[kvm-unit-tests PATCH v3 20/20] x86: Prevent realmode test code instrumentation with nop-mcount

[Sean Christopherson <seanjc@google.com>]

From: Mathias Krause <minipli@grsecurity.net>

Commit f01ea38a385a ("x86: Better backtraces for leaf functions") made
use of '-pg -mnop-mcount' to provide a lightweight way to force leaf
functions to emit a proper prologue for the backtracing code. However,
-mnop-mcount doesn't play well with 16-bit code generation for C code.
gcc happily emits a 5-byte NOP that transmutes to a 4-byte NOP followed
by a zero byte when executed in real mode, wrecking all code that
follows.

Fix that by selectively disabling '-mnop-mcount' for realmode.c, making
it call mcount(), which is provided as a stub function.

Note, a fix for the bad gcc behavior has been queued for gcc-16, i.e.
this workaround can be dropped when gcc-16 is the minimal supported
version for KUT (so in about 30 years).

Link: https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=114a19fae9bd [1]
Reported-by: Sean Christopherson <seanjc@google.com>
Fixes: f01ea38a385a ("x86: Better backtraces for leaf functions")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
[sean: add note regarding gcc bug]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 x86/Makefile.common | 5 ++++-
 x86/realmode.c      | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/x86/Makefile.common b/x86/Makefile.common
index f7e3ba78..f5cbd9cf 100644
--- a/x86/Makefile.common
+++ b/x86/Makefile.common
@@ -51,7 +51,9 @@ ifneq ($(KEEP_FRAME_POINTER),)
 # We need to defer the cc-option test until -fno-pic or -no-pie have been
 # added to CFLAGS as -mnop-mcount needs it. The lazy evaluation of CFLAGS
 # during compilation makes this do "The Right Thing."
-LATE_CFLAGS += $(call cc-option, -pg -mnop-mcount, "")
+NOP_PGFLAGS := -pg -mnop-mcount
+LATE_CFLAGS += $(call cc-option, $(NOP_PGFLAGS), "")
+NO_NOP_MCOUNT = $(if $(filter $(NOP_PGFLAGS),$(LATE_CFLAGS)),-mno-nop-mcount)
 endif
 
 FLATLIBS = lib/libcflat.a
@@ -123,6 +125,7 @@ $(TEST_DIR)/realmode.elf: $(TEST_DIR)/realmode.o $(SRCDIR)/$(TEST_DIR)/realmode.
 	      -T $(SRCDIR)/$(TEST_DIR)/realmode.lds $(filter %.o, $^)
 
 $(TEST_DIR)/realmode.o: bits = $(realmode_bits)
+$(TEST_DIR)/realmode.o: CFLAGS += $(NO_NOP_MCOUNT)
 
 $(TEST_DIR)/access_test.$(bin): $(TEST_DIR)/access.o
 
diff --git a/x86/realmode.c b/x86/realmode.c
index 7a4423ec..0a7104d4 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -23,6 +23,9 @@ void test_function(void);
 asm(
 	"test_function: \n\t"
 	"mov $0x1234, %eax \n\t"
+	"ret\n\t"
+	/* mcount() stub */
+	"mcount:\n\t"
 	"ret"
    );
 
-- 
2.54.0.563.g4f69b47b94-goog