[kvm-unit-tests PATCH v3 19/20] x86: Better backtraces for leaf functions

[Sean Christopherson <seanjc@google.com>]

From: Mathias Krause <minipli@grsecurity.net>

Leaf functions are problematic for backtraces as they lack the frame
pointer setup epilogue. If such a function causes a fault, the original
caller won't be part of the backtrace. That's problematic if, for
example, memcpy() is failing because it got passed a bad pointer. The
generated backtrace will look like this, providing no clue what the
issue may be:

	STACK: @401b31 4001ad
  0x0000000000401b31: memcpy at lib/string.c:136 (discriminator 3)
        	for (i = 0; i < n; ++i)
      > 		a[i] = b[i];

  0x00000000004001ac: gdt32_end at x86/cstart64.S:127
        	lea __environ(%rip), %rdx
      > 	call main
        	mov %eax, %edi

By abusing profiling, we can force the compiler to emit a frame pointer
setup epilogue even for leaf functions, making the above backtrace
change like this:

	STACK: @401c21 400512 4001ad
  0x0000000000401c21: memcpy at lib/string.c:136 (discriminator 3)
        	for (i = 0; i < n; ++i)
      > 		a[i] = b[i];

  0x0000000000400511: main at x86/hypercall.c:91 (discriminator 24)

      > 	memcpy((void *)~0xbadc0de, (void *)0xdeadbeef, 42);

  0x00000000004001ac: gdt32_end at x86/cstart64.S:127
        	lea __environ(%rip), %rdx
      > 	call main
        	mov %eax, %edi

Above backtrace includes the failing memcpy() call, making it much
easier to spot the bug.

Enable "fake profiling" if supported by the compiler to get better
backtraces. The runtime overhead should be negligible for the gained
debugability as the profiling call is actually a NOP.

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Tested-by: Andrew Jones <andrew.jones@linux.dev>
Fixes: f01ea38a385a ("x86: Better backtraces for leaf functions")
Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 x86/Makefile.common | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/x86/Makefile.common b/x86/Makefile.common
index ef0e09a6..f7e3ba78 100644
--- a/x86/Makefile.common
+++ b/x86/Makefile.common
@@ -43,6 +43,17 @@ COMMON_CFLAGS += -O1
 # stack.o relies on frame pointers.
 KEEP_FRAME_POINTER := y
 
+ifneq ($(KEEP_FRAME_POINTER),)
+# Fake profiling to force the compiler to emit a frame pointer setup also in
+# leaf function (-mno-omit-leaf-frame-pointer doesn't work, unfortunately).
+#
+# Note:
+# 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, "")
+endif
+
 FLATLIBS = lib/libcflat.a
 
 ifeq ($(CONFIG_EFI),y)
-- 
2.54.0.563.g4f69b47b94-goog