From 0705fe93566fca735bf0df155f71641df02fc273 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Fri, 2 Aug 2019 14:04:45 -0400 Subject: [PATCH] clock_gettime: add support for 32-bit vdso with 64-bit time_t this fixes a major upcoming performance regression introduced by commit 72f50245d018af0c31b38dec83c557a4e5dd1ea8, 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. --- src/time/clock_gettime.c | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) diff --git a/src/time/clock_gettime.c b/src/time/clock_gettime.c index 46083759..63e9f9c8 100644 --- a/src/time/clock_gettime.c +++ b/src/time/clock_gettime.c @@ -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); -- 2.25.1