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clock_gettime: add support for 32-bit vdso with 64-bit time_t

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this fixes a major upcoming performance regression introduced by
commit 72f50245d0, whereby 32-bit archs
would lose vdso clock_gettime after switching to 64-bit time_t, unless
the kernel supports time64 and provides a time64 version of the vdso
function. this would incur not just one but two syscalls: first, the
failed time64 syscall, then the fallback time32 one.

overflow of the 32-bit result is detected and triggers a revert to
syscalls. normally, on a system that's not Y2038-ready, this would
still overflow, but if the process has been migrated to a
time64-capable kernel or if the kernel has been hot-patched to add
time64 syscalls, it may conceivably work.
This commit is contained in:
Rich Felker 2019-08-02 14:04:45 -04:00
parent 006a75a997
commit 0705fe9356
1 changed files with 32 additions and 0 deletions
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32
src/time/clock_gettime.c
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@ -8,9 +8,41 @@
static void *volatile vdso_func;
#ifdef VDSO_CGT32_SYM
static void *volatile vdso_func_32;
static int cgt_time32_wrap(clockid_t clk, struct timespec *ts)
{
long ts32[2];
int (*f)(clockid_t, long[2]) =
(int (*)(clockid_t, long[2]))vdso_func_32;
int r = f(clk, ts32);
if (!r) {
/* Fallback to syscalls if time32 overflowed. Maybe
* we lucked out and somehow migrated to a kernel with
* time64 syscalls available. */
if (ts32[0] < 0) {
a_cas_p(&vdso_func, (void *)cgt_time32_wrap, 0);
return -ENOSYS;
}
ts->tv_sec = ts32[0];
ts->tv_nsec = ts32[1];
}
return r;
}
#endif
static int cgt_init(clockid_t clk, struct timespec *ts)
{
void *p = __vdsosym(VDSO_CGT_VER, VDSO_CGT_SYM);
#ifdef VDSO_CGT32_SYM
if (!p) {
void *q = __vdsosym(VDSO_CGT32_VER, VDSO_CGT32_SYM);
if (q) {
a_cas_p(&vdso_func_32, 0, q);
p = cgt_time32_wrap;
}
}
#endif
int (*f)(clockid_t, struct timespec *) =
(int (*)(clockid_t, struct timespec *))p;
a_cas_p(&vdso_func, (void *)cgt_init, p);