[v5] linux: Add support for getrandom vDSO

Message ID 20240910131456.2029980-1-adhemerval.zanella@linaro.org
State Superseded
Headers
Series [v5] linux: Add support for getrandom vDSO |

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Commit Message

Adhemerval Zanella Netto Sept. 10, 2024, 1:13 p.m. UTC
  Linux 6.11 will support calling getrandom() from the vDSO (commit
4ad10a5f5f78a5b3e525a63bd075a4eb1139dde1). 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. And the kernel's random.git tree has
support pending for arm64, powerpc32, powerpc64, and loongarch64.

Checked on x86_64-linux-gnu.

Co-authored-by: Jason A. Donenfeld <Jason@zx2c4.com>

[1] https://pubs.opengroup.org/onlinepubs/9799919799/nframe.html

--

Changes from v4:
* Handle cancellation by using GRND_NONBLOCK.

Changes from v3:
* Query the vgetrandom mmap parameters at loading time, instead
  of each block allocation.

Changes from v2:
* Move the getrandom opaque state buffer to 'struct pthread'.
* Move the state release to start_thread and after signals are
  blocked, to avoid a possible concurrency update.
* Move the state reset on fork() to reclaim_stack.  This makes
  all the logic of handling threads on one place, and simplify
  the __getrandom_fork_subprocess (no need to extra argument).
* Fixed some style issue on comments.
* Do not use mremap to reallocate the free states, it avoids
  a possible fork() issue where the allocator state pointer update
  is interrupted just after the mremap call returns, but before
  the assignment.  This is done by mmap/munmap a a new buffer with
  the new size, a fork() will not invalidate the old buffer.
* Block all signals before taking the lock to avoid the _Fork()
  issue if it interrupts getrandom while the lock is taken.

---
 include/sys/random.h                         |   4 +
 malloc/malloc.c                              |   4 +-
 nptl/allocatestack.c                         |   2 +
 nptl/descr.h                                 |   3 +
 nptl/pthread_create.c                        |   5 +
 sysdeps/generic/not-cancel.h                 |   4 +-
 sysdeps/mach/hurd/not-cancel.h               |   4 +-
 sysdeps/nptl/_Fork.c                         |   2 +
 sysdeps/nptl/fork.h                          |  12 +
 sysdeps/unix/sysv/linux/dl-vdso-setup.c      |  12 +
 sysdeps/unix/sysv/linux/dl-vdso-setup.h      |  17 ++
 sysdeps/unix/sysv/linux/getrandom.c          | 229 ++++++++++++++++++-
 sysdeps/unix/sysv/linux/getrandom_vdso.h     |  36 +++
 sysdeps/unix/sysv/linux/include/sys/random.h |  29 +++
 sysdeps/unix/sysv/linux/not-cancel.h         |   7 +-
 sysdeps/unix/sysv/linux/x86_64/sysdep.h      |   1 +
 16 files changed, 364 insertions(+), 7 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/getrandom_vdso.h
 create mode 100644 sysdeps/unix/sysv/linux/include/sys/random.h
  

Comments

Florian Weimer Sept. 10, 2024, 4:45 p.m. UTC | #1
* Adhemerval Zanella:

> 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()).

Just as a quick comment: I think the POSIX model is that a function is
either a cancellation point, or it isn't.  The programing model depends
on cancellation being predictable, and not just happening once slow-ish
or blocking system calls happen.  POSIX expects that if you write a loop
with a cancellation point in it, it gets terminated promptly.

Our 'c' flag for fopen deviates from that rule, but it's non-standard
and probably not a good idea.  I don't think it helps with the getrandom
situation because the defaults are not aligned: with getrandom, zero
flags means blocking behavior, so zero flags should act on cancellation
for consistency (whether there is blocking or not).

The hopefully not-too-costly fix is to have a pthread_testcancel call
before or after the initial GRND_NONBLOCK vsyscall.  Then the
cancellation behavior will be predictable again from the programmer
point of view.

Thanks,
Flroian
  
Adhemerval Zanella Netto Sept. 10, 2024, 8:12 p.m. UTC | #2
On 10/09/24 13:45, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> 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()).
> 
> Just as a quick comment: I think the POSIX model is that a function is
> either a cancellation point, or it isn't.  The programing model depends
> on cancellation being predictable, and not just happening once slow-ish
> or blocking system calls happen.  POSIX expects that if you write a loop
> with a cancellation point in it, it gets terminated promptly.

