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

[Mattias Rönnblom]

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 at 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).


> +		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.


> +	rc = 0;
> +	errno = 0;


Why are you changing errno? You should never touch errno on success.


> +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 "..".


> +	if (__rte_getentropy(&ge_seed, sizeof(ge_seed)) == 0)
> +		return ge_seed;
> +
>   	/* second fallback: seed using rdtsc */
>   	return rte_get_tsc_cycles();
>   }
> diff --git a/lib/librte_eal/meson.build b/lib/librte_eal/meson.build
> index 0267c3b9d..748359b8c 100644
> --- a/lib/librte_eal/meson.build
> +++ b/lib/librte_eal/meson.build
> @@ -15,9 +15,6 @@ deps += 'kvargs'
>   if dpdk_conf.has('RTE_USE_LIBBSD')
>   	ext_deps += libbsd
>   endif
> -if cc.has_function('getentropy', prefix : '#include <unistd.h>')
> -	cflags += '-DRTE_LIBEAL_USE_GETENTROPY'
> -endif
>   if cc.has_header('getopt.h')
>   	cflags += ['-DHAVE_GETOPT_H', '-DHAVE_GETOPT', '-DHAVE_GETOPT_LONG']
>   endif