Issue #29157: Prefer getrandom() over getentropy()
* dev_urandom() now calls py_getentropy(). Prepare the fallback to support getentropy() failure and falls back on reading from /dev/urandom. * Simplify dev_urandom(). pyurandom() is now responsible to call getentropy() or getrandom(). Enhance also dev_urandom() and pyurandom() documentation. * getrandom() is now preferred over getentropy(). The glibc 2.24 now implements getentropy() on Linux using the getrandom() syscall. But getentropy() doesn't support non-blocking mode. Since getrandom() is tried first, it's not more needed to explicitly exclude getentropy() on Solaris. Replace: "if defined(HAVE_GETENTROPY) && !defined(sun)" with "if defined(HAVE_GETENTROPY)" * Enhance py_getrandom() documentation. py_getentropy() now supports ENOSYS, EPERM & EINTR
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parent
84b6fb0eea
commit
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274
Python/random.c
274
Python/random.c
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@ -77,57 +77,23 @@ win32_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
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return 0;
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}
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/* Issue #25003: Don't use getentropy() on Solaris (available since
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* Solaris 11.3), it is blocking whereas os.urandom() should not block. */
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#elif defined(HAVE_GETENTROPY) && !defined(sun)
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#define PY_GETENTROPY 1
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/* Fill buffer with size pseudo-random bytes generated by getentropy().
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Return 0 on success, or raise an exception and return -1 on error.
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If raise is zero, don't raise an exception on error. */
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static int
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py_getentropy(char *buffer, Py_ssize_t size, int raise)
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{
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while (size > 0) {
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Py_ssize_t len = Py_MIN(size, 256);
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int res;
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if (raise) {
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Py_BEGIN_ALLOW_THREADS
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res = getentropy(buffer, len);
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Py_END_ALLOW_THREADS
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}
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else {
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res = getentropy(buffer, len);
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}
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if (res < 0) {
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if (raise) {
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PyErr_SetFromErrno(PyExc_OSError);
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}
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return -1;
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}
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buffer += len;
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size -= len;
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}
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return 0;
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}
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#else
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#else /* !MS_WINDOWS */
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#if defined(HAVE_GETRANDOM) || defined(HAVE_GETRANDOM_SYSCALL)
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#define PY_GETRANDOM 1
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/* Call getrandom()
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/* Call getrandom() to get random bytes:
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- Return 1 on success
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- Return 0 if getrandom() syscall is not available (failed with ENOSYS or
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EPERM) or if getrandom(GRND_NONBLOCK) failed with EAGAIN (system urandom
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not initialized yet) and raise=0.
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- Return 0 if getrandom() is not available (failed with ENOSYS or EPERM),
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or if getrandom(GRND_NONBLOCK) failed with EAGAIN (system urandom not
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initialized yet) and raise=0.
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- Raise an exception (if raise is non-zero) and return -1 on error:
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getrandom() failed with EINTR and the Python signal handler raised an
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exception, or getrandom() failed with a different error. */
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if getrandom() failed with EINTR, raise is non-zero and the Python signal
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handler raised an exception, or if getrandom() failed with a different
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error.
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getrandom() is retried if it failed with EINTR: interrupted by a signal. */
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static int
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py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
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{
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@ -148,7 +114,8 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
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while (0 < size) {
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#ifdef sun
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/* Issue #26735: On Solaris, getrandom() is limited to returning up
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to 1024 bytes */
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to 1024 bytes. Call it multiple times if more bytes are
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requested. */
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n = Py_MIN(size, 1024);
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#else
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n = Py_MIN(size, LONG_MAX);
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@ -179,18 +146,19 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
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#endif
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if (n < 0) {
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/* ENOSYS: getrandom() syscall not supported by the kernel (but
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* maybe supported by the host which built Python). EPERM:
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* getrandom() syscall blocked by SECCOMP or something else. */
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/* ENOSYS: the syscall is not supported by the kernel.
