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

[Dan Gora <dg@adax.com>]

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;
+		return -1;
+	}
+
+	fd = open("/dev/urandom", O_RDONLY);
+	if (fd < 0) {
+		errno = ENODEV;
+		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;
+	}
+	rc = 0;
+	errno = 0;
+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.. */
+	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
-- 
2.24.1.425.g7034cd094b