linux: Add openat2 (BZ 31664)HEADmaster

The openat2 syscall was added on Linux 5.6, as an extension of openat.
Unlike other open-like functions, the kernel only provides the LFS
variant (so files larger than 4GB always succeed, unlike other
functions with an offset larger than off_t).  Also, similar to other
open functions, the new symbol is a cancellable entrypoint.

The test case added only stress tests for some of the syscalls' provided
functionality, and it is based on an existing kernel self-test.

A fortify wrapper is added to verify the argument size if not larger
than the current support open_how struct.

Gnulib added an openat2 module, which uses read-only for the open_how
argument [1].  There is no clear indication whether the kernel will
indeed use the argument as in-out, how it would do so, or for which
kind of functionality [2]. Also, adding a potentially different prototype
than gnulib only would add extra unnecessary friction and extra
wrappers to handle it.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.

[1] https://gitweb.git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commit;h=0b97ffdf32bdab909d02449043447237273df75e
[2] https://sourceware.org/pipermail/libc-alpha/2025-September/169740.html

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index b70dc27..6964ca0 100644
--- a/NEWS
+++ b/NEWS
@@ -54,6 +54,10 @@ Major new features:
   targets, and libgcc compatible runtime (including libgcc_s.so for
   pthread cancellation and backtrace runtime support).
 
+* On Linux, the openat2 function has been added.  It is an extension of
+  openat and provides a superset of its functionality.  It is supported only
+  in LFS mode and it is a cancellable entrypoint.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * Support for dumped heaps has been removed - malloc_set_state() now always