krb5 commit: Use getentropy() when available

https://github.com/krb5/krb5/commit/ead2a8b253c7db8d17a0765044947c5fbd564bc4
commit ead2a8b253c7db8d17a0765044947c5fbd564bc4
Author: Greg Hudson <[email protected]>
Date:   Wed Oct 30 17:08:00 2024 -0400

    Use getentropy() when available
    
    When available, use getentropy() in preference to SYS_getrandom or
    /dev/urandom.  This binding is present in glibc 2.25 and the BSDs.
    
    ticket: 9149 (new)

 src/configure.ac          |  2 +-
 src/lib/crypto/krb/prng.c | 26 ++++++++++++++++++++++----
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/src/configure.ac b/src/configure.ac
index 464648079..10a5d1a74 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -466,7 +466,7 @@ AC_CONFIG_HEADERS(include/autoconf.h, [echo timestamp > include/autoconf.stamp])
 AC_C_CONST
 AC_HEADER_DIRENT
 AC_FUNC_STRERROR_R
-AC_CHECK_FUNCS(strdup setvbuf seteuid setresuid setreuid setegid setresgid setregid setsid flock fchmod chmod strptime geteuid setenv unsetenv getenv gmtime_r localtime_r bswap16 bswap64 mkstemp getusershell access getcwd srand48 srand srandom stat strchr strerror timegm explicit_bzero explicit_memset getresuid getresgid)
+AC_CHECK_FUNCS(strdup setvbuf seteuid setresuid setreuid setegid setresgid setregid setsid flock fchmod chmod strptime geteuid setenv unsetenv getenv gmtime_r localtime_r bswap16 bswap64 mkstemp getusershell access getcwd srand48 srand srandom stat strchr strerror timegm explicit_bzero explicit_memset getresuid getresgid getentropy)
 
 AC_CHECK_FUNC(mkstemp,
 [MKSTEMP_ST_OBJ=
diff --git a/src/lib/crypto/krb/prng.c b/src/lib/crypto/krb/prng.c
index d6b79e2de..a9c166815 100644
--- a/src/lib/crypto/krb/prng.c
+++ b/src/lib/crypto/krb/prng.c
@@ -96,7 +96,28 @@ cleanup:
 static krb5_boolean
 get_os_entropy(unsigned char *buf, size_t len)
 {
-#if defined(__linux__) && defined(SYS_getrandom)
+#if defined(HAVE_GETENTROPY)
+    int r;
+    size_t seg;
+
+    /* getentropy() has a maximum length of 256. */
+    while (len > 0) {
+        seg = (len > 256) ? 256 : len;
+        r = getentropy(buf, seg);
+        if (r != 0)
+            break;
+        len -= seg;
+        buf += seg;
+    }
+    if (len == 0)
+        return TRUE;
+#elif defined(__linux__) && defined(SYS_getrandom)
+    /*
+     * Linux added SYS_getrandom in 3.17 (2014), but glibc did not have a
+     * wrapper until 2.25 (2017).  This block can be deleted when that interval
+     * is far in the past, along with the conditional include of
+     * <sys/syscall.h> above.
+     */
     int r;
 
     while (len > 0) {
@@ -104,9 +125,6 @@ get_os_entropy(unsigned char *buf, size_t len)
          * Pull from the /dev/urandom pool, but require it to have been seeded.
          * This ensures strong randomness while only blocking during first
          * system boot.
-         *
-         * glibc does not currently provide a binding for getrandom:
-         * https://sourceware.org/bugzilla/show_bug.cgi?id=17252
          */
         errno = 0;
         r = syscall(SYS_getrandom, buf, len, 0);