linux-kernel - Re: [PATCH 03/12] futex: Flag conversion

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Message-ID: <87sf8s6qj3.fsf@suse.de>
Date:   Wed, 09 Aug 2023 14:04:32 -0400
From:   Gabriel Krisman Bertazi <krisman@...e.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, andres@...razel.de, tglx@...utronix.de
Subject: Re: [PATCH 03/12] futex: Flag conversion

Jens Axboe <axboe@...nel.dk> writes:

> From: Peter Zijlstra <peterz@...radead.org>
>
> Futex has 3 sets of flags:
>
>  - legacy futex op bits
>  - futex2 flags
>  - internal flags
>
> Add a few helpers to convert from the API flags into the internal
> flags.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Jens Axboe <axboe@...nel.dk>
> ---
>  kernel/futex/futex.h    | 64 +++++++++++++++++++++++++++++++++++++++--
>  kernel/futex/syscalls.c | 24 ++++++----------
>  kernel/futex/waitwake.c |  4 +--
>  3 files changed, 72 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
> index b5379c0e6d6d..c0e04599904a 100644
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -5,6 +5,7 @@
>  #include <linux/futex.h>
>  #include <linux/rtmutex.h>
>  #include <linux/sched/wake_q.h>
> +#include <linux/compat.h>
>  
>  #ifdef CONFIG_PREEMPT_RT
>  #include <linux/rcuwait.h>
> @@ -16,8 +17,15 @@
>   * Futex flags used to encode options to functions and preserve them across
>   * restarts.
>   */
> +#define FLAGS_SIZE_8		0x00
> +#define FLAGS_SIZE_16		0x01
> +#define FLAGS_SIZE_32		0x02
> +#define FLAGS_SIZE_64		0x03
> +
> +#define FLAGS_SIZE_MASK		0x03
> +
>  #ifdef CONFIG_MMU
> -# define FLAGS_SHARED		0x01
> +# define FLAGS_SHARED		0x10
>  #else
>  /*
>   * NOMMU does not have per process address space. Let the compiler optimize
> @@ -25,8 +33,58 @@
>   */
>  # define FLAGS_SHARED		0x00
>  #endif
> -#define FLAGS_CLOCKRT		0x02
> -#define FLAGS_HAS_TIMEOUT	0x04
> +#define FLAGS_CLOCKRT		0x20
> +#define FLAGS_HAS_TIMEOUT	0x40
> +#define FLAGS_NUMA		0x80
> +
> +/* FUTEX_ to FLAGS_ */
> +static inline unsigned int futex_to_flags(unsigned int op)
> +{
> +	unsigned int flags = FLAGS_SIZE_32;
> +
> +	if (!(op & FUTEX_PRIVATE_FLAG))
> +		flags |= FLAGS_SHARED;
> +
> +	if (op & FUTEX_CLOCK_REALTIME)
> +		flags |= FLAGS_CLOCKRT;
> +
> +	return flags;
> +}
> +
> +/* FUTEX2_ to FLAGS_ */
> +static inline unsigned int futex2_to_flags(unsigned int flags2)
> +{
> +	unsigned int flags = flags2 & FUTEX2_64;

FUTEX2_64 -> FLAGS_SIZE_MASK

> +
> +	if (!(flags2 & FUTEX2_PRIVATE))
> +		flags |= FLAGS_SHARED;
> +
> +	if (flags2 & FUTEX2_NUMA)
> +		flags |= FLAGS_NUMA;
> +
> +	return flags;
> +}
> +
> +static inline bool futex_flags_valid(unsigned int flags)
> +{
> +	/* Only 64bit futexes for 64bit code */
> +	if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
> +		if ((flags & FLAGS_SIZE_MASK) == FLAGS_SIZE_64)
> +			return false;
> +	}

I read the comment above as '64bit code can only have 64bit futexes',
which is obviously wrong and not what the code is checking.
Something like this would be better:

/* Reject 64bit futexes on !64bit code. */

Or Perhaps make it generic:

/* Don't allow futexes larger than the word size */
  if (futex_size(flags) > (__WORDSIZE/8) || in_compat_syscall())

-- 
Gabriel Krisman Bertazi