3.16.72-rc1 review patch. If anyone has any objections, please let me know.
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From: Michael Ellerman mpe@ellerman.id.au
commit b5b4453e7912f056da1ca7572574cada32ecb60c upstream.
Jakub Drnec reported: Setting the realtime clock can sometimes make the monotonic clock go back by over a hundred years. Decreasing the realtime clock across the y2k38 threshold is one reliable way to reproduce. Allegedly this can also happen just by running ntpd, I have not managed to reproduce that other than booting with rtc at >2038 and then running ntp. When this happens, anything with timers (e.g. openjdk) breaks rather badly.
And included a test case (slightly edited for brevity): #define _POSIX_C_SOURCE 199309L #include <stdio.h> #include <time.h> #include <stdlib.h> #include <unistd.h>
long get_time(void) { struct timespec tp; clock_gettime(CLOCK_MONOTONIC, &tp); return tp.tv_sec + tp.tv_nsec / 1000000000; }
int main(void) { long last = get_time(); while(1) { long now = get_time(); if (now < last) { printf("clock went backwards by %ld seconds!\n", last - now); } last = now; sleep(1); } return 0; }
Which when run concurrently with: # date -s 2040-1-1 # date -s 2037-1-1
Will detect the clock going backward.
The root cause is that wtom_clock_sec in struct vdso_data is only a 32-bit signed value, even though we set its value to be equal to tk->wall_to_monotonic.tv_sec which is 64-bits.
Because the monotonic clock starts at zero when the system boots the wall_to_montonic.tv_sec offset is negative for current and future dates. Currently on a freshly booted system the offset will be in the vicinity of negative 1.5 billion seconds.
However if the wall clock is set past the Y2038 boundary, the offset from wall to monotonic becomes less than negative 2^31, and no longer fits in 32-bits. When that value is assigned to wtom_clock_sec it is truncated and becomes positive, causing the VDSO assembly code to calculate CLOCK_MONOTONIC incorrectly.
That causes CLOCK_MONOTONIC to jump ahead by ~4 billion seconds which it is not meant to do. Worse, if the time is then set back before the Y2038 boundary CLOCK_MONOTONIC will jump backward.
We can fix it simply by storing the full 64-bit offset in the vdso_data, and using that in the VDSO assembly code. We also shuffle some of the fields in vdso_data to avoid creating a hole.
The original commit that added the CLOCK_MONOTONIC support to the VDSO did actually use a 64-bit value for wtom_clock_sec, see commit a7f290dad32e ("[PATCH] powerpc: Merge vdso's and add vdso support to 32 bits kernel") (Nov 2005). However just 3 days later it was converted to 32-bits in commit 0c37ec2aa88b ("[PATCH] powerpc: vdso fixes (take #2)"), and the bug has existed since then AFAICS.
Fixes: 0c37ec2aa88b ("[PATCH] powerpc: vdso fixes (take #2)") Link: http://lkml.kernel.org/r/HaC.ZfES.62bwlnvAvMP.1STMMj@seznam.cz Reported-by: Jakub Drnec jaydee@email.cz Signed-off-by: Michael Ellerman mpe@ellerman.id.au [bwh: Backported to 3.16: CLOCK_MONOTONIC_COARSE is not handled by this vDSO] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/powerpc/include/asm/vdso_datapage.h | 8 ++++---- arch/powerpc/kernel/vdso64/gettimeofday.S | 4 ++-- 2 files changed, 6 insertions(+), 6 deletions(-)
--- a/arch/powerpc/include/asm/vdso_datapage.h +++ b/arch/powerpc/include/asm/vdso_datapage.h @@ -82,10 +82,10 @@ struct vdso_data { __u32 icache_block_size; /* L1 i-cache block size */ __u32 dcache_log_block_size; /* L1 d-cache log block size */ __u32 icache_log_block_size; /* L1 i-cache log block size */ - __s32 wtom_clock_sec; /* Wall to monotonic clock */ - __s32 wtom_clock_nsec; - struct timespec stamp_xtime; /* xtime as at tb_orig_stamp */ - __u32 stamp_sec_fraction; /* fractional seconds of stamp_xtime */ + __u32 stamp_sec_fraction; /* fractional seconds of stamp_xtime */ + __s32 wtom_clock_nsec; /* Wall to monotonic clock nsec */ + __s64 wtom_clock_sec; /* Wall to monotonic clock sec */ + struct timespec stamp_xtime; /* xtime as at tb_orig_stamp */ __u32 syscall_map_64[SYSCALL_MAP_SIZE]; /* map of syscalls */ __u32 syscall_map_32[SYSCALL_MAP_SIZE]; /* map of syscalls */ }; --- a/arch/powerpc/kernel/vdso64/gettimeofday.S +++ b/arch/powerpc/kernel/vdso64/gettimeofday.S @@ -85,7 +85,7 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime) * At this point, r4,r5 contain our sec/nsec values. */
- lwa r6,WTOM_CLOCK_SEC(r3) + ld r6,WTOM_CLOCK_SEC(r3) lwa r9,WTOM_CLOCK_NSEC(r3)
/* We now have our result in r6,r9. We create a fake dependency