linux: Add support for getrandom vDSO

Linux 6.11 has getrandom() in vDSO. It operates on a thread-local opaque
state allocated with mmap using flags specified by the vDSO.

Multiple states are allocated at once, as many as fit into a page, and
these are held in an array of available states to be doled out to each
thread upon first use, and recycled when a thread terminates. As these
states run low, more are allocated.

To make this procedure async-signal-safe, a simple guard is used in the
LSB of the opaque state address, falling back to the syscall if there's
reentrancy contention.

Also, _Fork() is handled by blocking signals on opaque state allocation
(so _Fork() always sees a consistent state even if it interrupts a
getrandom() call) and by iterating over the thread stack cache on
reclaim_stack. Each opaque state will be in the free states list
(grnd_alloc.states) or allocated to a running thread.

The cancellation is handled by always using GRND_NONBLOCK flags while
calling the vDSO, and falling back to the cancellable syscall if the
kernel returns EAGAIN (would block). Since getrandom is not defined by
POSIX and cancellation is supported as an extension, the cancellation is
handled as 'may occur' instead of 'shall occur' [1], meaning that if
vDSO does not block (the expected behavior) getrandom will not act as a
cancellation entrypoint. It avoids a pthread_testcancel call on the fast
path (different than 'shall occur' functions, like sem_wait()).

It is currently enabled for x86_64, which is available in Linux 6.11,
and aarch64, powerpc32, powerpc64, loongarch64, and s390x, which are
available in Linux 6.12.

Link: https://pubs.opengroup.org/onlinepubs/9799919799/nframe.html [1]
Co-developed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com> # x86_64
Tested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org> # x86_64, aarch64
Tested-by: Xi Ruoyao <xry111@xry111.site> # x86_64, aarch64, loongarch64
Tested-by: Stefan Liebler <stli@linux.ibm.com> # s390x

NEWS

@@ -49,7 +49,9 @@ Deprecated and removed features, and other changes affecting compatibility:
 
 Changes to build and runtime requirements:
 
-  [Add changes to build and runtime requirements here]
+* On recent Linux kernels with vDSO getrandom support, getrandom does not
+  act as a "shall occur" cancellation point, in which case it might
+  not issue a syscall or trigger a deferred cancellation event.
 
 Security related changes:
 

elf/libc_early_init.c

@@ -23,6 +23,7 @@
 #include <lowlevellock.h>
 #include <pthread_early_init.h>
 #include <sys/single_threaded.h>
+#include <getrandom-internal.h>
 
 #ifdef SHARED
 _Bool __libc_initial;
@@ -43,6 +44,8 @@ __libc_early_init (_Bool initial)
 
   __pthread_early_init ();
 
+  __getrandom_early_init (initial);
+
 #if ENABLE_ELISION_SUPPORT
   __lll_elision_init ();
 #endif

malloc/malloc.c

@@ -3140,7 +3140,7 @@ static void
 tcache_key_initialize (void)
 {
   /* We need to use the _nostatus version here, see BZ 29624.  */
-  if (__getrandom_nocancel_nostatus (&tcache_key, sizeof(tcache_key),
+  if (__getrandom_nocancel_nostatus_direct (&tcache_key, sizeof(tcache_key),
 					    GRND_NONBLOCK)
       != sizeof (tcache_key))
     {

nptl/allocatestack.c

@@ -132,6 +132,8 @@ get_cached_stack (size_t *sizep, void **memp)
   __libc_lock_init (result->exit_lock);
   memset (&result->tls_state, 0, sizeof result->tls_state);
 
+  result->getrandom_buf = NULL;
+
   /* Clear the DTV.  */
   dtv_t *dtv = GET_DTV (TLS_TPADJ (result));
   for (size_t cnt = 0; cnt < dtv[-1].counter; ++cnt)

nptl/descr.h

@@ -404,6 +404,9 @@ struct pthread
   /* Used on strsignal.  */
   struct tls_internal_t tls_state;
 
