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libc: Fix possible overflow in binuptime().

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This is an application of the kernel overflow fix from r357948 to
userspace, based on the algorithm developed by Bruce Evans. To keep
the ABI of the vds_timekeep stable, instead of adding the large_delta
member, MSB of both multipliers are added to quickly estimate the overflow.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
This commit is contained in:
Konstantin Belousov 2020-04-09 23:22:35 +00:00
parent c948a17a52
commit 09bae0a023
1 changed files with 16 additions and 2 deletions
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18
lib/libc/sys/__vdso_gettimeofday.c
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@ -32,6 +32,7 @@ __FBSDID("$FreeBSD$");
#include <sys/time.h>
#include <sys/vdso.h>
#include <errno.h>
#include <strings.h>
#include <time.h>
#include <machine/atomic.h>
#include "libc_private.h"
@ -62,7 +63,8 @@ binuptime(struct bintime *bt, struct vdso_timekeep *tk, int abs)
{
struct vdso_timehands *th;
uint32_t curr, gen;
u_int delta;
uint64_t scale, x;
u_int delta, scale_bits;
int error;
do {
@ -78,7 +80,19 @@ binuptime(struct bintime *bt, struct vdso_timekeep *tk, int abs)
continue;
if (error != 0)
return (error);
bintime_addx(bt, th->th_scale * delta);
scale = th->th_scale;
#ifdef _LP64
scale_bits = ffsl(scale);
#else
scale_bits = ffsll(scale);
#endif
if (__predict_false(scale_bits + fls(delta) > 63)) {
x = (scale >> 32) * delta;
scale &= 0xffffffff;
bt->sec += x >> 32;
bintime_addx(bt, x << 32);
}
bintime_addx(bt, scale * delta);
if (abs)
bintime_add(bt, &th->th_boottime);