[PATCH v4 55/58] objtool/klp: Add test for absolute and __ADDRESSABLE symbols

[Song Liu <song@kernel.org>]

A SHN_ABS symbol has no section, so any walk of sym->sec which does not
check dereferences NULL, and the kernel has plenty of them -- from linker
scripts and from .set in assembly.  __ADDRESSABLE() emits a pointer into
.discard.addressable purely to keep a symbol referenced; it means nothing
to a livepatch and is discarded at link time, but it is a relocation like
any other and gets looked at.

Neither is what the patch changes.  What this guards against is not a wrong
answer but a crash or an error on input the kernel produces routinely,
which would make every function near one unpatchable.

Not isolated to a single line, and the test says so: the absolute symbol
here has zero length, so it is excluded before the section check is reached
and removing that check alone changes nothing observable.

Assisted-by: Claude:claude-opus-4
Based-on-test-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Song Liu <song@kernel.org>
---
 .../generic/fixtures/abs_and_addressable.c    | 44 ++++++++++++++++
 .../tests/generic/test-abs-and-addressable.sh | 50 +++++++++++++++++++
 2 files changed, 94 insertions(+)
 create mode 100644 tools/objtool/tests/generic/fixtures/abs_and_addressable.c
 create mode 100755 tools/objtool/tests/generic/test-abs-and-addressable.sh

diff --git a/tools/objtool/tests/generic/fixtures/abs_and_addressable.c b/tools/objtool/tests/generic/fixtures/abs_and_addressable.c
new file mode 100644
index 000000000000..6392ff99af42
--- /dev/null
+++ b/tools/objtool/tests/generic/fixtures/abs_and_addressable.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Two constructs which appear all over the kernel and must not upset klp
+ * checksum or klp diff.
+ *
+ * An absolute symbol (SHN_ABS) has no section, so anything walking sym->sec
+ * without checking dereferences NULL.  The kernel makes them with linker
+ * scripts and with .set in asm; VDSO and the fixed-address per-cpu bases are
+ * the usual sources.
+ *
+ * __ADDRESSABLE() emits a pointer into .discard.addressable purely to keep a
+ * symbol referenced.  It is discarded at link time and means nothing to a
+ * livepatch, but the pointer is a relocation like any other and has to survive
+ * being looked at.
+ *
+ * Neither is the subject of the patch; the point is that their presence does
+ * not disturb the function that is.
+ */
+
+static const char __modinfo[]
+	__attribute__((section(".modinfo"), used, aligned(1))) = "\0name=vmlinux";
+
+/* SHN_ABS, referenced from code. */
+extern char abs_sym[];
+__asm__(".globl abs_sym\n"
+	".set abs_sym, 0x1234\n");
+
+int helper(int x);
+int helper(int x) { return x + 1; }
+
+/* The shape of __ADDRESSABLE(helper). */
+__asm__(".pushsection .discard.addressable, \"aw\"\n"
+	".balign 8\n"
+	".quad helper\n"
+	".popsection\n");
+
+int target(int x)
+{
+#ifdef PATCHED
+	return helper(x) + (int)(long)abs_sym + 1;
+#else
+	return helper(x) + (int)(long)abs_sym;
+#endif
+}
diff --git a/tools/objtool/tests/generic/test-abs-and-addressable.sh b/tools/objtool/tests/generic/test-abs-and-addressable.sh
new file mode 100755
index 000000000000..6adb23ed4b88
--- /dev/null
+++ b/tools/objtool/tests/generic/test-abs-and-addressable.sh
@@ -0,0 +1,50 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# An absolute symbol and an __ADDRESSABLE() pointer must not disturb the
+# function being patched.
+#
+# A SHN_ABS symbol has no section, so any walk of sym->sec which does not check
+# dereferences NULL -- and the kernel has plenty, from linker scripts and from
+# .set in assembly.  __ADDRESSABLE() emits a pointer into .discard.addressable
+# to keep a symbol referenced; it means nothing to a livepatch and is discarded
+# at link time, but it is a relocation like any other and gets looked at.
+#
+# Neither is what the patch changes.  The failure this guards against is not a
+# wrong answer but a crash or an error on input the kernel produces routinely,
+# which would make any function near one unpatchable.
+#
+# Not isolated to a single guard: the absolute symbol here has zero length, so
+# it is excluded before the section check is reached and removing that check
+# alone changes nothing observable.  This stands as a check on the behaviour
+# rather than on the line which produces it.
+#
+# Covers the same ground as corpus/x86_64/checksum-abs-sym-skip and
+# addressable-symbols in Joe Lawrence's klp-build unit test corpus.
+
+. "$(dirname "$0")/../lib.sh"
+
+setup
+build_pair abs_and_addressable.c
+
+# The premise: the fixture really did produce both.
+in_symbols orig.o | grep -q 'ABS.*abs_sym' ||
+	probe_skip "assembler did not make abs_sym absolute here"
+assert_input_section .discard.addressable
+
+# Checksumming has to survive them, and still see the function that changed.
+run_checksum
+assert_checksum_differs target
+assert_checksum_matches helper
+
+# So does the diff.
+run_diff
+assert_patched target
+assert_not_patched helper
+
+# An absolute symbol has no address to record a checksum against, so it gets
+# no entry -- the reference to it is what mattered, not the symbol itself.
+in_relocs orig.o | awk '/rela\.discard\.sym_checksum/,/^$/' | grep -qw abs_sym &&
+	fail "absolute symbol got a checksum entry"
+
+pass "absolute and __ADDRESSABLE symbols do not disturb the patched function"
-- 
2.53.0-Meta