[PATCH dwarves 2/2] tests: Add test for parameters described by entry values

[Yonghong Song <yonghong.song@linux.dev>]

Cover the case where clang leaves the entry PC out of a parameter's
location list because the parameter is moved into a callee-saved register
during the prologue.  f_lazy() reproduces the shape found in the kernel:
@lazy arrives in RSI, the first location list entry starts past the
function entry and names the callee-saved register, and only the
DW_OP_entry_value entries name RSI.

Without the entry value being preferred, @lazy looks like it is in an
unexpected register and f_lazy() is dropped from BTF altogether.

The test results on x86_64:
  $ VERBOSE=1 ./clang_parm_entry_value.sh
  Validation of BTF encoding of parameters described by entry values.
     BTF: bool f_lazy(struct task * p, bool lazy, const struct aux  * aux);
  Test ./clang_parm_entry_value.sh passed

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tests/clang_parm_entry_value.sh | 84 +++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100755 tests/clang_parm_entry_value.sh

diff --git a/tests/clang_parm_entry_value.sh b/tests/clang_parm_entry_value.sh
new file mode 100755
index 0000000..9fe81a7
--- /dev/null
+++ b/tests/clang_parm_entry_value.sh
@@ -0,0 +1,84 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-only
+
+source "$(dirname "$0")/test_lib.sh"
+
+outdir=$(make_tmpdir)
+
+# Comment this out to save test data.
+trap cleanup EXIT
+
+title_log "Validation of BTF encoding of parameters described by entry values."
+
+entry_value="${outdir}/entry_value"
+CC=$(which clang 2>/dev/null)
+
+if [[ -z "$CC" ]]; then
+	info_log "skip: clang not available"
+	test_skip
+fi
+
+arch=$(uname -m)
+if [[ "$arch" != "x86_64" ]]; then
+	info_log "skip: test is x86_64 only, running on $arch"
+	test_skip
+fi
+
+# clang can leave the entry PC out of a parameter's location list when the
+# parameter is moved into a callee-saved register during the prologue.  The
+# first location list entry then names that callee-saved register rather than
+# the ABI argument register, and only the DW_OP_entry_value entries tell which
+# argument register the parameter really arrived in.  pahole has to use the
+# entry value, otherwise the parameter looks like it is in an unexpected
+# register and the whole function is dropped from BTF.
+cat > ${entry_value}.c << EOF
+typedef _Bool bool;
+struct sched;
+struct aux;
+struct task { long a; bool lazy; };
+
+extern struct sched *get_sched(const struct aux *aux);
+extern void rcu_lock(void);
+extern void rcu_unlock(void);
+extern int on_sched(struct sched *s, struct task *p);
+
+__attribute__((noinline)) bool f_lazy(struct task *p, bool lazy, const struct aux *aux)
+{
+	struct sched *sch;
+
+	rcu_lock();
+	sch = get_sched(aux);
+	if (__builtin_expect(!(sch && on_sched(sch, p)), 0)) {
+		rcu_unlock();
+		return 0;
+	}
+	__atomic_store_n(&p->lazy, lazy, __ATOMIC_RELAXED);
+	rcu_unlock();
+	return 1;
+}
+
+bool (*keep[])() = { (bool(*)())f_lazy };
+EOF
+
+${CC} -g -O2 -c -o ${entry_value}.o ${entry_value}.c 2>/dev/null
+if [[ $? -ne 0 ]]; then
+	info_log "skip: clang could not compile ${entry_value}.c"
+	test_skip
+fi
+
+LLVM_OBJCOPY=objcopy pahole -J --btf_features=consistent_func ${entry_value}.o
+if [[ $? -ne 0 ]]; then
+	error_log "Could not encode BTF for ${entry_value}.o"
+	test_fail
+fi
+
+encoded=$(pfunct --all --format_path=btf ${entry_value}.o | grep " f_lazy(")
+if [[ -n "$VERBOSE" ]]; then
+	printf "   BTF: %s\n" "$encoded"
+fi
+if [[ -z "$encoded" ]]; then
+	error_log "f_lazy() is missing from BTF"
+	test_fail
+fi
+
+test_pass
-- 
2.53.0-Meta