linux-kernel - Re: [patch 3/5] lib/vdso/32: Provide legacy syscall fallbacks

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Date:   Mon, 29 Jul 2019 07:48:31 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Kees Cook <keescook@...omium.org>,
        Paul Bolle <pebolle@...cali.nl>, Will Deacon <will@...nel.org>
Subject: Re: [patch 3/5] lib/vdso/32: Provide legacy syscall fallbacks

On Sun, Jul 28, 2019 at 03:12:54PM +0200, Thomas Gleixner wrote:
> To address the regression which causes seccomp to deny applications the
> access to clock_gettime64() and clock_getres64() syscalls because they
> are not enabled in the existing filters.
> 
> That trips over the fact that 32bit VDSOs use the new clock_gettime64() and
> clock_getres64() syscalls in the fallback path.
> 
> Implement a __cvdso_clock_get*time32() variants which invokes the legacy
> 32bit syscalls when the architecture requests it.
> 
> The conditional can go away once all architectures are converted.
> 
> Fixes: 00b26474c2f1 ("lib/vdso: Provide generic VDSO implementation")
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  lib/vdso/gettimeofday.c |   46 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
> 
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -117,6 +117,8 @@ static __maybe_unused int
>  	return 0;
>  }
>  
> +#ifndef VDSO_HAS_32BIT_FALLBACK
> +
>  static __maybe_unused int
>  __cvdso_clock_gettime32(clockid_t clock, struct old_timespec32 *res)
>  {
> @@ -132,10 +134,29 @@ static __maybe_unused int
>  		res->tv_sec = ts.tv_sec;
>  		res->tv_nsec = ts.tv_nsec;
>  	}
> -
>  	return ret;
>  }
>  
> +#else
> +
> +static __maybe_unused int
> +__cvdso_clock_gettime32(clockid_t clock, struct old_timespec32 *res)
> +{
> +	struct __kernel_timespec ts;
> +	int ret;
> +
> +	ret = __cvdso_clock_gettime_common(clock, &ts);
> +
> +	if (likely(!ret)) {
> +		res->tv_sec = ts.tv_sec;
> +		res->tv_nsec = ts.tv_nsec;
> +		return 0;
> +	}
> +	return clock_gettime32_fallback(clock, res);
> +}
> +
> +#endif
> +
>  static __maybe_unused int
>  __cvdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz)
>  {
> @@ -225,6 +246,8 @@ int __cvdso_clock_getres(clockid_t clock
>  	return 0;
>  }
>  
> +#ifndef VDSO_HAS_32BIT_FALLBACK
> +
>  static __maybe_unused int
>  __cvdso_clock_getres_time32(clockid_t clock, struct old_timespec32 *res)
>  {
> @@ -241,4 +264,25 @@ static __maybe_unused int
>  	}
>  	return ret;
>  }
> +
> +#else
> +
> +static __maybe_unused int
> +__cvdso_clock_getres_time32(clockid_t clock, struct old_timespec32 *res)
> +{
> +	struct __kernel_timespec ts;
> +	int ret;
> +
> +	ret = __cvdso_clock_getres_common(clock, &ts);
> +
> +	if (likely(!ret)) {
> +		res->tv_sec = ts.tv_sec;
> +		res->tv_nsec = ts.tv_nsec;
> +		return 0;
> +	}
> +
> +	return clock_getres32_fallback(clock, res);
> +}
> +#endif
> +
>  #endif /* VDSO_HAS_CLOCK_GETRES */

Any reason not to have the #ifndef apply only to the fallback?  Wrapping
the entire function and flipping the order of handling 'ret' makes it a bit
difficult to discern that the only difference is the fallback invocation.

static __maybe_unused int
__cvdso_clock_gettime32(clockid_t clock, struct old_timespec32 *res)
{
        struct __kernel_timespec ts;
        int ret;

        ret = __cvdso_clock_gettime_common(clock, &ts);

        if (unlikely(ret))
#ifndef VDSO_HAS_32BIT_FALLBACK
                ret = clock_gettime_fallback(clock, &ts);
#else
                return clock_gettime32_fallback(clock, res);
#endif

        if (likely(!ret)) {
                res->tv_sec = ts.tv_sec;
                res->tv_nsec = ts.tv_nsec;
        }
        return ret;
}

static __maybe_unused int
__cvdso_clock_getres_time32(clockid_t clock, struct old_timespec32 *res)
{
        struct __kernel_timespec ts;
        int ret;

        ret = __cvdso_clock_getres_common(clock, &ts);
        if (unlikely(ret))
#ifndef VDSO_HAS_32BIT_FALLBACK
                ret = clock_getres_fallback(clock, &ts);
#else
                return clock_getres32_fallback(clock, res);
#endif

        if (likely(!ret)) {
                res->tv_sec = ts.tv_sec;
                res->tv_nsec = ts.tv_nsec;
        }
        return ret;
}