[PATCH -tip v3 1/2] kcov: 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, and mark them noinstr.

Declare write_comp_data() as __always_inline to ensure it is inlined,
which also reduces stack usage and removes 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>
---
Applies to -tip only currently, because of the use of instrumentation.h
markers.

v3:
* Remove objtool hack, and instead properly mark __sanitizer_cov
  functions as noinstr.
* Add comment about .entry.text.

v2: https://lkml.kernel.org/r/20200604145635.21565-1-elver@xxxxxxxxxx
* 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 | 59 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 45 insertions(+), 14 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 8accc9722a81..84cdc30d478e 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -6,6 +6,7 @@
 #include <linux/compiler.h>
 #include <linux/errno.h>
 #include <linux/export.h>
+#include <linux/instrumentation.h>
 #include <linux/types.h>
 #include <linux/file.h>
 #include <linux/fs.h>
@@ -24,6 +25,7 @@
 #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__)
 
@@ -172,20 +174,38 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
 	return ip;
 }
 
+/* Return true if @ip is within a noinstr section. */
+static __always_inline bool within_noinstr_section(unsigned long ip)
+{
+	/*
+	 * Note: .entry.text is also considered noinstr, but for now, since all
+	 * .entry.text code lives in .S files, these are never instrumented.
+	 */
+	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.
  */
-void notrace __sanitizer_cov_trace_pc(void)
+void noinstr __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(within_noinstr_section(_RET_IP_)))
+		return;
+
+	instrumentation_begin();
+
 	t = current;
 	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
-		return;
+		goto out;
+
+	ip = canonicalize_ip(_RET_IP_);
 
 	area = t->kcov_area;
 	/* The first 64-bit word is the number of subsequent PCs. */
@@ -194,19 +214,27 @@ void notrace __sanitizer_cov_trace_pc(void)
 		area[pos] = ip;
 		WRITE_ONCE(area[0], pos);
 	}
+
+out:
+	instrumentation_end();
 }
 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(within_noinstr_section(ip)))
+		return;
+
+	instrumentation_begin();
+
 	t = current;
 	if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
-		return;
+		goto out;
 
 	ip = canonicalize_ip(ip);
 
@@ -229,61 +257,64 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 		area[start_index + 3] = ip;
 		WRITE_ONCE(area[0], count + 1);
 	}
+
+out:
+	instrumentation_end();
 }
 
-void notrace __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2)
+void noinstr __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(0), arg1, arg2, _RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_cmp1);
 
-void notrace __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2)
+void noinstr __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(1), arg1, arg2, _RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_cmp2);
 
-void notrace __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2)
+void noinstr __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(2), arg1, arg2, _RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_cmp4);
 
-void notrace __sanitizer_cov_trace_cmp8(u64 arg1, u64 arg2)
+void noinstr __sanitizer_cov_trace_cmp8(u64 arg1, u64 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(3), arg1, arg2, _RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_cmp8);
 
-void notrace __sanitizer_cov_trace_const_cmp1(u8 arg1, u8 arg2)
+void noinstr __sanitizer_cov_trace_const_cmp1(u8 arg1, u8 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(0) | KCOV_CMP_CONST, arg1, arg2,
 			_RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp1);
 
-void notrace __sanitizer_cov_trace_const_cmp2(u16 arg1, u16 arg2)
+void noinstr __sanitizer_cov_trace_const_cmp2(u16 arg1, u16 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(1) | KCOV_CMP_CONST, arg1, arg2,
 			_RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp2);
 
-void notrace __sanitizer_cov_trace_const_cmp4(u32 arg1, u32 arg2)
+void noinstr __sanitizer_cov_trace_const_cmp4(u32 arg1, u32 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(2) | KCOV_CMP_CONST, arg1, arg2,
 			_RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp4);
 
-void notrace __sanitizer_cov_trace_const_cmp8(u64 arg1, u64 arg2)
+void noinstr __sanitizer_cov_trace_const_cmp8(u64 arg1, u64 arg2)
 {
 	write_comp_data(KCOV_CMP_SIZE(3) | KCOV_CMP_CONST, arg1, arg2,
 			_RET_IP_);
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_const_cmp8);
 
-void notrace __sanitizer_cov_trace_switch(u64 val, u64 *cases)
+void noinstr __sanitizer_cov_trace_switch(u64 val, u64 *cases)
 {
 	u64 i;
 	u64 count = cases[0];
-- 
2.27.0.278.ge193c7cf3a9-goog