futex(2) says that 'utime' is a pointer to 'const'.
The implementation doesn't use 'const';
however, it _never_ modifies the contents of utime.
- futex() either uses 'utime' as a pointer to struct or as a 'u32'.
- In case it's used as a 'u32', it makes a copy of it,
and of course it is not dereferenced.
- In case it's used as a 'struct __kernel_timespec __user *',
the pointer is not dereferenced inside the futex() definition,
and it is only passed to a function: get_timespec64(),
which accepts a 'const struct __kernel_timespec __user *'.
context:
........
[[
FUTEX(2) Linux Programmer's Manual FUTEX(2)
NAME
futex - fast user-space locking
SYNOPSIS
#include <linux/futex.h>
#include <stdint.h>
#include <sys/time.h>
long futex(uint32_t *uaddr, int futex_op, uint32_t val,
const struct timespec *timeout, /* or: uint32_t val2 */
uint32_t *uaddr2, uint32_t val3);
Note: There is no glibc wrapper for this system call; see
NOTES.
]]
$ sed -n '/SYSCALL_DEFINE.(futex\>/,/^}/p' linux/kernel/futex.c
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct __kernel_timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)
{
struct timespec64 ts;
ktime_t t, *tp = NULL;
u32 val2 = 0;
int cmd = op & FUTEX_CMD_MASK;
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
cmd == FUTEX_WAIT_REQUEUE_PI)) {
if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
return -EFAULT;
if (get_timespec64(&ts, utime))
return -EFAULT;
if (!timespec64_valid(&ts))
return -EINVAL;
t = timespec64_to_ktime(ts);
if (cmd == FUTEX_WAIT)
t = ktime_add_safe(ktime_get(), t);
else if (!(op & FUTEX_CLOCK_REALTIME))
t = timens_ktime_to_host(CLOCK_MONOTONIC, t);
tp = &t;
}
/*
* requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
* number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
*/
if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
val2 = (u32) (unsigned long) utime;
return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}
$ sed -n '/get_timespec64(/,/;/p' linux/include/linux/time.h
int get_timespec64(struct timespec64 *ts,
const struct __kernel_timespec __user *uts);
...
Signed-off-by: Alejandro Colomar <alx.manpa...@gmail.com>
---
Hello Thomas & Ingo,
I'm sorry I couldn't test the change in my computers,
as there is a bug since Linux 5.7 where I can't boot
(https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=974166).
Alex
kernel/futex.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 00259c7e288e..28577c7d2805 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3792,8 +3792,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t
*timeout,
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
- struct __kernel_timespec __user *, utime, u32 __user *, uaddr2,
- u32, val3)
+ const struct __kernel_timespec __user *, utime,
+ u32 __user *, uaddr2, u32, val3)
{
struct timespec64 ts;
ktime_t t, *tp = NULL;
--
2.29.2
