This is the first half of CONFIG_KCOV_EXT_RECORDS.

Set the appropriate compiler flags to call separate hooks for function
entry/exit, and provide these hooks, but don't make it visible in the KCOV
UAPI yet.

With -fsanitize-coverage=trace-pc-entry-exit, the compiler behavior changes
as follows:

- The __sanitizer_cov_trace_pc() call on function entry is replaced with a
  call to __sanitizer_cov_trace_pc_entry(); so for now,
  __sanitizer_cov_trace_pc_entry() must be treated the same way as
  __sanitizer_cov_trace_pc().
 - On function exit, an extra call to __sanitizer_cov_trace_pc_exit()
   happens; since function exit produced no coverage in the old UAPI,
   __sanitizer_cov_trace_pc_exit() should do nothing for now.

This feature was added to LLVM in commit:
https://github.com/llvm/llvm-project/commit/dc5c6d008f487eea8f5d646011f9b3dca6caebd7

Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jann Horn <jannh@google.com>
---
 include/linux/kcov.h  |  2 ++
 kernel/kcov.c         | 34 +++++++++++++++++++++++++++-------
 lib/Kconfig.debug     | 12 ++++++++++++
 scripts/Makefile.kcov |  2 ++
 tools/objtool/check.c |  2 ++
 5 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index 0143358874b0..e5502d674029 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -81,6 +81,8 @@ typedef unsigned long long kcov_u64;
 #endif
 
 void __sanitizer_cov_trace_pc(void);
+void __sanitizer_cov_trace_pc_entry(void);
+void __sanitizer_cov_trace_pc_exit(void);
 void __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2);
 void __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2);
 void __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2);
diff --git a/kernel/kcov.c b/kernel/kcov.c
index 0b369e88c7c9..86b681c7865c 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -202,15 +202,10 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
 	return ip;
 }
 
-/*
- * Entry point from instrumented code.
- * This is called once per basic-block/edge.
- */
-void notrace __sanitizer_cov_trace_pc(void)
+static __always_inline void notrace kcov_add_pc_record(unsigned long record)
 {
 	struct task_struct *t;
 	unsigned long *area;
-	unsigned long ip = canonicalize_ip(_RET_IP_);
 	unsigned long pos;
 
 	t = current;
@@ -230,11 +225,36 @@ void notrace __sanitizer_cov_trace_pc(void)
 		 */
 		WRITE_ONCE(area[0], pos);
 		barrier();
-		area[pos] = ip;
+		area[pos] = record;
 	}
 }
+
+/*
+ * Entry point from instrumented code.
+ * This is called once per basic-block/edge.
+ */
+void notrace __sanitizer_cov_trace_pc(void)
+{
+	kcov_add_pc_record(canonicalize_ip(_RET_IP_));
+}
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
 
+#ifdef CONFIG_KCOV_EXT_RECORDS
+void notrace __sanitizer_cov_trace_pc_entry(void)
+{
+	unsigned long record = canonicalize_ip(_RET_IP_);
+
+	/*
+	 * This hook replaces __sanitizer_cov_trace_pc() for the function entry
+	 * basic block; it should still emit a record even in classic kcov mode.
+	 */
+	kcov_add_pc_record(record);
+}
+void notrace __sanitizer_cov_trace_pc_exit(void)
+{
+}
+#endif
+
 #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
 static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 {
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 93f356d2b3d9..58686a99c40a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2215,6 +2215,18 @@ config KCOV
 
 	  For more details, see Documentation/dev-tools/kcov.rst.
 
+config KCOV_EXT_RECORDS
+	bool "Support extended KCOV records with function entry/exit records"
+	depends on KCOV
+	depends on 64BIT
+	depends on $(cc-option,-fsanitize-coverage=trace-pc-entry-exit)
+	help
+	  Extended KCOV records allow distinguishing between multiple types of
+	  records: Normal edge coverage, function entry, and function exit.
+
+	  This will likely cause a small additional slowdown compared to normal
+	  KCOV.
+
 config KCOV_ENABLE_COMPARISONS
 	bool "Enable comparison operands collection by KCOV"
 	depends on KCOV
diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 78305a84ba9d..aa0be904268f 100644
--- a/scripts/Makefile.kcov
+++ b/scripts/Makefile.kcov
@@ -1,10 +1,12 @@
 # SPDX-License-Identifier: GPL-2.0-only
 kcov-flags-y					+= -fsanitize-coverage=trace-pc
+kcov-flags-$(CONFIG_KCOV_EXT_RECORDS)		+= -fsanitize-coverage=trace-pc-entry-exit
 kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)	+= -fsanitize-coverage=trace-cmp
 
 kcov-rflags-y					+= -Cpasses=sancov-module
 kcov-rflags-y					+= -Cllvm-args=-sanitizer-coverage-level=3
 kcov-rflags-y					+= -Cllvm-args=-sanitizer-coverage-trace-pc
+kcov-rflags-$(CONFIG_KCOV_EXT_RECORDS)		+= -Cllvm-args=-sanitizer-coverage-trace-pc-entry-exit
 kcov-rflags-$(CONFIG_KCOV_ENABLE_COMPARISONS)	+= -Cllvm-args=-sanitizer-coverage-trace-compares
 
 export CFLAGS_KCOV := $(kcov-flags-y)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index a30379e4ff97..ae3127227621 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1251,6 +1251,8 @@ static const char *uaccess_safe_builtin[] = {
 	"write_comp_data",
 	"check_kcov_mode",
 	"__sanitizer_cov_trace_pc",
+	"__sanitizer_cov_trace_pc_entry",
+	"__sanitizer_cov_trace_pc_exit",
 	"__sanitizer_cov_trace_const_cmp1",
 	"__sanitizer_cov_trace_const_cmp2",
 	"__sanitizer_cov_trace_const_cmp4",

-- 
2.53.0.851.ga537e3e6e9-goog