That is not really what POSIX states [1], in 'Thread Cancellation Overview':

  Cancellation points are points inside of certain functions where a thread 
  has to act on any pending cancellation request when cancelability is enabled.
  For functions in the "shall occur" list, a cancellation check must be performed 
  on every call regardless of whether, absent the cancellation, the call would
  have blocked. For functions in the "may occur" list, a cancellation check may 
  be performed on some calls but not others; i.e., whether or not a cancellation
  point occurs when one of these functions is being executed can depend on 
  current conditions.

This was previously discussed on BZ#18243 [2].

> 
> Our 'c' flag for fopen deviates from that rule, but it's non-standard
> and probably not a good idea.  I don't think it helps with the getrandom
> situation because the defaults are not aligned: with getrandom, zero
> flags means blocking behavior, so zero flags should act on cancellation
> for consistency (whether there is blocking or not).
> 
> The hopefully not-too-costly fix is to have a pthread_testcancel call
> before or after the initial GRND_NONBLOCK vsyscall.  Then the
> cancellation behavior will be predictable again from the programmer
> point of view.

I don't think zero flag means 'blocking' behavior, the idea is the caller
*may* expect blocking.  The whole GRND_NONBLOCK flag indicates that, under 
certain conditions, the kernel may not block.

So I think what we need to define is if getrandom should act as "shall occur"
or as "may occur" wrt cancellation.  It is not clear how POSIX sets the category
for a cancellable entrypoint.

> 
> Thanks,
> Flroian
> 

[1] https://pubs.opengroup.org/onlinepubs/9799919799/xrat/V4_xsh_chap01.html
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=18243#c4
  

Patch

diff --git a/include/sys/random.h b/include/sys/random.h
index 6aa313d35d..35f64a0339 100644
--- a/include/sys/random.h
+++ b/include/sys/random.h
@@ -1,8 +1,12 @@ 
 #ifndef _SYS_RANDOM_H
 #include <stdlib/sys/random.h>
 
+#include_next <sys/random.h>
+
 # ifndef _ISOMAC
 
+# include <stdbool.h>
+
 extern ssize_t __getrandom (void *__buffer, size_t __length,
                             unsigned int __flags) __wur;
 libc_hidden_proto (__getrandom)
diff --git a/malloc/malloc.c b/malloc/malloc.c
index bcb6e5b83c..9e577ab900 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3140,8 +3140,8 @@  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),
-				     GRND_NONBLOCK)
+  if (__getrandom_nocancel_nostatus_direct (&tcache_key, sizeof(tcache_key),
+					    GRND_NONBLOCK)
       != sizeof (tcache_key))
     {
       tcache_key = random_bits ();
diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index 2cb562f8ea..d9adb5856c 100644
--- a/nptl/allocatestack.c
+++ b/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)
diff --git a/nptl/descr.h b/nptl/descr.h
index 65d3baaee3..989995262b 100644
--- a/nptl/descr.h
+++ b/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;
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index 1d3665d5ed..d1f5568b3b 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -38,6 +38,7 @@ 
 #include <version.h>
 #include <clone_internal.h>
 #include <futex-internal.h>
+#include <sys/random.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);
diff --git a/sysdeps/generic/not-cancel.h b/sysdeps/generic/not-cancel.h
index 2dd1064600..8e3f49cc07 100644
--- a/sysdeps/generic/not-cancel.h
+++ b/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)
diff --git a/sysdeps/mach/hurd/not-cancel.h b/sysdeps/mach/hurd/not-cancel.h
index 69fb3c00ef..ec5f5aa895 100644
--- a/sysdeps/mach/hurd/not-cancel.h
+++ b/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)
diff --git a/sysdeps/nptl/_Fork.c b/sysdeps/nptl/_Fork.c
index ef199ddbc3..adb7c18b29 100644
--- a/sysdeps/nptl/_Fork.c
+++ b/sysdeps/nptl/_Fork.c
@@ -18,6 +18,7 @@ 
 
 #include <arch-fork.h>
 #include <pthreadP.h>
+#include <sys/random.h>
 
 pid_t
 _Fork (void)
@@ -43,6 +44,7 @@  _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);
     }
   return pid;
 }
diff --git a/sysdeps/nptl/fork.h b/sysdeps/nptl/fork.h
index 7643926df9..106b2cf71d 100644
--- a/sysdeps/nptl/fork.h
+++ b/sysdeps/nptl/fork.h
@@ -26,6 +26,7 @@ 
 #include <mqueue.h>
 #include <pthreadP.h>
 #include <sysdep.h>
+#include <sys/random.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));
 
