Re: [PATCH v2] uapi: futex: Add a futex syscall

[Alistair Francis]

On Tue, Oct 26, 2021 at 2:34 AM André Almeida <andrealmeid@xxxxxxxxxxxxx> wrote:
>
> Hi Alistair,
>
> Às 02:54 de 21/10/21, Alistair Francis escreveu:
> > From: Alistair Francis <alistair.francis@xxxxxxx>
> >
> > This commit adds two futex syscall wrappers that are exposed to
> > userspace.
> >
> > Neither the kernel or glibc currently expose a futex wrapper, so
> > userspace is left performing raw syscalls. This has mostly been becuase
>
>                                                                   because
>
> > the overloading of one of the arguments makes it impossible to provide a
> > single type safe function.
> >
> > Until recently the single syscall has worked fine. With the introduction
> > of a 64-bit time_t futex call on 32-bit architectures, this has become
> > more complex. The logic of handling the two possible futex syscalls is
> > complex and often implemented incorrectly.
> >
> > This patch adds two futux syscall functions that correctly handle the
> > time_t complexity for userspace.
> >
> > This idea is based on previous discussions: https://lkml.org/lkml/2021/9/21/143
>
> I would use lore
> https://lore.kernel.org/lkml/CAK8P3a3x_EyCiPDpMK54y=Rtm-Wb08ym2TNiuAZgXhYrThcWTw@xxxxxxxxxxxxxx/
>
> >
> > Signed-off-by: Alistair Francis <alistair.francis@xxxxxxx>
>
> Thanks for working on that :)
>
> > ---
> >  include/uapi/linux/futex_syscall.h | 81 ++++++++++++++++++++++++++++++
> >  1 file changed, 81 insertions(+)
> >  create mode 100644 include/uapi/linux/futex_syscall.h
> >
> > diff --git a/include/uapi/linux/futex_syscall.h b/include/uapi/linux/futex_syscall.h
> > new file mode 100644
> > index 0000000000000..f84a0c68baf78
> > --- /dev/null
> > +++ b/include/uapi/linux/futex_syscall.h
> > @@ -0,0 +1,81 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#ifndef _UAPI_LINUX_FUTEX_SYSCALL_H
> > +#define _UAPI_LINUX_FUTEX_SYSCALL_H
> > +
> > +#include <asm/unistd.h>
> > +#include <errno.h>
> > +#include <linux/types.h>
> > +#include <linux/time_types.h>
> > +#include <sys/syscall.h>
> > +
> > +/**
> > + * futex_syscall_timeout() - __NR_futex/__NR_futex_time64 syscall wrapper
> > + * @uaddr:  address of first futex
> > + * @op:   futex op code
> > + * @val:  typically expected value of uaddr, but varies by op
> > + * @timeout:  an absolute struct timespec
> > + * @uaddr2: address of second futex for some ops
> > + * @val3: varies by op
> > + */
> > +static inline int
> > +__kernel_futex_syscall_timeout(volatile u_int32_t *uaddr, int op, u_int32_t val,
> > +                   struct timespec *timeout, volatile u_int32_t *uaddr2, int val3)
>
> I tried to write an example[0] that uses this header, but I can't
> compile given that u_int32_t isn't defined. Maybe change to uint32_t and
> include <stdint.h>?
>
> Also, I got some invalid use of undefined type 'struct timespec', and
> #include <time.h> solved.
>
> [0] https://paste.debian.net/1216834/
>
> > +{
> > +#if defined(__NR_futex_time64)
> > +     if (sizeof(*timeout) != sizeof(struct __kernel_old_timespec)) {
> > +             int ret =  syscall(__NR_futex_time64, uaddr, op, val, timeout, uaddr2, val3);
> > +
> > +             if (ret == 0 || errno != ENOSYS)
> > +                     return ret;
> > +     }
> > +#endif
> > +
> > +#if defined(__NR_futex)
> > +     if (sizeof(*timeout) == sizeof(struct __kernel_old_timespec))
> > +             return syscall(__NR_futex, uaddr, op, val, timeout, uaddr2, val3);
> > +
> > +     if (timeout && timeout->tv_sec == (long)timeout->tv_sec) {
> > +             struct __kernel_old_timespec ts32;
> > +
> > +             ts32.tv_sec = (__kernel_long_t) timeout->tv_sec;> +             ts32.tv_nsec = (__kernel_long_t) timeout->tv_nsec;
> > +
> > +             return syscall(__NR_futex, uaddr, op, val, &ts32, uaddr2, val3);
> > +     } else if (!timeout) {
> > +             return syscall(__NR_futex, uaddr, op, val, NULL, uaddr2, val3);
> > +     }
> > +#endif
>
> If I read this part right, you will always use ts32 for __NR_futex. I
> know that it can be misleading, but __NR_futex uses ts64 in 64-bit
> archs, so they shouldn't be converted to ts32 in those cases.
>
> Just to make it clear, there's no __NR_futex_time64 at 64-bit archs.
>
> > +
> > +     errno = ENOSYS;
> > +     return -1;
> > +}
> > +
> > +/**
> > + * futex_syscall_nr_requeue() - __NR_futex/__NR_futex_time64 syscall wrapper
> > + * @uaddr:  address of first futex
> > + * @op:   futex op code
> > + * @val:  typically expected value of uaddr, but varies by op
> > + * @nr_requeue:  an op specific meaning
> > + * @uaddr2: address of second futex for some ops
> > + * @val3: varies by op
> > + */
> > +static inline int
> > +__kernel_futex_syscall_nr_requeue(volatile u_int32_t *uaddr, int op, u_int32_t val,
> > +                      u_int32_t nr_requeue, volatile u_int32_t *uaddr2, int val3)
>
> I would always assume that op is FUTEX_CMP_REQUEUE, given that
> FUTEX_REQUEUE is racy. From `man futex`:

There are other ops that this could be though. From just the kernel
futex self tests it could be FUTEX_WAKE_OP, FUTEX_WAIT_REQUEUE_PI or
FUTEX_CMP_REQUEUE_PI

Alistair

>
> The  FUTEX_CMP_REQUEUE operation was added as a replacement for the
> earlier FUTEX_REQUEUE.  The difference is that the check of the value at
> uaddr can be used to ensure that requeueing happens only under certain
> conditions, which allows race conditions to be avoided in certain use cases.
>
> And then we can drop `int op` from the args and give defined
> descriptions for the args.
>