Re: [dpdk-dev] [PATCH v4 2/2] eal: emulate glibc getentropy for initial random seed

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2020-06-29 19:57, Dan Gora wrote:
> On Mon, Jun 29, 2020 at 6:30 AM Mattias Rönnblom
> <mattias.ronnblom@ericsson.com> wrote:
>> On 2020-04-23 01:42, Dan Gora wrote:
>>> The getentropy() function was introduced into glibc v2.25 and so is
>>> not available on all supported platforms.  Previously, if DPDK was
>>> compiled (using meson) on a system which has getentropy(), it would
>>> introduce a dependency on glibc v2.25 which would prevent that binary
>>> from running on a system with an older glibc.  Similarly if DPDK was
>>> compiled on a system which did not have getentropy(), getentropy()
>>> could not be used even if the execution system supported it.
>>>
>>> Introduce a new static function, __rte_getentropy() to emulate the
>>> glibc getentropy() function by reading from /dev/urandom to remove
>>> this dependency on the glibc version.
>>>
>>> Since __rte_genentropy() should never fail, the rdseed method is
>>> tried first.
>>>
>>> Signed-off-by: Dan Gora <dg@adax.com>
>>> ---
>>>    lib/librte_eal/common/rte_random.c | 62 ++++++++++++++++++++++++++----
>>>    lib/librte_eal/meson.build         |  3 --
>>>    2 files changed, 54 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/lib/librte_eal/common/rte_random.c b/lib/librte_eal/common/rte_random.c
>>> index 2c84c8527..f043adf03 100644
>>> --- a/lib/librte_eal/common/rte_random.c
>>> +++ b/lib/librte_eal/common/rte_random.c
>>> @@ -7,6 +7,7 @@
>>>    #endif
>>>    #include <stdlib.h>
>>>    #include <unistd.h>
>>> +#include <fcntl.h>
>>>
>>>    #include <rte_branch_prediction.h>
>>>    #include <rte_cycles.h>
>>> @@ -176,20 +177,61 @@ rte_rand_max(uint64_t upper_bound)
>>>        return res;
>>>    }
>>>
>>> +/* Emulate glibc getentropy() using /dev/urandom */
>>> +static int
>>> +__rte_getentropy(void *buffer, size_t length)
>>> +{
>>> +     uint8_t *start = buffer;
>>> +     uint8_t *end;
>>> +     ssize_t bytes;
>>> +     int fd;
>>> +     int rc = -1;
>>> +
>>> +     if (length > 256) {
>>> +             errno = EIO;
>>
>> First of all; only the return code is needed, so why bother with errno?
>> If you would, I suspect it should be rte_errno and not errno (which is
>> already set).
> Because, as I thought that I clearly explained in the previous email
> in this thread:
>
> https://protect2.fireeye.com/v1/url?k=64eebf70-3a4e5fe4-64eeffeb-86d2114eab2f-e9077eca0a4dd2ab&q=1&e=2360d5cd-0b70-4aa9-86f1-f72782986b27&u=https%3A%2F%2Fwww.mail-archive.com%2Fdev%40dpdk.org%2Fmsg164646.html
>
> this function is emulating the getentropy() system call.  Since we
> want it to have to the same semantics as getentropy() and since
> getentropy() is a system call, it clears and sets errno, just like
> getentropy():


Since you've replaced getentropy() altogether for all builds, there's no 
need to be API-compatible. Just do an as-simple-as-possible function 
that reads 8 bytes from /dev/urandom.