+  /* getrandom vDSO per-thread opaque state.  */
+  void *getrandom_buf;
+
   /* rseq area registered with the kernel.  Use a custom definition
      here to isolate from kernel struct rseq changes.  The
      implementation of sched_getcpu needs acccess to the cpu_id field;

nptl/pthread_create.c

@@ -38,6 +38,7 @@
 #include <version.h>
 #include <clone_internal.h>
 #include <futex-internal.h>
+#include <getrandom-internal.h>
 
 #include <shlib-compat.h>
 
@@ -549,6 +550,10 @@ start_thread (void *arg)
     }
 #endif
 
+  /* Release the vDSO getrandom per-thread buffer with all signal blocked,
+     to avoid creating a new free-state block during thread release.  */
+  __getrandom_vdso_release (pd);
+
   if (!pd->user_stack)
     advise_stack_range (pd->stackblock, pd->stackblock_size, (uintptr_t) pd,
 			pd->guardsize);

stdlib/Makefile

@@ -278,6 +278,7 @@ tests := \
   tst-cxa_atexit \
   tst-environ \
   tst-getrandom \
+  tst-getrandom2 \
   tst-labs \
   tst-limits \
   tst-llabs \
@@ -627,3 +628,4 @@ $(objpfx)tst-setcontext3.out: tst-setcontext3.sh $(objpfx)tst-setcontext3
 $(objpfx)tst-qsort5: $(libm)
 $(objpfx)tst-concurrent-exit: $(shared-thread-library)
 $(objpfx)tst-concurrent-quick_exit: $(shared-thread-library)
+$(objpfx)tst-getrandom2: $(shared-thread-library)

stdlib/tst-getrandom2.c

@@ -0,0 +1,47 @@
+/* Tests for the getrandom functions.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <gnu/lib-names.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+#include <sys/random.h>
+
+static __typeof (getrandom) *getrandom_ptr;
+
+static void *
+threadfunc (void *ignored)
+{
+  char buffer;
+  TEST_COMPARE (getrandom_ptr (&buffer, 1, 0), 1);
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  /* Check if issuing getrandom in the secondary libc.so works when
+     the vDSO might be potentially used.  */
+  void *handle = xdlmopen (LM_ID_NEWLM, LIBC_SO, RTLD_NOW);
+  getrandom_ptr = xdlsym (handle, "getrandom");
+  for (int i = 0; i < 1000; ++i)
+    xpthread_join (xpthread_create (NULL, threadfunc, NULL));
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/generic/getrandom-internal.h

@@ -0,0 +1,26 @@
+/* Internal definitions for getrandom implementation.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _GETRANDOM_INTERNAL_H
+#define _GETRANDOM_INTERNAL_H
+
+static inline void __getrandom_early_init (_Bool)
+{
+}
+
+#endif

sysdeps/generic/not-cancel.h

@@ -51,7 +51,9 @@
   __fcntl64 (fd, cmd, __VA_ARGS__)
 #define __getrandom_nocancel(buf, size, flags) \
   __getrandom (buf, size, flags)
-#define __getrandom_nocancel_nostatus(buf, size, flags) \
+#define __getrandom_nocancel_direct(buf, size, flags) \
+  __getrandom (buf, size, flags)
+#define __getrandom_nocancel_nostatus_direct(buf, size, flags) \
   __getrandom (buf, size, flags)
 #define __poll_infinity_nocancel(fds, nfds) \
   __poll (fds, nfds, -1)

sysdeps/mach/hurd/not-cancel.h

@@ -79,7 +79,7 @@ __typeof (__fcntl) __fcntl_nocancel;
 /* Non cancellable getrandom syscall that does not also set errno in case of
    failure.  */
 static inline ssize_t
-__getrandom_nocancel_nostatus (void *buf, size_t buflen, unsigned int flags)
+__getrandom_nocancel_nostatus_direct (void *buf, size_t buflen, unsigned int flags)
 {
   int save_errno = errno;
   ssize_t r = __getrandom (buf, buflen, flags);
@@ -90,6 +90,8 @@ __getrandom_nocancel_nostatus (void *buf, size_t buflen, unsigned int flags)
 
