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    Subject[PATCH 4.19 143/234] arm: use fallback for random_get_entropy() instead of zero
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit ff8a8f59c99f6a7c656387addc4d9f2247d75077 upstream.

    In the event that random_get_entropy() can't access a cycle counter or
    similar, falling back to returning 0 is really not the best we can do.
    Instead, at least calling random_get_entropy_fallback() would be
    preferable, because that always needs to return _something_, even
    falling back to jiffies eventually. It's not as though
    random_get_entropy_fallback() is super high precision or guaranteed to
    be entropic, but basically anything that's not zero all the time is
    better than returning zero all the time.

    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm/include/asm/timex.h | 1 +
    1 file changed, 1 insertion(+)

    --- a/arch/arm/include/asm/timex.h
    +++ b/arch/arm/include/asm/timex.h
    @@ -14,5 +14,6 @@

    typedef unsigned long cycles_t;
    #define get_cycles() ({ cycles_t c; read_current_timer(&c) ? 0 : c; })
    +#define random_get_entropy() (((unsigned long)get_cycles()) ?: random_get_entropy_fallback())

    #endif

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