um: Support kcov

This adds support for kcov to UML.

There is a small problem where UML will randomly segfault during boot;
this is because current_thread_info() occasionally returns an invalid
(non-NULL) pointer and we try to dereference it in
__sanitizer_cov_trace_pc(). I consider this a bug in UML itself and this
patch merely exposes it.

[v2: disable instrumentation in UML-specific code]

Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Thomas Meyer <thomas@m3y3r.de>
Cc: user-mode-linux-devel <user-mode-linux-devel@lists.sourceforge.net>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

3 files changed, 9 insertions, 0 deletions

diff --git a/arch/um/Kconfig.common b/arch/um/Kconfig.common
index 08d931c98621..f8530d866c9f 100644
--- a/arch/um/Kconfig.common
+++ b/arch/um/Kconfig.common
@@ -1,6 +1,7 @@
 config UML
         bool
         default y
+        select ARCH_HAS_KCOV
         select HAVE_ARCH_AUDITSYSCALL
         select HAVE_ARCH_SECCOMP_FILTER
         select HAVE_UID16
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index a6a5e42caaef..2f36d515762e 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -3,6 +3,11 @@
 # Licensed under the GPL
 #
 
+# Don't instrument UML-specific code; without this, we may crash when
+# accessing the instrumentation buffer for the first time from the
+# kernel.
+KCOV_INSTRUMENT                := n
+
 CPPFLAGS_vmlinux.lds := -DSTART=$(LDS_START)            \
                         -DELF_ARCH=$(LDS_ELF_ARCH)      \
                         -DELF_FORMAT=$(LDS_ELF_FORMAT)  \
diff --git a/arch/um/os-Linux/Makefile b/arch/um/os-Linux/Makefile
index 08ff5094fcdd..ada473bf6f46 100644
--- a/arch/um/os-Linux/Makefile
+++ b/arch/um/os-Linux/Makefile
@@ -3,6 +3,9 @@
 # Licensed under the GPL
 #
 
+# Don't instrument UML-specific code
+KCOV_INSTRUMENT                := n
+
 obj-y = aio.o execvp.o file.o helper.o irq.o main.o mem.o process.o \
         registers.o sigio.o signal.o start_up.o time.o tty.o \
         umid.o user_syms.o util.o drivers/ skas/