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libc, librthread: _twait: subtraction is not comparison

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Compare the current time with the absolute timeout before computing
the relative timeout to avoid arithmetic overflow.  Fixes a bug where
large negative absolute timeouts are subtracted into large positive
relative timeouts and incorrectly cause the caller to block.

While here, use timespeccmp(3) and timespecsub(3) to simplify the
code.

Thread: https://marc.info/?l=openbsd-tech&m=169945962503129&w=2
This commit is contained in:
cheloha 2024-01-07 19:44:28 +00:00
parent cec65c07f7
commit 2e686a7fd2
2 changed files with 10 additions and 18 deletions
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14
lib/libc/thread/synch.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: synch.h,v 1.8 2023/11/08 15:51:28 cheloha Exp $ */
/* $OpenBSD: synch.h,v 1.9 2024/01/07 19:44:28 cheloha Exp $ */
/*
* Copyright (c) 2017 Martin Pieuchot
*
@ -28,7 +28,7 @@ _wake(volatile uint32_t *p, int n)
static inline int
_twait(volatile uint32_t *p, int val, clockid_t clockid, const struct timespec *abs)
{
struct timespec rel;
struct timespec now, rel;
int saved_errno = errno;
int error;
@ -41,16 +41,12 @@ _twait(volatile uint32_t *p, int val, clockid_t clockid, const struct timespec *
return error;
}
if (!timespecisvalid(abs) || WRAP(clock_gettime)(clockid, &rel))
if (!timespecisvalid(abs) || WRAP(clock_gettime)(clockid, &now))
return EINVAL;
rel.tv_sec = abs->tv_sec - rel.tv_sec;
if ((rel.tv_nsec = abs->tv_nsec - rel.tv_nsec) < 0) {
rel.tv_sec--;
rel.tv_nsec += 1000000000;
}
if (rel.tv_sec < 0)
if (timespeccmp(abs, &now, <=))
return ETIMEDOUT;
timespecsub(abs, &now, &rel);
error = futex(p, FUTEX_WAIT_PRIVATE, val, &rel, NULL);
if (error == -1) {

14
lib/librthread/synch.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: synch.h,v 1.9 2023/11/08 15:51:28 cheloha Exp $ */
/* $OpenBSD: synch.h,v 1.10 2024/01/07 19:44:28 cheloha Exp $ */
/*
* Copyright (c) 2017 Martin Pieuchot
*
@ -28,7 +28,7 @@ _wake(volatile uint32_t *p, int n)
static inline int
_twait(volatile uint32_t *p, int val, clockid_t clockid, const struct timespec *abs)
{
struct timespec rel;
struct timespec now, rel;
int saved_errno = errno;
int error;
@ -41,16 +41,12 @@ _twait(volatile uint32_t *p, int val, clockid_t clockid, const struct timespec *
return error;
}
if (!timespecisvalid(abs) || clock_gettime(clockid, &rel))
if (!timespecisvalid(abs) || clock_gettime(clockid, &now))
return EINVAL;
rel.tv_sec = abs->tv_sec - rel.tv_sec;
if ((rel.tv_nsec = abs->tv_nsec - rel.tv_nsec) < 0) {
rel.tv_sec--;
rel.tv_nsec += 1000000000;
}
if (rel.tv_sec < 0)
if (timespeccmp(abs, &now, <=))
return ETIMEDOUT;
timespecsub(abs, &now, &rel);
error = futex(p, FUTEX_WAIT, val, &rel, NULL);
if (error == -1) {