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EPERM: the syscall is blocked by a security policy (ex: SECCOMP)
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or something else. */
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if (errno == ENOSYS || errno == EPERM) {
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getrandom_works = 0;
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return 0;
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}
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/* getrandom(GRND_NONBLOCK) fails with EAGAIN if the system urandom
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is not initialiazed yet. For _PyRandom_Init(), we ignore their
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is not initialiazed yet. For _PyRandom_Init(), we ignore the
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error and fall back on reading /dev/urandom which never blocks,
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even if the system urandom is not initialized yet. */
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even if the system urandom is not initialized yet:
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see the PEP 524. */
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if (errno == EAGAIN && !raise && !blocking) {
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return 0;
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}
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@ -217,7 +185,80 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
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}
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return 1;
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}
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#endif
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#elif defined(HAVE_GETENTROPY)
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#define PY_GETENTROPY 1
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/* Fill buffer with size pseudo-random bytes generated by getentropy():
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- Return 1 on success
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- Return 0 if getentropy() syscall is not available (failed with ENOSYS or
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EPERM).
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- Raise an exception (if raise is non-zero) and return -1 on error:
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if getentropy() failed with EINTR, raise is non-zero and the Python signal
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handler raised an exception, or if getentropy() failed with a different
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error.
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getentropy() is retried if it failed with EINTR: interrupted by a signal. */
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static int
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py_getentropy(char *buffer, Py_ssize_t size, int raise)
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{
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/* Is getentropy() supported by the running kernel? Set to 0 if
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getentropy() failed with ENOSYS or EPERM. */
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static int getentropy_works = 1;
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if (!getentropy_works) {
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return 0;
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}
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while (size > 0) {
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/* getentropy() is limited to returning up to 256 bytes. Call it
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multiple times if more bytes are requested. */
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Py_ssize_t len = Py_MIN(size, 256);
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int res;
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if (raise) {
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Py_BEGIN_ALLOW_THREADS
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res = getentropy(buffer, len);
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Py_END_ALLOW_THREADS
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}
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else {
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res = getentropy(buffer, len);
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}
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if (res < 0) {
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/* ENOSYS: the syscall is not supported by the running kernel.
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EPERM: the syscall is blocked by a security policy (ex: SECCOMP)
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or something else. */
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if (errno == ENOSYS || errno == EPERM) {
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getentropy_works = 0;
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return 0;
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}
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if (errno == EINTR) {
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if (raise) {
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if (PyErr_CheckSignals()) {
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return -1;
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}
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}
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/* retry getentropy() if it was interrupted by a signal */
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continue;
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}
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if (raise) {
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PyErr_SetFromErrno(PyExc_OSError);
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}
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return -1;
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}
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buffer += len;
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size -= len;
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}
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return 1;
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}
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#endif /* defined(HAVE_GETENTROPY) && !defined(sun) */
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static struct {
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int fd;
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@ -225,35 +266,38 @@ static struct {
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ino_t st_ino;
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} urandom_cache = { -1 };
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/* Read random bytes from the /dev/urandom device:
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/* Read 'size' random bytes from py_getrandom(). Fall back on reading from
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/dev/urandom if getrandom() is not available.
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- Return 0 on success
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- Raise an exception (if raise is non-zero) and return -1 on error
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Return 0 on success. Raise an exception (if raise is non-zero) and return -1
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on error. */
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Possible causes of errors:
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- open() failed with ENOENT, ENXIO, ENODEV, EACCES: the /dev/urandom device
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was not found. For example, it was removed manually or not exposed in a
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chroot or container.
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- open() failed with a different error
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- fstat() failed
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- read() failed or returned 0
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read() is retried if it failed with EINTR: interrupted by a signal.
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The file descriptor of the device is kept open between calls to avoid using
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many file descriptors when run in parallel from multiple threads:
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see the issue #18756.
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st_dev and st_ino fields of the file descriptor (from fstat()) are cached to
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check if the file descriptor was replaced by a different file (which is
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likely a bug in the application): see the issue #21207.
