir SIGFE
 closelog SIGFE
 cnd_broadcast SIGFE
diff --git a/winsup/cygwin/include/cygwin/version.h b/winsup/cygwin/include/cygwin/version.h
index c8177c2b1..3036878c4 100644
--- a/winsup/cygwin/include/cygwin/version.h
+++ b/winsup/cygwin/include/cygwin/version.h
@@ -484,12 +484,13 @@ details. */
   347: Add c16rtomb, c32rtomb, mbrtoc16, mbrtoc32.
   348: Add c8rtomb, mbrtoc.
   349: Add fallocate.
+  350: Add close_range.
 
   Note that we forgot to bump the api for ualarm, strtoll, strtoull,
   sigaltstack, sethostname. */
 
 #define CYGWIN_VERSION_API_MAJOR 0
-#define CYGWIN_VERSION_API_MINOR 349
+#define CYGWIN_VERSION_API_MINOR 350
 
 /* There is also a compatibity version number associated with the shared memory
    regions.  It is incremented when incompatible changes are made to the shared
diff --git a/winsup/cygwin/release/3.5.0 b/winsup/cygwin/release/3.5.0
index d0a6c2fc8..6209064a6 100644
--- a/winsup/cygwin/release/3.5.0
+++ b/winsup/cygwin/release/3.5.0
@@ -43,6 +43,8 @@ What's new:
 
 - New API calls: c8rtomb, c16rtomb, c32rtomb, mbrtoc8, mbrtoc16, mbrtoc32.
 
+- New API call: close_range (available on FreeBSD and Linux).
+
 - New API call: fallocate (Linux-specific).
 
 - Implement OSS-based sound mixer device (/dev/mixer).
diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 486db1db6..9d88b60b0 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -85,6 +85,48 @@ close_all_files (bool norelease)
   cygheap->fdtab.unlock ();
 }
 
+/* Close or set the close-on-exec flag for all open file descriptors
+   from firstfd to lastfd.  CLOSE_RANGE_UNSHARE is not supported.
+   Available on FreeBSD since 13 and Linux since 5.9 */
+extern "C" int
+close_range (unsigned int firstfd, unsigned int lastfd, int flags)
+{
+  pthread_testcancel ();
+
+  if (!(firstfd <= lastfd && !(flags & ~CLOSE_RANGE_CLOEXEC)))
+    {
+      set_errno (EINVAL);
+      return -1;
+    }
+
+  cygheap->fdtab.lock ();
+
+  unsigned int size = (lastfd < cygheap->fdtab.size ? lastfd + 1 :
+		      cygheap->fdtab.size);
+
+  for (unsigned int i = firstfd; i < size; i++)
+    {
+      cygheap_fdget cfd ((int) i, false, false);
+      if (cfd < 0)
+	continue;
+
+      if (flags & CLOSE_RANGE_CLOEXEC)
+	{
+	  syscall_printf ("set FD_CLOEXEC on fd %u", i);
+	  cfd->fcntl (F_SETFD, FD_CLOEXEC);
+	}
+      else
+	{
+	  syscall_printf ("closing fd %u", i);
+	  cfd->close_with_arch ();
+	  cfd.release ();
+	}
+    }
+
+  cygheap->fdtab.unlock ();
+  return 0;
+}
+
 extern "C" int
 dup (int fd)
 {
diff --git a/winsup/doc/new-features.xml b/winsup/doc/new-features.xml
index 6ae420031..0abe1c41c 100644
--- a/winsup/doc/new-features.xml
+++ b/winsup/doc/new-features.xml
@@ -74,6 +74,10 @@ posix_spawn_file_actions_addfchdir_np.
 New API calls: c8rtomb, c16rtomb, c32rtomb, mbrtoc8, mbrtoc16, mbrtoc32.
 </para></listitem>
 
+<listitem><para>
+New API call: close_range (available on FreeBSD and Linux).
+</para></listitem>
+
 <listitem><para>
 New API call: fallocate (Linux-specific).
 </para></listitem>
diff --git a/winsup/doc/posix.xml b/winsup/doc/posix.xml
index 1a4eee1ab..89056915b 100644
--- a/winsup/doc/posix.xml
+++ b/winsup/doc/posix.xml
@@ -1143,6 +1143,7 @@ also IEEE Std 1003.1-2017 (POSIX.1-2017).</para>
     cfmakeraw
     cfsetspeed
     clearerr_unlocked
+    close_range
     daemon
     dn_comp
     dn_expand
@@ -1297,6 +1298,7 @@ also IEEE Std 1003.1-2017 (POSIX.1-2017).</para>
     clog10
     clog10f
     clog10l
+    close_range			(see <xref linkend="std-notes">chapter "Implementation Notes"</xref>)
     crypt_r			(available in external "crypt" library)
     dladdr			(see <xref linkend="std-notes">chapter "Implementation Notes"</xref>)
     dremf
@@ -1656,6 +1658,9 @@ CLOCK_REALTIME and CLOCK_MONOTONIC.  <function>clock_setres</function>,
 <function>clock_settime</function>, and <function>timer_create</function>
 currently support only CLOCK_REALTIME.</para>
 
+<para><function>close_range</function> does not support the Linux-specific
+flag CLOSE_RANGE_UNSHARE.</para>
+
 <para>POSIX file locks via <function>fcntl</function> or
 <function>lockf</function>, as well as BSD <function>flock</function> locks
 are advisory locks.  They don't interact with Windows mandatory locks, nor
-- 
2.42.1