[PATCH v2 2/5] random: always use getentropy

[Stephen Hemminger <stephen@networkplumber.org>]

The current meson build check for getentropy support is broken
and getentropy never gets used!

The cc.has_function() checks with -std=c11 but glibc hides
getentropy behind _DEFAULT_SOURCE. Therefore the probe always
failed, and code would fallback to RDSEED or TSC
for the initial PRNG seed.

Now that a getentropy() wrapper exists for Windows it can always be used.

Falling back to the TSC now logs at ERR level, including the errno
from getentropy() so that a blocked syscall can be told apart from
other failures. A TSC derived seed only varies by process start time,
which is easily determined by local attacker.

A backport needs the preceding patch which adds the Windows
getentropy() shim, otherwise the Windows build breaks.

Bugzilla ID: 2035
Fixes: faf8fd252785 ("eal: improve entropy for initial PRNG seed")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 doc/guides/rel_notes/release_26_11.rst | 4 ++++
 lib/eal/common/rte_random.c            | 6 ++++--
 lib/eal/include/rte_random.h           | 7 ++++---
 lib/eal/meson.build                    | 3 ---
 4 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/doc/guides/rel_notes/release_26_11.rst b/doc/guides/rel_notes/release_26_11.rst
index 88c06025b0..9bef8b4774 100644
--- a/doc/guides/rel_notes/release_26_11.rst
+++ b/doc/guides/rel_notes/release_26_11.rst
@@ -158,6 +158,10 @@ New Features
   Added ``rte_bbdev_queue_stats_get()`` function to retrieve statistics
   for a specific queue, complementing the existing device-level statistics API.
 
+* **Updated random number generation.**
+
+  * The initial seed is now always taken from ``getentropy()``.
+
 
 Removed Items
 -------------
diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index 537fa035ee..b036c50349 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -7,6 +7,8 @@
 #include <x86intrin.h>
 #endif
 #endif
+#include <errno.h>
+#include <string.h>
 #include <unistd.h>
 
 #include <rte_bitops.h>
@@ -219,7 +221,6 @@ rte_drand(void)
 static uint64_t
 __rte_random_initial_seed(void)
 {
-#ifdef RTE_LIBEAL_USE_GETENTROPY
 	int ge_rc;
 	uint64_t ge_seed;
 
@@ -227,7 +228,6 @@ __rte_random_initial_seed(void)
 
 	if (ge_rc == 0)
 		return ge_seed;
-#endif
 #ifdef __RDSEED__
 	unsigned int rdseed_low;
 	unsigned int rdseed_high;
@@ -238,6 +238,8 @@ __rte_random_initial_seed(void)
 		return (uint64_t)rdseed_low | ((uint64_t)rdseed_high << 32);
 #endif
 	/* second fallback: seed using rdtsc */
+	EAL_LOG(ERR, "getentropy() failed (%s), seeding PRNG from TSC: seed has low entropy",
+		strerror(errno));
 	return rte_get_tsc_cycles();
 }
 
diff --git a/lib/eal/include/rte_random.h b/lib/eal/include/rte_random.h
index 15cbe6215a..978ecda203 100644
--- a/lib/eal/include/rte_random.h
+++ b/lib/eal/include/rte_random.h
@@ -20,9 +20,10 @@ extern "C" {
 /**
  * Seed the pseudo-random generator.
  *
- * The generator is automatically seeded by the EAL init with a timer
- * value. It may need to be re-seeded by the user with a real random
- * value.
+ * The generator is automatically seeded by the EAL init from the
+ * random source provided by the operating system, so there is no need
+ * to re-seed it to get unpredictable values. Seeding it explicitly is
+ * useful to make a run repeatable.
  *
  * This function is not multi-thread safe in regards to other
  * rte_srand() calls, nor is it in relation to concurrent rte_rand(),
diff --git a/lib/eal/meson.build b/lib/eal/meson.build
index 092f5c5261..a261bdd48b 100644
--- a/lib/eal/meson.build
+++ b/lib/eal/meson.build
@@ -22,9 +22,6 @@ endif
 if dpdk_conf.has('RTE_HAS_LIBARCHIVE')
     ext_deps += libarchive
 endif
-if cc.has_function('getentropy', prefix : '#include <unistd.h>')
-    cflags += '-DRTE_LIBEAL_USE_GETENTROPY'
-endif
 
 if is_freebsd
     annotate_locks = false
-- 
2.53.0