 #define __getrandom_nocancel(buf, size, flags) \
   __getrandom (buf, size, flags)
+#define __getrandom_nocancel_direct(buf, size, flags) \
+  __getrandom (buf, size, flags)
 
 #define __poll_infinity_nocancel(fds, nfds) \
   __poll (fds, nfds, -1)

sysdeps/nptl/_Fork.c

@@ -19,6 +19,7 @@
 #include <arch-fork.h>
 #include <libc-lock.h>
 #include <pthreadP.h>
+#include <getrandom-internal.h>
 
 pid_t
 _Fork (void)
@@ -50,6 +51,7 @@ _Fork (void)
       self->robust_head.list = &self->robust_head;
       INTERNAL_SYSCALL_CALL (set_robust_list, &self->robust_head,
 			     sizeof (struct robust_list_head));
+      call_function_static_weak (__getrandom_fork_subprocess);
     }
 
   __abort_lock_unlock (&original_sigmask);

sysdeps/nptl/fork.h

@@ -26,6 +26,7 @@
 #include <mqueue.h>
 #include <pthreadP.h>
 #include <sysdep.h>
+#include <getrandom-internal.h>
 
 static inline void
 fork_system_setup (void)
@@ -46,6 +47,7 @@ fork_system_setup_after_fork (void)
 
   call_function_static_weak (__mq_notify_fork_subprocess);
   call_function_static_weak (__timer_fork_subprocess);
+  call_function_static_weak (__getrandom_fork_subprocess);
 }
 
 /* In case of a fork() call the memory allocation in the child will be
@@ -128,9 +130,19 @@ reclaim_stacks (void)
 		    curp->specific_used = true;
 		  }
 	    }
+
+	  call_function_static_weak (__getrandom_reset_state, curp);
 	}
     }
 
+  /* Also reset stale getrandom states for user stack threads.  */
+  list_for_each (runp, &GL (dl_stack_user))
+    {
+      struct pthread *curp = list_entry (runp, struct pthread, list);
+      if (curp != self)
+	call_function_static_weak (__getrandom_reset_state, curp);
+    }
+
   /* Add the stack of all running threads to the cache.  */
   list_splice (&GL (dl_stack_used), &GL (dl_stack_cache));
 

sysdeps/unix/sysv/linux/aarch64/sysdep.h

@@ -164,6 +164,7 @@
 # define HAVE_CLOCK_GETRES64_VSYSCALL	"__kernel_clock_getres"
 # define HAVE_CLOCK_GETTIME64_VSYSCALL	"__kernel_clock_gettime"
 # define HAVE_GETTIMEOFDAY_VSYSCALL	"__kernel_gettimeofday"
+# define HAVE_GETRANDOM_VSYSCALL        "__kernel_getrandom"
 
 # define HAVE_CLONE3_WRAPPER		1
 

sysdeps/unix/sysv/linux/dl-vdso-setup.c

@@ -66,6 +66,11 @@ PROCINFO_CLASS int (*_dl_vdso_clock_getres) (clockid_t,
 PROCINFO_CLASS int (*_dl_vdso_clock_getres_time64) (clockid_t,
 						    struct __timespec64 *) RELRO;
 # endif
+# ifdef HAVE_GETRANDOM_VSYSCALL
+PROCINFO_CLASS ssize_t (*_dl_vdso_getrandom) (void *buffer, size_t len,
+                                              unsigned int flags, void *state,
+                                              size_t state_len) RELRO;
+# endif
 
 /* PowerPC specific ones.  */
 # ifdef HAVE_GET_TBFREQ

sysdeps/unix/sysv/linux/dl-vdso-setup.h

@@ -50,6 +50,9 @@ setup_vdso_pointers (void)
 #ifdef HAVE_RISCV_HWPROBE
   GLRO(dl_vdso_riscv_hwprobe) = dl_vdso_vsym (HAVE_RISCV_HWPROBE);
 #endif
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  GLRO(dl_vdso_getrandom) = dl_vdso_vsym (HAVE_GETRANDOM_VSYSCALL);
+#endif
 }
 
