[PATCH v2 1/2] kcov, objtool: Make runtime functions noinstr-compatible

[Marco Elver]

While we lack a compiler attribute to add to noinstr that would disable
KCOV, make the KCOV runtime functions return if the caller is in a
noinstr section. We then whitelist __sanitizer_cov_*() functions in
objtool. __sanitizer_cov_*() cannot safely become safe noinstr functions
as-is, as they may fault due to accesses to vmalloc's memory.

Declare write_comp_data() as __always_inline to ensure it is inlined,
and reduce stack usage and remove one extra call from the fast-path.

In future, our compilers may provide an attribute to implement
__no_sanitize_coverage, which can then be added to noinstr, and the
checks added in this patch can be guarded by an #ifdef checking if the
compiler has such an attribute or not.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
Apply after:
https://lkml.kernel.org/r/20200604102241.466509982@xxxxxxxxxxxxx

v2:
* Rewrite based on Peter's and Andrey's feedback -- v1 worked because we
  got lucky. Let's not rely on luck, as it will be difficult to ensure the
  same conditions remain true in future.

v1: https://lkml.kernel.org/r/20200604095057.259452-1-elver@xxxxxxxxxx

Note: There are a set of KCOV patches from Andrey in -next:
https://lkml.kernel.org/r/cover.1585233617.git.andreyknvl@xxxxxxxxxx --
Git cleanly merges this patch with those patches, and no merge conflict
is expected.
---
 kernel/kcov.c         | 19 +++++++++++++++++--
 tools/objtool/check.c |  7 +++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 8accc9722a81..3329a0fdb868 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -24,6 +24,7 @@
 #include <linux/refcount.h>
 #include <linux/log2.h>
 #include <asm/setup.h>
+#include <asm/sections.h>
 
 #define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
 
@@ -172,6 +173,12 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
 	return ip;
 }
 
+static __always_inline bool in_noinstr_section(unsigned long ip)
+{
+	return (unsigned long)__noinstr_text_start <= ip &&
+	       ip < (unsigned long)__noinstr_text_end;
+}
+
 /*
  * Entry point from instrumented code.
  * This is called once per basic-block/edge.
@@ -180,13 +187,18 @@ void notrace __sanitizer_cov_trace_pc(void)
 {
 	struct task_struct *t;
 	unsigned long *area;
-	unsigned long ip = canonicalize_ip(_RET_IP_);
+	unsigned long ip;
 	unsigned long pos;
 
+	if (unlikely(in_noinstr_section(_RET_IP_)))
+		return;
+
 	t = current;
 	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
 		return;
 
+	ip = canonicalize_ip(_RET_IP_);
+
 	area = t->kcov_area;
 	/* The first 64-bit word is the number of subsequent PCs. */
 	pos = READ_ONCE(area[0]) + 1;
@@ -198,12 +210,15 @@ void notrace __sanitizer_cov_trace_pc(void)
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
 
 #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
-static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
+static __always_inline void write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 {
 	struct task_struct *t;
 	u64 *area;
 	u64 count, start_index, end_pos, max_pos;
 
+	if (unlikely(in_noinstr_section(ip)))
+		return;
+
 	t = current;
 	if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
 		return;
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 3e214f879ada..cb208959f560 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2213,6 +2213,13 @@ static inline bool noinstr_call_dest(struct symbol *func)
 	if (!strncmp(func->name, "__ubsan_handle_", 15))
 		return true;
 
+	/*
+	 * The __sanitizer_cov_*() calls include a check if the caller is in the
+	 * noinstr section, and simply return if that is the case.
+	 */
+	if (!strncmp(func->name, "__sanitizer_cov_", 16))
+		return true;
+
 	return false;
 }
 
-- 
2.27.0.rc2.251.g90737beb825-goog