objtool: Validate disassembler headers in libopcodes probegrafted

commit 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
tested for libopcodes availability by linking a test snippet with a forward
declaration of disassemble_init_for_target().

However, testing symbol linkage with an extern declaration only verifies
the presence of the library (.so/.a) and bypasses checking for development
headers (binutils-dev). On systems where libopcodes is present without
development headers installed, the probe succeeds, enabling BUILD_DISAS.
Subsequent compilation of objtool then fails:

  fatal error: 'bfd.h' file not found
    113 | #include <bfd.h>

Additionally, the probe invokes $(HOSTCC) without $(HOSTCFLAGS), ignoring
any sysroot or include flags specified for the host compiler.

Fix this by including <bfd.h> and <dis-asm.h> directly in the test snippet,
passing $(HOSTCFLAGS) so host compiler options are respected, and defining
PACKAGE="objtool" to satisfy the configuration check in <bfd.h>.

Fixes: 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
Fixes: 436326bc525d ("objtool: fix build failure due to missing libopcodes check")
Reported-by: Alice Ryhl <aliceryhl@google.com>
Assisted-by: Antigravity:Gemini-Next
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>
Link: https://patch.msgid.link/20260904150710.2997558-1-umendez@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

1 files changed, 86 insertions, 0 deletions

diff --git a/Documentation/userspace-api/futex2.rst b/Documentation/userspace-api/futex2.rst
new file mode 100644
index 000000000..9693f47a7
--- /dev/null
+++ b/Documentation/userspace-api/futex2.rst
@@ -0,0 +1,86 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======
+futex2
+======
+
+:Author: André Almeida <andrealmeid@collabora.com>
+
+futex, or fast user mutex, is a set of syscalls to allow userspace to create
+performant synchronization mechanisms, such as mutexes, semaphores and
+conditional variables in userspace. C standard libraries, like glibc, uses it
+as a means to implement more high level interfaces like pthreads.
+
+futex2 is a followup version of the initial futex syscall, designed to overcome
+limitations of the original interface.
+
+User API
+========
+
+``futex_waitv()``
+-----------------
+
+Wait on an array of futexes, wake on any::
+
+  futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes,
+              unsigned int flags, struct timespec *timeout, clockid_t clockid)
+
+  struct futex_waitv {
+        __u64 val;
+        __u64 uaddr;
+        __u32 flags;
+        __u32 __reserved;
+  };
+
+Userspace sets an array of struct futex_waitv (up to a max of 128 entries),
+using ``uaddr`` for the address to wait for, ``val`` for the expected value
+and ``flags`` to specify the type (e.g. private) and size of futex.
+``__reserved`` needs to be 0, but it can be used for future extension. The
+pointer for the first item of the array is passed as ``waiters``. An invalid
+address for ``waiters`` or for any ``uaddr`` returns ``-EFAULT``.
+
+If userspace has 32-bit pointers, it should do a explicit cast to make sure
+the upper bits are zeroed. ``uintptr_t`` does the tricky and it works for
+both 32/64-bit pointers.
+
+``nr_futexes`` specifies the size of the array. Numbers out of [1, 128]
+interval will make the syscall return ``-EINVAL``.
+
+The ``flags`` argument of the syscall needs to be 0, but it can be used for
+future extension.
+
+For each entry in ``waiters`` array, the current value at ``uaddr`` is compared
+to ``val``. If it's different, the syscall undo all the work done so far and
+return ``-EAGAIN``. If all tests and verifications succeeds, syscall waits until
+one of the following happens:
+
+- The timeout expires, returning ``-ETIMEOUT``.
+- A signal was sent to the sleeping task, returning ``-ERESTARTSYS``.
+- Some futex at the list was woken, returning the index of some waked futex.
+
+An example of how to use the interface can be found at ``tools/testing/selftests/futex/functional/futex_waitv.c``.
+
+Timeout
+-------
+
+``struct timespec *timeout`` argument is an optional argument that points to an
+absolute timeout. You need to specify the type of clock being used at
+``clockid`` argument. ``CLOCK_MONOTONIC`` and ``CLOCK_REALTIME`` are supported.
+This syscall accepts only 64bit timespec structs.
+
+Types of futex
+--------------
+
+A futex can be either private or shared. Private is used for processes that
+shares the same memory space and the virtual address of the futex will be the
+same for all processes. This allows for optimizations in the kernel. To use
+private futexes, it's necessary to specify ``FUTEX_PRIVATE_FLAG`` in the futex
+flag. For processes that doesn't share the same memory space and therefore can
+have different virtual addresses for the same futex (using, for instance, a
+file-backed shared memory) requires different internal mechanisms to be get
+properly enqueued. This is the default behavior, and it works with both private
+and shared futexes.
+
+Futexes can be of different sizes: 8, 16, 32 or 64 bits. Currently, the only
+supported one is 32 bit sized futex, and it need to be specified using
+``FUTEX_32`` flag.