 #endif

sysdeps/unix/sysv/linux/getrandom-internal.h

@@ -0,0 +1,29 @@
+/* Internal definitions for Linux getrandom implementation.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _GETRANDOM_INTERNAL_H
+#define _GETRANDOM_INTERNAL_H
+
+#include <pthreadP.h>
+
+extern void __getrandom_early_init (_Bool) attribute_hidden;
+
+extern void __getrandom_fork_subprocess (void) attribute_hidden;
+extern void __getrandom_vdso_release (struct pthread *curp) attribute_hidden;
+extern void __getrandom_reset_state (struct pthread *curp) attribute_hidden;
+#endif

sysdeps/unix/sysv/linux/getrandom.c

@@ -21,12 +21,314 @@
 #include <unistd.h>
 #include <sysdep-cancel.h>
 
+static inline ssize_t
+getrandom_syscall (void *buffer, size_t length, unsigned int flags,
+		   bool cancel)
+{
+  return cancel
+	 ? SYSCALL_CANCEL (getrandom, buffer, length, flags)
+	 : INLINE_SYSCALL_CALL (getrandom, buffer, length, flags);
+}
+
+#ifdef HAVE_GETRANDOM_VSYSCALL
+# include <assert.h>
+# include <ldsodefs.h>
+# include <libc-lock.h>
+# include <list.h>
+# include <setvmaname.h>
+# include <sys/mman.h>
+# include <sys/sysinfo.h>
+# include <tls-internal.h>
+
+/* These values will be initialized at loading time by calling the
+   _dl_vdso_getrandom with a special value.  The 'state_size' is the opaque
+   state size per-thread allocated with a mmap using 'mmap_prot' and
+   'mmap_flags' argument.  */
+static uint32_t state_size;
+static uint32_t state_size_cache_aligned;
+static uint32_t mmap_prot;
+static uint32_t mmap_flags;
+
+/* The function below are used on reentracy handling with (i.e. SA_NODEFER).
+   Before allocating a new state or issue the vDSO, atomically read the
+   current thread buffer, and if this is already reserved (is_reserved_ptr)
+   fallback to the syscall.  Otherwise, reserve the buffer by atomically
+   setting the LSB of the opaque state pointer.  The bit is cleared after the
+   vDSO is called, or before issuing the fallback syscall.  */
+
+static inline void *reserve_ptr (void *p)
+{
+  return (void *) ((uintptr_t) (p) | 1UL);
+}
+
+static inline void *release_ptr (void *p)
+{
+  return (void *) ((uintptr_t) (p) & ~1UL);
+}
+
+static inline bool is_reserved_ptr (void *p)
+{
+  return (uintptr_t) (p) & 1UL;
+}
+
+static struct
+{
+  __libc_lock_define (, lock);
+
+  void **states;  /* Queue of opaque states allocated with the kernel
+		     provided flags and used on getrandom vDSO call.  */
+  size_t len;	  /* Number of available free states in the queue.  */
+  size_t total;	  /* Number of states allocated from the kernel.  */
+  size_t cap;     /* Total number of states that 'states' can hold before
+		     needed to be resized.  */
+} grnd_alloc = {
+  .lock = LLL_LOCK_INITIALIZER
+};
+
+static bool
+vgetrandom_get_state_alloc (void)
+{
+  /* Start by allocating one page for the opaque states.  */
+  size_t block_size = ALIGN_UP (state_size_cache_aligned, GLRO(dl_pagesize));
+  size_t states_per_page = GLRO (dl_pagesize) / state_size_cache_aligned;
+  void *block = __mmap (NULL, GLRO(dl_pagesize), mmap_prot, mmap_flags, -1, 0);
+  if (block == MAP_FAILED)
+    return false;
+  __set_vma_name (block, block_size, " glibc: getrandom");
+
+  if (grnd_alloc.total + states_per_page > grnd_alloc.cap)
+    {
+      /* Use a new mmap instead of trying to mremap.  It avoids a
+	 potential multithread fork issue where fork is called just after
+	 mremap returns but before assigning to the grnd_alloc.states,
+	 thus making the its value invalid in the child.  */
+      void *old_states = grnd_alloc.states;
+      size_t new_states_size = ALIGN_UP ((grnd_alloc.total + states_per_page)
+					 * sizeof (*grnd_alloc.states),
+					 GLRO(dl_pagesize));
+
+      /* There is no need to memcpy any opaque state information because
