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    Subject[PATCH 4.19 071/234] random: simplify arithmetic function flow in account()
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit a254a0e4093fce8c832414a83940736067eed515 upstream.

    Now that have_bytes is never modified, we can simplify this function.
    First, we move the check for negative entropy_count to be first. That
    ensures that subsequent reads of this will be non-negative. Then,
    have_bytes and ibytes can be folded into their one use site in the
    min_t() function.

    Suggested-by: Dominik Brodowski <linux@dominikbrodowski.net>
    Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 17 ++++++-----------
    1 file changed, 6 insertions(+), 11 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1293,7 +1293,7 @@ EXPORT_SYMBOL_GPL(add_disk_randomness);
    */
    static size_t account(size_t nbytes, int min)
    {
    - int entropy_count, orig, have_bytes;
    + int entropy_count, orig;
    size_t ibytes, nfrac;

    BUG_ON(input_pool.entropy_count > POOL_FRACBITS);
    @@ -1301,20 +1301,15 @@ static size_t account(size_t nbytes, int
    /* Can we pull enough? */
    retry:
    entropy_count = orig = READ_ONCE(input_pool.entropy_count);
    - ibytes = nbytes;
    - /* never pull more than available */
    - have_bytes = entropy_count >> (POOL_ENTROPY_SHIFT + 3);
    -
    - if (have_bytes < 0)
    - have_bytes = 0;
    - ibytes = min_t(size_t, ibytes, have_bytes);
    - if (ibytes < min)
    - ibytes = 0;
    -
    if (WARN_ON(entropy_count < 0)) {
    pr_warn("negative entropy count: count %d\n", entropy_count);
    entropy_count = 0;
    }
    +
    + /* never pull more than available */
    + ibytes = min_t(size_t, nbytes, entropy_count >> (POOL_ENTROPY_SHIFT + 3));
    + if (ibytes < min)
    + ibytes = 0;
    nfrac = ibytes << (POOL_ENTROPY_SHIFT + 3);
    if ((size_t)entropy_count > nfrac)
    entropy_count -= nfrac;

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