Date: Wed, 21 Jul 2021 00:10:29 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, Alexander Motin <mav@freebsd.org> Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 28d70deaafa6 - main - Fix race between first rand(3) calls. Message-ID: <df8b37ed-d115-4361-d8e1-aa80a4a601af@FreeBSD.org> In-Reply-To: <YPcmvk924TKbOMu9@kib.kiev.ua> References: <202107201730.16KHUXPM057550@gitrepo.freebsd.org> <YPcmvk924TKbOMu9@kib.kiev.ua>
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On 20/07/2021 22:40, Konstantin Belousov wrote: > On Tue, Jul 20, 2021 at 05:30:33PM +0000, Alexander Motin wrote: >> The branch main has been updated by mav: >> >> URL: https://cgit.FreeBSD.org/src/commit/?id=28d70deaafa62c5d1602de5272c0aad0fcca8aff >> >> commit 28d70deaafa62c5d1602de5272c0aad0fcca8aff >> Author: Alexander Motin <mav@FreeBSD.org> >> AuthorDate: 2021-07-20 17:15:08 +0000 >> Commit: Alexander Motin <mav@FreeBSD.org> >> CommitDate: 2021-07-20 17:30:28 +0000 >> >> Fix race between first rand(3) calls. >> >> Before this patch there was a chance for thread that called rand(3) >> slightly later to see rand3_state already allocated, but not yet >> initialized. While this API is not expected to be thread-safe, it >> is not expected to crash. ztest on 64-thread system reproduced it >> reliably for me. >> >> MFC after: 1 month >> --- >> lib/libc/stdlib/rand.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c >> index bddb0f040302..353f59349e1d 100644 >> --- a/lib/libc/stdlib/rand.c >> +++ b/lib/libc/stdlib/rand.c >> @@ -44,6 +44,7 @@ __FBSDID("$FreeBSD$"); >> #include <stdbool.h> >> #include <stdlib.h> >> #include <syslog.h> >> +#include <machine/atomic.h> >> #include "un-namespace.h" >> >> #include "random.h" >> @@ -68,11 +69,15 @@ static struct __random_state *rand3_state; >> static void >> initialize_rand3(void) >> { >> + struct __random_state *state; >> int error; >> >> - rand3_state = allocatestate(TYPE_3); >> - error = initstate_r(rand3_state, 1, rand3_state->rst_randtbl, BREAK_3); >> + state = allocatestate(TYPE_3); >> + error = initstate_r(state, 1, state->rst_randtbl, BREAK_3); >> assert(error == 0); >> + if (!atomic_cmpset_rel_ptr((volatile uintptr_t *)&rand3_state, >> + (uintptr_t)NULL, (uintptr_t)state)) >> + free(state); > For this to have effect on less ordered architectures (AKA non-x86), at > least reads of rand3_state should be atomic_load_acq(). > > But consider using _once() in libc, which takes care about ordering/threads. FWIW, I've been using the following patch locally. And the reason for the patched is discussed here: https://www.mail-archive.com/freebsd-current@freebsd.org/msg182365.html commit cc38290d14a5bbab64e419a2d6d8c34f61840829 Author: Andriy Gapon <avg@FreeBSD.org> AuthorDate: Mon Apr 5 17:33:07 2021 +0300 Commit: Andriy Gapon <avg@FreeBSD.org> CommitDate: Tue Jun 8 10:34:28 2021 +0300 [tmp?] make rand/srand initialization thread safe But rand(3) still remains non-thread-safe in general. This is only to prevent an initialization time crash in ztest. Probably should be fixed there, e.g. by using a different random number API or by adding some locking. diff --git a/lib/libc/stdlib/rand.c b/lib/libc/stdlib/rand.c index bddb0f040302..0d3ed2230e10 100644 --- a/lib/libc/stdlib/rand.c +++ b/lib/libc/stdlib/rand.c @@ -41,11 +41,13 @@ __FBSDID("$FreeBSD$"); #include <sys/param.h> #include <sys/sysctl.h> #include <assert.h> +#include <pthread.h> #include <stdbool.h> #include <stdlib.h> #include <syslog.h> #include "un-namespace.h" +#include "libc_private.h" #include "random.h" /* @@ -64,6 +66,7 @@ __FBSDID("$FreeBSD$"); * the advantage of being the one already in the tree. */ static struct __random_state *rand3_state; +static pthread_once_t __random_state_once = PTHREAD_ONCE_INIT; static void initialize_rand3(void) @@ -78,16 +81,14 @@ initialize_rand3(void) int rand(void) { - if (rand3_state == NULL) - initialize_rand3(); + _once(&__random_state_once, initialize_rand3); return ((int)random_r(rand3_state)); } void srand(unsigned seed) { - if (rand3_state == NULL) - initialize_rand3(); + _once(&__random_state_once, initialize_rand3); srandom_r(rand3_state, seed); } -- Andriy Gapon
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