+	 all the allocated opaque states are assigned to running threads
+	 (meaning that if we iterate over them we can reconstruct the state
+	 list).  */
+      void **states = __mmap (NULL, new_states_size, PROT_READ | PROT_WRITE,
+			      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+      if (states == MAP_FAILED)
+	{
+	  __munmap (block, block_size);
+	  return false;
+	}
+
+      /* Atomically replace the old state, so if a fork happens the child
+	 process will see a consistent free state buffer.  The size might
+	 not be updated, but it does not really matter since the buffer is
+	 always increased.  */
+      grnd_alloc.states = states;
+      atomic_thread_fence_seq_cst ();
+      if (old_states != NULL)
+	__munmap (old_states, grnd_alloc.cap * sizeof (*grnd_alloc.states));
+
+      __set_vma_name (states, new_states_size, " glibc: getrandom states");
+      grnd_alloc.cap = new_states_size / sizeof (*grnd_alloc.states);
+      atomic_thread_fence_seq_cst ();
+    }
+
+  for (size_t i = 0; i < states_per_page; ++i)
+    {
+      /* There is no need to handle states that straddle a page because
+	 we allocate only one page.  */
+      grnd_alloc.states[i] = block;
+      block += state_size_cache_aligned;
+    }
+  /* Concurrent fork should not observe the previous pointer value.  */
+  grnd_alloc.len = states_per_page;
+  grnd_alloc.total += states_per_page;
+  atomic_thread_fence_seq_cst ();
+
+  return true;
+}
+
+/* Allocate an opaque state for vgetrandom.  If the grnd_alloc does not have
+   any, mmap() another page of them using the vgetrandom parameters.  */
+static void *
+vgetrandom_get_state (void)
+{
+  void *state = NULL;
+
+  /* The signal blocking avoid the potential issue where _Fork() (which is
+     async-signal-safe) is called with the lock taken.  The function is
+     called only once during thread lifetime, so the overhead should be
+     minimal.  */
+  internal_sigset_t set;
+  internal_signal_block_all (&set);
+  __libc_lock_lock (grnd_alloc.lock);
+
+  if (grnd_alloc.len > 0 || vgetrandom_get_state_alloc ())
+    state = grnd_alloc.states[--grnd_alloc.len];
+
+  __libc_lock_unlock (grnd_alloc.lock);
+  internal_signal_restore_set (&set);
+
+  return state;
+}
+
+/* Returns true when vgetrandom is used successfully.  Returns false if the
+   syscall fallback should be issued in the case the vDSO is not present, in
+   the case of reentrancy, or if any memory allocation fails.  */
+static ssize_t
+getrandom_vdso (void *buffer, size_t length, unsigned int flags, bool cancel)
+{
+  if (__glibc_unlikely (state_size == 0))
+    return getrandom_syscall (buffer, length, flags, cancel);
+
+  struct pthread *self = THREAD_SELF;
+
+  void *state = atomic_load_relaxed (&self->getrandom_buf);
+  if (is_reserved_ptr (state))
+    return getrandom_syscall (buffer, length, flags, cancel);
+  atomic_store_relaxed (&self->getrandom_buf, reserve_ptr (state));
+  __atomic_signal_fence (__ATOMIC_ACQ_REL);
+
+  bool r = false;
+  if (state == NULL)
+    {
+      state = vgetrandom_get_state ();
+      if (state == NULL)
+        goto out;
+    }
+
+  /* Since the vDSO implementation does not issue the syscall with the
+     cancellation bridge (__syscall_cancel_arch), use GRND_NONBLOCK so there
+     is no potential unbounded blocking in the kernel.  It should be a rare
+     situation, only at system startup when RNG is not initialized.  */
+  ssize_t ret = GLRO (dl_vdso_getrandom) (buffer,
+					  length,
+					  flags | GRND_NONBLOCK,
+					  state,
+					  state_size);
+  if (INTERNAL_SYSCALL_ERROR_P (ret))
+    {
+      /* Fallback to the syscall if the kernel would block.  */
+      int err = INTERNAL_SYSCALL_ERRNO (ret);
+      if (err == EAGAIN && !(flags & GRND_NONBLOCK))
+        goto out;
+