diff --git a/sysdeps/unix/sysv/linux/dl-vdso-setup.c b/sysdeps/unix/sysv/linux/dl-vdso-setup.c
index 3a44944dbb..b117a25922 100644
--- a/sysdeps/unix/sysv/linux/dl-vdso-setup.c
+++ b/sysdeps/unix/sysv/linux/dl-vdso-setup.c
@@ -66,6 +66,18 @@  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;
+/* 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.  */
+PROCINFO_CLASS uint32_t _dl_vdso_getrandom_state_size RELRO;
+PROCINFO_CLASS uint32_t _dl_vdso_getrandom_mmap_prot RELRO;
+PROCINFO_CLASS uint32_t _dl_vdso_getrandom_mmap_flags RELRO;
+# endif
 
 /* PowerPC specific ones.  */
 # ifdef HAVE_GET_TBFREQ
diff --git a/sysdeps/unix/sysv/linux/dl-vdso-setup.h b/sysdeps/unix/sysv/linux/dl-vdso-setup.h
index 8aee5a8212..c63b7689e5 100644
--- a/sysdeps/unix/sysv/linux/dl-vdso-setup.h
+++ b/sysdeps/unix/sysv/linux/dl-vdso-setup.h
@@ -19,6 +19,10 @@ 
 #ifndef _DL_VDSO_INIT_H
 #define _DL_VDSO_INIT_H
 
+#ifdef HAVE_GETRANDOM_VSYSCALL
+# include <getrandom_vdso.h>
+#endif
+
 /* Initialize the VDSO functions pointers.  */
 static inline void __attribute__ ((always_inline))
 setup_vdso_pointers (void)
@@ -50,6 +54,19 @@  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);
+  if (GLRO(dl_vdso_getrandom) != NULL)
+    {
+      struct vgetrandom_opaque_params params;
+      if (GLRO(dl_vdso_getrandom) (NULL, 0, 0, &params, ~0UL) == 0)
+	{
+	  GLRO(dl_vdso_getrandom_state_size) = params.size_of_opaque_state;
+	  GLRO(dl_vdso_getrandom_mmap_prot) = params.mmap_prot;
+	  GLRO(dl_vdso_getrandom_mmap_flags) = params.mmap_flags;
+	}
+    }
+#endif
 }
 
 #endif
diff --git a/sysdeps/unix/sysv/linux/getrandom.c b/sysdeps/unix/sysv/linux/getrandom.c
index 777d1decf0..15ecfa987f 100644
--- a/sysdeps/unix/sysv/linux/getrandom.c
+++ b/sysdeps/unix/sysv/linux/getrandom.c
@@ -21,12 +21,239 @@ 
 #include <unistd.h>
 #include <sysdep-cancel.h>
 
