[PATCH v2 1/4] MIPS: VDSO: Fix conversions in do_monotonic()/do_monotonic_coarse()

[Aleksandar Markovic]

From: Goran Ferenc <goran.ferenc@xxxxxxxxxx>

Fix incorrect calculation in do_monotonic() and do_monotonic_coarse()
function that in turn caused incorrect values returned by the vdso
version of system call clock_gettime() on mips64 if its system clock
ID parameter was CLOCK_MONOTONIC or CLOCK_MONOTONIC_COARSE.

Consider these variables and their types on mips32 and mips64:

tk->wall_to_monotonic.tv_sec  s64, s64   (kernel/vdso.c)
vdso_data.wall_to_mono_sec    u32, u32   (kernel/vdso.c)
to_mono_sec                   u32, u32   (vdso/gettimeofday.c)
ts->tv_sec                    s32, s64   (vdso/gettimeofday.c)

For mips64 case, u32 vdso_data.wall_to_mono_sec variable is updated
from the 64-bit signed variable tk->wall_to_monotonic.tv_sec
(kernel/vdso.c:76) which is a negative number holding the time passed
from 1970-01-01 to the time boot started. This 64-bit signed value is
currently around 47+ years, in seconds. For instance, let this value
be:

-1489757461

or

11111111111111111111111111111111 10100111001101000001101011101011

By updating 32-bit vdso_data.wall_to_mono_sec variable, we lose upper
32 bits (signed 1's).

to_mono_sec variable is a parameter of do_monotonic() and
do_monotonic_coarse() functions which holds vdso_data.wall_to_mono_sec
value. Its value needs to be added (or subtracted considering it holds
negative value from the tk->wall_to_monotonic.tv_sec) to the current
time passed from 1970-01-01 (ts->tv_sec), which is again something like
47+ years, but increased by the time passed from the boot to the
current time. ts->tv_sec is 32-bit long in case of 32-bit architecture
and 64-bit long in case of 64-bit architecture. Consider the update of
ts->tv_sec (vdso/gettimeofday.c:55 & 167):

ts->tv_sec += to_mono_sec;

mips32 case: This update will be performed correctly, since both
ts->tv_sec and to_mono_sec are 32-bit long and the sign in to_mono_sec
is preserved. Implicit conversion from u32 to s32 will be done
correctly.

mips64 case: This update will be wrong, since the implicit conversion
will not be done correctly. The reason is that the conversion will be
from u32 to s64. This is because to_mono_sec is 32-bit long for both
mips32 and mips64 cases and s64..33 bits of converted to_mono_sec
variable will be zeros.

So, in order to make MIPS64 implementation work properly for
MONOTONIC and MONOTONIC_COARSE clock ids on mips64, the size of
wall_to_mono_sec variable in mips_vdso_data union and respective
parameters in do_monotonic() and do_monotonic_coarse() functions
should be changed from u32 to u64. Because of consistency, this
size change from u32 and u64 is also done for wall_to_mono_nsec
variable and corresponding function parameters.

As far as similar situations for other architectures are concerned,
let's take a look at arm. Arm has two distinct vdso_data structures
for 32-bit & 64-bit cases, and arm's wall_to_mono_sec and
wall_to_mono_nsec are u32 for 32-bit and u64 for 64-bit cases.
On the other hand, MIPS has only one structure (mips_vdso_data),
hence the need for changing the size of above mentioned parameters.

Signed-off-by: Goran Ferenc <goran.ferenc@xxxxxxxxxx>
Signed-off-by: Miodrag Dinic <miodrag.dinic@xxxxxxxxxx>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@xxxxxxxxxx>
---
 arch/mips/include/asm/vdso.h  | 4 ++--
 arch/mips/vdso/gettimeofday.c | 8 ++++----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/mips/include/asm/vdso.h b/arch/mips/include/asm/vdso.h
index 8f4ca5d..b7cd6cf 100644
--- a/arch/mips/include/asm/vdso.h
+++ b/arch/mips/include/asm/vdso.h
@@ -79,8 +79,8 @@ union mips_vdso_data {
 	struct {
 		u64 xtime_sec;
 		u64 xtime_nsec;
-		u32 wall_to_mono_sec;
-		u32 wall_to_mono_nsec;
+		u64 wall_to_mono_sec;
+		u64 wall_to_mono_nsec;
 		u32 seq_count;
 		u32 cs_shift;
 		u8 clock_mode;
diff --git a/arch/mips/vdso/gettimeofday.c b/arch/mips/vdso/gettimeofday.c
index ce89c9e..fd7d433 100644
--- a/arch/mips/vdso/gettimeofday.c
+++ b/arch/mips/vdso/gettimeofday.c
@@ -39,8 +39,8 @@ static __always_inline int do_monotonic_coarse(struct timespec *ts,
 					       const union mips_vdso_data *data)
 {
 	u32 start_seq;
-	u32 to_mono_sec;
-	u32 to_mono_nsec;
+	u64 to_mono_sec;
+	u64 to_mono_nsec;
 
 	do {
 		start_seq = vdso_data_read_begin(data);
@@ -148,8 +148,8 @@ static __always_inline int do_monotonic(struct timespec *ts,
 {
 	u32 start_seq;
 	u64 ns;
-	u32 to_mono_sec;
-	u32 to_mono_nsec;
+	u64 to_mono_sec;
+	u64 to_mono_nsec;
 
 	do {
 		start_seq = vdso_data_read_begin(data);
-- 
2.7.4