> https://protect2.fireeye.com/v1/url?k=7d08ee94-23a80e00-7d08ae0f-86d2114eab2f-0d7c5c2b9ffa9874&q=1&e=2360d5cd-0b70-4aa9-86f1-f72782986b27&u=https%3A%2F%2Fsourceware.org%2Fgit%2F%3Fp%3Dglibc.git%3Ba%3Dblob%3Bf%3Dsysdeps%2Funix%2Fsysv%2Flinux%2Fgetentropy.c%3Bh%3D1778632ff1f1fd77019401c3fbaa164c167248b0%3Bhb%3D92dcaa3e2f7bf0f7f1c04cd2fb6a317df1a4e225
>
>>
>>> +             return -1;
>>> +     }
>>> +
>>> +     fd = open("/dev/urandom", O_RDONLY);
>>> +     if (fd < 0) {
>>> +             errno = ENODEV;
>>
>> See above.
>>
>>
>>> +             return -1;
>>> +     }
>>> +
>>> +     end = start + length;
>>> +     while (start < end) {
>>> +             bytes = read(fd, start, end - start);
>>> +             if (bytes < 0) {
>>> +                     if (errno == EINTR)
>>> +                             /* Supposedly cannot be interrupted by
>>> +                              * a signal, but just in case...
>>> +                              */
>>> +                             continue;
>>> +                     else
>>> +                             goto out;
>>> +             }
>>> +             if (bytes == 0) {
>>> +                     /* no more bytes available, should not happen under
>>> +                      * normal circumstances.
>>> +                      */
>>> +                     errno = EIO;
>>> +                     goto out;
>>> +             }
>>> +             start += bytes;
>>> +     }
>>
>> There's no need for this loop. A /dev/urandom read() is guaranteed to
>> return as many bytes as requested, up to 256 bytes. See random(4) for
>> details.
> It can't be interrupted by a signal?  Are you _sure_ that it cannot
> return less than the requested number of bytes and has been that was
> forever and always?  Why does getentropy() check this then?  In the
> case where it does not fail this error checking makes no difference
> other than a couple extra instructions.  In the case that it does, it
> saves your bacon.


The random(4) manual page says it can't be interrupted for small 
requests, which seems to hold true for Linux 3.17 and later. I don't 
know the hows and whys of glibc getentropy(). Studying LGPL code before 
implementing BSD licensed code performing the same function might not be 
the best of ideas.


>>
>>> +     rc = 0;
>>> +     errno = 0;
>>
>> Why are you changing errno? You should never touch errno on success.
> Because getentropy() does and we are emulating getentropy() and want
> to have the same semantics:
> https://protect2.fireeye.com/v1/url?k=44546baa-1af48b3e-44542b31-86d2114eab2f-bc2d2a695ed31cdc&q=1&e=2360d5cd-0b70-4aa9-86f1-f72782986b27&u=https%3A%2F%2Fsourceware.org%2Fgit%2F%3Fp%3Dglibc.git%3Ba%3Dblob%3Bf%3Dsysdeps%2Funix%2Fsysv%2Flinux%2Fgetentropy.c%3Bh%3D1778632ff1f1fd77019401c3fbaa164c167248b0%3Bhb%3D92dcaa3e2f7bf0f7f1c04cd2fb6a317df1a4e225
>
>>
>>> +out:
>>> +     close(fd);
>>> +     return rc;
>>> +}
>>> +
>>>    static uint64_t
>>>    __rte_random_initial_seed(void)
>>>    {
>>> -#ifdef RTE_LIBEAL_USE_GETENTROPY
>>> -     int ge_rc;
>>>        uint64_t ge_seed;
>>>
>>> -     ge_rc = getentropy(&ge_seed, sizeof(ge_seed));
>>> -
>>> -     if (ge_rc == 0)
>>> -             return ge_seed;
>>> -#endif
>>>    #if defined(RTE_ARCH_X86)
>>> -     /* first fallback: rdseed instruction, if available */
>>>        if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_RDSEED)) {
>>>                unsigned int rdseed_low;
>>>                unsigned int rdseed_high;
>>> @@ -200,6 +242,10 @@ __rte_random_initial_seed(void)
>>>                                ((uint64_t)rdseed_high << 32);
>>>        }
>>>    #endif
>>> +     /* first fallback: read from /dev/urandom.. */
>>
>> Remove "..".
> *sigh*.....
>
> thanks
>
> dan