+#ifdef HAVE_GETRANDOM_VSYSCALL
+# include <getrandom_vdso.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>
+
+# define ALIGN_PAGE(p)		PTR_ALIGN_UP (p, GLRO (dl_pagesize))
+# define READ_ONCE(p)		(*((volatile typeof (p) *) (&(p))))
+# define WRITE_ONCE(p, v)	(*((volatile typeof (p) *) (&(p))) = (v))
+# define RESERVE_PTR(p)		((void *) ((uintptr_t) (p) | 1UL))
+# define RELEASE_PTR(p)		((void *) ((uintptr_t) (p) & ~1UL))
+# define IS_RESERVED_PTR(p)	(!!((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 numver of states that 'states' can hold before
+		     needed to be resized.  */
+} grnd_alloc = {
+  .lock = LLL_LOCK_INITIALIZER
+};
+
+static bool
+vgetrandom_get_state_alloc (void)
+{
+  size_t num = __get_nprocs (); /* Just a decent heuristic.  */
+
+  size_t block_size = ALIGN_PAGE (num * GLRO(dl_vdso_getrandom_state_size));
+  num = (GLRO (dl_pagesize) / GLRO(dl_vdso_getrandom_state_size)) *
+	(block_size / GLRO (dl_pagesize));
+  void *block = __mmap (NULL, block_size, GLRO(dl_vdso_getrandom_mmap_prot),
+			GLRO(dl_vdso_getrandom_mmap_flags), -1, 0);
+  if (block == MAP_FAILED)
+    return false;
+  __set_vma_name (block, block_size, " glibc: getrandom");
+
+  if (grnd_alloc.total + num > 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 old_states_size = ALIGN_PAGE (sizeof (*grnd_alloc.states) *
+					   grnd_alloc.total + num);
+      size_t states_size;
+      if (grnd_alloc.states == NULL)
+	states_size = old_states_size;
+      else
+	states_size = ALIGN_PAGE (sizeof (*grnd_alloc.states)
+				  * grnd_alloc.cap);
+
+      void **states = __mmap (NULL, 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.  */
+      atomic_store_relaxed (&grnd_alloc.states, states);
+      if (old_states != NULL)
+	__munmap (old_states, old_states_size);
+
+      __set_vma_name (states, states_size, " glibc: getrandom states");
+      grnd_alloc.cap = states_size / sizeof (*grnd_alloc.states);
+    }
+
+  for (size_t i = 0; i < num; ++i)
+    {
+      /* States should not straddle a page.  */
+      if (((uintptr_t) block & (GLRO (dl_pagesize) - 1)) +
+	  GLRO(dl_vdso_getrandom_state_size) > GLRO (dl_pagesize))
+	block = ALIGN_PAGE (block);
+      grnd_alloc.states[i] = block;
+      block += GLRO(dl_vdso_getrandom_state_size);
+    }
+  grnd_alloc.len = num;
+  grnd_alloc.total += num;
+
+  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 bool
+getrandom_vdso (ssize_t *ret, void *buffer, size_t length, unsigned int flags)
+{
+  if (GLRO (dl_vdso_getrandom_state_size) == 0)
+    return false;
+
+  struct pthread *self = THREAD_SELF;
+
+  /* If the LSB of getrandom_buf is set, then this function is already being
+     called, and we have a reentrant call from a signal handler.  In this case
+     fallback to the syscall.  */
+  void *state = READ_ONCE (self->getrandom_buf);
+  if (IS_RESERVED_PTR (state))
+    return false;
+  WRITE_ONCE (self->getrandom_buf, RESERVE_PTR (state));
+
+  bool r = false;
+  if (state == NULL)
+    {
+      state = vgetrandom_get_state ();
+      if (state == NULL)
+        goto out;
+    }
+
+  /* Since the vDSO fallback 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.  */
+  *ret = GLRO (dl_vdso_getrandom) (buffer, length, flags | GRND_NONBLOCK,
+				   state, GLRO(dl_vdso_getrandom_state_size));
+  if (INTERNAL_SYSCALL_ERROR_P (*ret))
+    {
+      /* Fallback to the cancellable 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:
+  WRITE_ONCE (self->getrandom_buf, state);
+  return r;
+}
+#endif
+
+/* Re-add the state state from CURP on the free list.  */
+void
+__getrandom_reset_state (struct pthread *curp)
+{
+#ifdef HAVE_GETRANDOM_VSYSCALL
+  if (grnd_alloc.states == NULL || curp->getrandom_buf == NULL)
+    return;
+  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.  */
+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
+  ssize_t r;
+  if (getrandom_vdso (&r, buffer, length, flags))
+    return r;
+#endif
+
+  return INLINE_SYSCALL_CALL (getrandom, buffer, length, flags);
+}
+
 /* 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
+  ssize_t r;
+  if (getrandom_vdso (&r, buffer, length, flags))
+    return r;
+#endif
+
+  return INTERNAL_SYSCALL_CALL (getrandom, buffer, length, flags);
 }
 libc_hidden_def (__getrandom)
 weak_alias (__getrandom, getrandom)
diff --git a/sysdeps/unix/sysv/linux/getrandom_vdso.h b/sysdeps/unix/sysv/linux/getrandom_vdso.h
new file mode 100644
index 0000000000..d1ef690e50
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/getrandom_vdso.h
@@ -0,0 +1,36 @@ 
+/* Linux getrandom vDSO support.
+   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_VDSO_H
+#define _GETRANDOM_VDSO_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <sys/types.h>
+
+/* 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];
+};
+
+#endif
diff --git a/sysdeps/unix/sysv/linux/include/sys/random.h b/sysdeps/unix/sysv/linux/include/sys/random.h
new file mode 100644
index 0000000000..5a48de2d29
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/include/sys/random.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 _LINUX_SYS_RANDOM_H
+#define _LINUX_SYS_RANDOM_H
+
+# ifndef _ISOMAC
+# include <pthreadP.h>
+
+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
+#endif
diff --git a/sysdeps/unix/sysv/linux/not-cancel.h b/sysdeps/unix/sysv/linux/not-cancel.h
index 2a7585b73f..12f26912d3 100644
--- a/sysdeps/unix/sysv/linux/not-cancel.h
+++ b/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);
 }
diff --git a/sysdeps/unix/sysv/linux/x86_64/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
index a2b021bd86..7dc072ae2d 100644
--- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
+++ b/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