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If the file descriptor was closed or replaced, open a new file descriptor
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but don't close the old file descriptor: it probably points to something
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important for some third-party code. */
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static int
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dev_urandom(char *buffer, Py_ssize_t size, int blocking, int raise)
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dev_urandom(char *buffer, Py_ssize_t size, int raise)
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{
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int fd;
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Py_ssize_t n;
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#ifdef PY_GETRANDOM
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int res;
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#endif
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assert(size > 0);
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#ifdef PY_GETRANDOM
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res = py_getrandom(buffer, size, blocking, raise);
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if (res < 0) {
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return -1;
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}
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if (res == 1) {
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return 0;
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}
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/* getrandom() failed with ENOSYS or EPERM,
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fall back on reading /dev/urandom */
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#endif
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if (raise) {
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struct _Py_stat_struct st;
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fd = _Py_open("/dev/urandom", O_RDONLY);
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if (fd < 0) {
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if (errno == ENOENT || errno == ENXIO ||
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errno == ENODEV || errno == EACCES)
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errno == ENODEV || errno == EACCES) {
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PyErr_SetString(PyExc_NotImplementedError,
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"/dev/urandom (or equivalent) not found");
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}
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/* otherwise, keep the OSError exception raised by _Py_open() */
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return -1;
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}
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urandom_cache.fd = -1;
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}
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}
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#endif /* !MS_WINDOWS */
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#endif
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/* Fill buffer with pseudo-random bytes generated by a linear congruent
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generator (LCG):
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}
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}
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/* If raise is zero:
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- Don't raise exceptions on error
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- Don't call PyErr_CheckSignals() on EINTR (retry directly the interrupted
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syscall)
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- Don't release the GIL to call syscalls. */
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/* Read random bytes:
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- Return 0 on success
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- Raise an exception (if raise is non-zero) and return -1 on error
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Used sources of entropy ordered by preference, preferred source first:
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- CryptGenRandom() on Windows
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- getrandom() function (ex: Linux and Solaris): call py_getrandom()
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- getentropy() function (ex: OpenBSD): call py_getentropy()
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- /dev/urandom device
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Read from the /dev/urandom device if getrandom() or getentropy() function
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is not available or does not work.
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Prefer getrandom() over getentropy() because getrandom() supports blocking
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and non-blocking mode: see the PEP 524. Python requires non-blocking RNG at
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startup to initialize its hash secret, but os.urandom() must block until the
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system urandom is initialized (at least on Linux 3.17 and newer).
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Prefer getrandom() and getentropy() over reading directly /dev/urandom
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because these functions don't need file descriptors and so avoid ENFILE or
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EMFILE errors (too many open files): see the issue #18756.
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Only the getrandom() function supports non-blocking mode.
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Only use RNG running in the kernel. They are more secure because it is
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harder to get the internal state of a RNG running in the kernel land than a
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RNG running in the user land. The kernel has a direct access to the hardware
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and has access to hardware RNG, they are used as entropy sources.
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Note: the OpenSSL RAND_pseudo_bytes() function does not automatically reseed
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its RNG on fork(), two child processes (with the same pid) generate the same
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random numbers: see issue #18747. Kernel RNGs don't have this issue,
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they have access to good quality entropy sources.
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If raise is zero:
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- Don't raise an exception on error
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- Don't call the Python signal handler (don't call PyErr_CheckSignals()) if
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a function fails with EINTR: retry directly the interrupted function
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- Don't release the GIL to call functions.
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*/
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static int
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pyurandom(void *buffer, Py_ssize_t size, int blocking, int raise)
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{
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#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
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int res;
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#endif
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if (size < 0) {
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if (raise) {
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PyErr_Format(PyExc_ValueError,
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#ifdef MS_WINDOWS
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return win32_urandom((unsigned char *)buffer, size, raise);
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#elif defined(PY_GETENTROPY)
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return py_getentropy(buffer, size, raise);
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#else
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return dev_urandom(buffer, size, blocking, raise);
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#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
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#ifdef PY_GETRANDOM
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res = py_getrandom(buffer, size, blocking, raise);
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#else
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res = py_getentropy(buffer, size, raise);
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#endif
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if (res < 0) {
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return -1;
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}
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if (res == 1) {
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return 0;
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}
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/* getrandom() or getentropy() function is not available: failed with
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ENOSYS or EPERM. Fall back on reading from /dev/urandom. */
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#endif
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return dev_urandom(buffer, size, raise);
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#endif
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}
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CryptReleaseContext(hCryptProv, 0);
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hCryptProv = 0;
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}
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#elif defined(PY_GETENTROPY)
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/* nothing to clean */
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#else
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dev_urandom_close();
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#endif
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