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, 96 insertions, 0 deletions

diff --git a/Documentation/maintainer/messy-diffstat.rst b/Documentation/maintainer/messy-diffstat.rst
new file mode 100644
index 000000000..c015f66d7
--- /dev/null
+++ b/Documentation/maintainer/messy-diffstat.rst
@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+Handling messy pull-request diffstats
+=====================================
+
+Subsystem maintainers routinely use ``git request-pull`` as part of the
+process of sending work upstream.  Normally, the result includes a nice
+diffstat that shows which files will be touched and how much of each will
+be changed.  Occasionally, though, a repository with a relatively
+complicated development history will yield a massive diffstat containing a
+great deal of unrelated work.  The result looks ugly and obscures what the
+pull request is actually doing.  This document describes what is happening
+and how to fix things up; it is derived from The Wisdom of Linus Torvalds,
+found in Linus1_ and Linus2_.
+
+.. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
+.. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/
+
+A Git development history proceeds as a series of commits.  In a simplified
+manner, mainline kernel development looks like this::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+
+If one wants to see what has changed between two points, a command like
+this will do the job::
+
+  $ git diff --stat --summary vN-rc2..vN-rc3
+
+Here, there are two clear points in the history; Git will essentially
+"subtract" the beginning point from the end point and display the resulting
+differences.  The requested operation is unambiguous and easy enough to
+understand.
+
+When a subsystem maintainer creates a branch and commits changes to it, the
+result in the simplest case is a history that looks like::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                          |
+                          +-- c1 --- c2 --- ... --- cN
+
+If that maintainer now uses ``git diff`` to see what has changed between
+the mainline branch (let's call it "linus") and cN, there are still two
+clear endpoints, and the result is as expected.  So a pull request
+generated with ``git request-pull`` will also be as expected.  But now
+consider a slightly more complex development history::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                |         |
+                |         +-- c1 --- c2 --- ... --- cN
+                |                   /
+                +-- x1 --- x2 --- x3
+
+Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the
+two were then subsequently merged into c2.  Now a pull request generated
+for cN may end up being messy indeed, and developers often end up wondering
+why.
+
+What is happening here is that there are no longer two clear end points for
+the ``git diff`` operation to use.  The development culminating in cN
+started in two different places; to generate the diffstat, ``git diff``
+ends up having pick one of them and hoping for the best.  If the diffstat
+starts at vN-rc1, it may end up including all of the changes between there
+and the second origin end point (vN-rc2), which is certainly not what our
+maintainer had in mind.  With all of that extra junk in the diffstat, it
+may be impossible to tell what actually happened in the changes leading up
+to cN.
+
+Maintainers often try to resolve this problem by, for example, rebasing the
+branch or performing another merge with the linus branch, then recreating
+the pull request.  This approach tends not to lead to joy at the receiving
+end of that pull request; rebasing and/or merging just before pushing
+upstream is a well-known way to get a grumpy response.
+
+So what is to be done?  The best response when confronted with this
+situation is to indeed to do a merge with the branch you intend your work
+to be pulled into, but to do it privately, as if it were the source of
+shame.  Create a new, throwaway branch and do the merge there::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                |         |                                      |
+                |         +-- c1 --- c2 --- ... --- cN           |
+                |                   /               |            |
+                +-- x1 --- x2 --- x3                +------------+-- TEMP
+
+The merge operation resolves all of the complications resulting from the
+multiple beginning points, yielding a coherent result that contains only
+the differences from the mainline branch.  Now it will be possible to
+generate a diffstat with the desired information::
+
+  $ git diff -C --stat --summary linus..TEMP
+
+Save the output from this command, then simply delete the TEMP branch;
+definitely do not expose it to the outside world.  Take the saved diffstat
+output and edit it into the messy pull request, yielding a result that
+shows what is really going on.  That request can then be sent upstream.