+      __set_errno (err);
+      ret = -1;
+    }
+  r = true;
+
+out:
+  __atomic_signal_fence (__ATOMIC_ACQ_REL);
+  atomic_store_relaxed (&self->getrandom_buf, state);
+  return r ? ret : getrandom_syscall (buffer, length, flags, cancel);
+}
+#endif
+
+void
+__getrandom_early_init (_Bool initial)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  /* libcs loaded for audit modules, dlmopen, etc. fallback to syscall.  */
+  if (initial && (GLRO (dl_vdso_getrandom) != NULL))
+    {
+      /* Used to query the vDSO for the required mmap flags and the opaque
+	 per-thread state size.  Defined by linux/random.h.  */
+      struct vgetrandom_opaque_params
+      {
+	uint32_t size_of_opaque_state;
+	uint32_t mmap_prot;
+	uint32_t mmap_flags;
+	uint32_t reserved[13];
+      } params;
+      if (GLRO(dl_vdso_getrandom) (NULL, 0, 0, &params, ~0UL) == 0)
+	{
+	  /* Align each opaque state to L1 data cache size to avoid false
+	     sharing.  If the size can not be obtained, use the kernel
+	     provided one.  */
+	  state_size = params.size_of_opaque_state;
+
+	  long int ld1sz = __sysconf (_SC_LEVEL1_DCACHE_LINESIZE);
+	  if (ld1sz <= 0)
+	    ld1sz = 1;
+	  state_size_cache_aligned = ALIGN_UP (state_size, ld1sz);
+	  /* Do not enable vDSO if the required opaque state size is larger
+	     than a page because we only allocate one page per time to hold
+	     the states.  */
+	  if (state_size_cache_aligned > GLRO(dl_pagesize))
+	    {
+	      state_size = 0;
+	      return;
+	    }
+	  mmap_prot = params.mmap_prot;
+	  mmap_flags = params.mmap_flags;
+	}
+    }
+#endif
+}
+
+/* Re-add the state state from CURP on the free list.  This function is
+   called after fork returns in the child, so no locking is required.  */
+void
+__getrandom_reset_state (struct pthread *curp)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  if (grnd_alloc.states == NULL || curp->getrandom_buf == NULL)
+    return;
+  assert (grnd_alloc.len < grnd_alloc.cap);
+  grnd_alloc.states[grnd_alloc.len++] = release_ptr (curp->getrandom_buf);
+  curp->getrandom_buf = NULL;
+#endif
+}
+
+/* Called when a thread terminates, and adds its random buffer back into the
+   allocator pool for use in a future thread.  This is called by
+   pthread_create during thread termination, and after signal has been
+   blocked. */
+void
+__getrandom_vdso_release (struct pthread *curp)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  if (curp->getrandom_buf == NULL)
+    return;
+
+  __libc_lock_lock (grnd_alloc.lock);
+  grnd_alloc.states[grnd_alloc.len++] = curp->getrandom_buf;
+  __libc_lock_unlock (grnd_alloc.lock);
+#endif
+}
+
+/* Reset the internal lock state in case another thread has locked while
+   this thread calls fork.  The stale thread states will be handled by
+   reclaim_stacks which calls __getrandom_reset_state on each thread.  */
+void
+__getrandom_fork_subprocess (void)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  grnd_alloc.lock = LLL_LOCK_INITIALIZER;
+#endif
+}
+
+ssize_t
+__getrandom_nocancel (void *buffer, size_t length, unsigned int flags)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  return getrandom_vdso (buffer, length, flags, false);
+#else
+  return getrandom_syscall (buffer, length, flags, false);
+#endif
+}
+
 /* Write up to LENGTH bytes of randomness starting at BUFFER.
    Return the number of bytes written, or -1 on error.  */
 ssize_t
 __getrandom (void *buffer, size_t length, unsigned int flags)
 {
-  return SYSCALL_CANCEL (getrandom, buffer, length, flags);
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  return getrandom_vdso (buffer, length, flags, true);
+#else
+  return getrandom_syscall (buffer, length, flags, true);
+#endif
 }
 libc_hidden_def (__getrandom)
 weak_alias (__getrandom, getrandom)

