Messages in this thread | | | Date | Mon, 23 Dec 2019 14:08:34 +0100 | From | "Jason A. Donenfeld" <> | Subject | vdso-related userspace crashes on 5.5 mips64 |
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Hi,
I'm experiencing VDSO-related crashes on 5.5 with MIPS64. The MIPS64 builders on build.wireguard.com are all red at the moment.
It looks like libc is crashing with a null pointer dereference when doing any work after returning from clock_gettime. This manifests itself, for me, with calls to clock_gettime(CLOCK_PROCESS_CPUTIME_ID), because CLOCK_PROCESS_CPUTIME_ID is not in the VDSO. It looks in the VDSO, doesn't find it, and then proceeds to make the real syscall, when it crashes. I can simulate the same crash by simply adding a printf after a successful call to the vdso before returning. For example:
int __clock_gettime(clockid_t clk, struct timespec *ts) { int r;
#ifdef VDSO_CGT_SYM int (*f)(clockid_t, struct timespec *) = (int (*)(clockid_t, struct timespec *))vdso_func; printf("vdso %p\n", f); // <-- this line does NOT crash. if (f) { r = f(clk, ts); if (!r) { printf("ret %d\n", r); // <-- this line DOES crash. return r; } if (r == -EINVAL) return __syscall_ret(r); } #endif printf("falling through\n"); // <--- this line DOES crash. r = __syscall(SYS_clock_gettime, clk, ts); // <-- also, this line will crash too if (r == -ENOSYS) { if (clk == CLOCK_REALTIME) { __syscall(SYS_gettimeofday, ts, 0); ts->tv_nsec = (int)ts->tv_nsec * 1000; return 0; } r = -EINVAL; } return __syscall_ret(r); }
It seems like somehow the stack frame is corrupted/unusable after a call to the vdso. But, returning immediately from clock_gettime after a call to the vdso allows the program to continue. Thus, this problem only manifests itself when using clocks that aren't handled by the vdso.
It's possible this is due to some compiler ABI mismatch situation between userspace and kernelspace. However, I've only started seeing this happen with 5.5 and not on 5.4.
Does the above description immediately point to some recognizable change? If not, I'll keep debugging.
Thanks, Jason
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