sysdeps/unix/sysv/linux/loongarch/sysdep.h

@@ -119,6 +119,7 @@
 #define HAVE_CLOCK_GETTIME64_VSYSCALL "__vdso_clock_gettime"
 #define HAVE_GETTIMEOFDAY_VSYSCALL "__vdso_gettimeofday"
 #define HAVE_GETCPU_VSYSCALL "__vdso_getcpu"
+#define HAVE_GETRANDOM_VSYSCALL "__vdso_getrandom"
 
 #define HAVE_CLONE3_WRAPPER 1
 

sysdeps/unix/sysv/linux/not-cancel.h

@@ -27,6 +27,7 @@
 #include <sys/syscall.h>
 #include <sys/wait.h>
 #include <time.h>
+#include <sys/random.h>
 
 /* Non cancellable open syscall.  */
 __typeof (open) __open_nocancel;
@@ -84,15 +85,17 @@ __writev_nocancel_nostatus (int fd, const struct iovec *iov, int iovcnt)
 }
 
 static inline ssize_t
-__getrandom_nocancel (void *buf, size_t buflen, unsigned int flags)
+__getrandom_nocancel_direct (void *buf, size_t buflen, unsigned int flags)
 {
   return INLINE_SYSCALL_CALL (getrandom, buf, buflen, flags);
 }
 
+__typeof (getrandom) __getrandom_nocancel attribute_hidden;
+
 /* Non cancellable getrandom syscall that does not also set errno in case of
    failure.  */
 static inline ssize_t
-__getrandom_nocancel_nostatus (void *buf, size_t buflen, unsigned int flags)
+__getrandom_nocancel_nostatus_direct (void *buf, size_t buflen, unsigned int flags)
 {
   return INTERNAL_SYSCALL_CALL (getrandom, buf, buflen, flags);
 }

sysdeps/unix/sysv/linux/powerpc/sysdep.h

@@ -223,5 +223,6 @@
 #define HAVE_TIME_VSYSCALL		"__kernel_time"
 #define HAVE_GETTIMEOFDAY_VSYSCALL      "__kernel_gettimeofday"
 #define HAVE_GET_TBFREQ                 "__kernel_get_tbfreq"
+#define HAVE_GETRANDOM_VSYSCALL         "__kernel_getrandom"
 
 #endif /* _LINUX_POWERPC_SYSDEP_H  */

sysdeps/unix/sysv/linux/s390/sysdep.h

@@ -72,6 +72,7 @@
 #ifdef __s390x__
 #define HAVE_CLOCK_GETRES64_VSYSCALL	"__kernel_clock_getres"
 #define HAVE_CLOCK_GETTIME64_VSYSCALL	"__kernel_clock_gettime"
+#define HAVE_GETRANDOM_VSYSCALL		"__kernel_getrandom"
 #else
 #define HAVE_CLOCK_GETRES_VSYSCALL	"__kernel_clock_getres"
 #define HAVE_CLOCK_GETTIME_VSYSCALL	"__kernel_clock_gettime"

sysdeps/unix/sysv/linux/x86_64/sysdep.h

@@ -376,6 +376,7 @@
 # define HAVE_TIME_VSYSCALL             "__vdso_time"
 # define HAVE_GETCPU_VSYSCALL		"__vdso_getcpu"
 # define HAVE_CLOCK_GETRES64_VSYSCALL   "__vdso_clock_getres"
+# define HAVE_GETRANDOM_VSYSCALL        "__vdso_getrandom"
 
 # define HAVE_CLONE3_WRAPPER			1