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| 1 | +/* Using ptrace, catch when a process in a process tree is about to |
| 2 | + * segfault from an attempted vsyscall, and fix it up to use the vDSO |
| 3 | + * instead. |
| 4 | + * |
| 5 | + * usage: vsyscall_trace -p <pid>... |
| 6 | + * vsyscall_trace <cmd> [args...] |
| 7 | + * |
| 8 | + * In the first mode, traces a process and all its children, until they |
| 9 | + * exit. In the second mode, run and trace a child process -- unless |
| 10 | + * vsyscalls are enabled, in which case it will just exec the child |
| 11 | + * process directly. Because the second mode waits on child processes (as |
| 12 | + * required by the ptrace API), it is usable as init inside a container. |
| 13 | + * Whether or not it runs as init, it will block until all descendant |
| 14 | + * processes exit. |
| 15 | + * |
| 16 | + * This program itself uses no vsyscalls, so it can be safely |
| 17 | + * dynamically linked against an older glibc. |
| 18 | + */ |
| 19 | + |
| 20 | +#define _GNU_SOURCE |
| 21 | +#include <sys/auxv.h> |
| 22 | +#include <sys/ptrace.h> |
| 23 | +#include <sys/types.h> |
| 24 | +#include <sys/stat.h> |
| 25 | +#include <sys/wait.h> |
| 26 | +#include <sys/user.h> |
| 27 | +#include <dlfcn.h> |
| 28 | +#include <errno.h> |
| 29 | +#include <fcntl.h> |
| 30 | +#include <signal.h> |
| 31 | +#include <stdlib.h> |
| 32 | +#include <stdio.h> |
| 33 | +#include <string.h> |
| 34 | +#include <unistd.h> |
| 35 | + |
| 36 | +#ifdef DEBUG |
| 37 | +#define debug_printf printf |
| 38 | +#else |
| 39 | +#define debug_printf(...) 0 |
| 40 | +#endif |
| 41 | + |
| 42 | +/* These are ABI constants: see arch/x86/include/uapi/asm/vsyscall.h |
| 43 | + * in the kernel source (probably installed on your system as |
| 44 | + * <asm/vsyscall.h>). They start at VSYSCALL_ADDR, and |
| 45 | + * increase by 1024 for each call. */ |
| 46 | +const unsigned long VSYS_gettimeofday = 0xffffffffff600000, |
| 47 | + VSYS_time = 0xffffffffff600400, |
| 48 | + VSYS_getcpu = 0xffffffffff600800; |
| 49 | + |
| 50 | +/* The vDSO is an area of memory that looks like a normal relocatable |
| 51 | + * dynamic library, magically placed in your address space by the |
| 52 | + * kernel. While it's mapped at a different address in each process when |
| 53 | + * ASLR is enabled, the relative offsets are the same, since the kernel |
| 54 | + * only contains one vDSO. These variables contain the relative offsets |
| 55 | + * as found in the current process. */ |
| 56 | +unsigned long VDSO_gettimeofday, VDSO_time, VDSO_getcpu; |
| 57 | + |
| 58 | +/* Look up the vDSO base address for a process in its auxiliary vector. |
| 59 | + * See proc(5) and getauxval(3). If we can ptrace the process, we should |
| 60 | + * have permissions to do this. */ |
| 61 | +unsigned long vdso_address(pid_t pid) { |
| 62 | + char *filename; |
| 63 | + asprintf(&filename, "/proc/%d/auxv", pid); |
| 64 | + int fd = open(filename, O_RDONLY); |
| 65 | + if (fd == -1) { |
| 66 | + return 0; |
| 67 | + } |
| 68 | + unsigned long buf[128]; |
| 69 | + int i; |
| 70 | + if (read(fd, buf, sizeof(buf)) == -1) { |
| 71 | + close(fd); |
| 72 | + return 0; |
| 73 | + } |
| 74 | + close(fd); |
| 75 | + free(filename); |
| 76 | + |
| 77 | + for (i = 0; i < 128; i += 2) { |
| 78 | + if (buf[i] == AT_SYSINFO_EHDR) { |
| 79 | + return buf[i+1]; |
| 80 | + } else if (buf[i] == 0) { |
| 81 | + return 0; |
| 82 | + } |
| 83 | + } |
| 84 | +} |
| 85 | + |
| 86 | +/* If the ptraced process segfaulted because it tried to call one of the |
| 87 | + * three vsyscalls, redirect its instruction pointer to the |
| 88 | + * corresponding vDSO address. The calling conventions are the same, so |
| 89 | + * we don't need to change / inspect arguments or do any other safety |
| 90 | + * checks - the process could have gotten here on its own. */ |
| 91 | +int handle_vsyscall(pid_t pid) { |
| 92 | + struct user_regs_struct regs; |
| 93 | + ptrace(PTRACE_GETREGS, pid, 0, ®s); |
| 94 | + if ((regs.rip & 0xfffffffffffff0ff) == 0xffffffffff600000) { |
| 95 | + debug_printf("handling vsyscall for %d\n", pid); |
| 96 | + unsigned long vdso = vdso_address(pid); |
| 97 | + if (vdso_address == 0) { |
| 98 | + debug_printf("couldn't find vdso\n"); |
| 99 | + return 0; |
| 100 | + } |
| 101 | + |
| 102 | + if (regs.rip == VSYS_gettimeofday) { |
| 103 | + regs.rip = vdso | VDSO_gettimeofday; |
| 104 | + } else if (regs.rip == VSYS_time) { |
| 105 | + regs.rip = vdso | VDSO_time; |
| 106 | + } else if (regs.rip == VSYS_getcpu) { |
| 107 | + regs.rip = vdso | VDSO_getcpu; |
| 108 | + } else { |
| 109 | + debug_printf("invalid vsyscall %x\n", regs.rip); |
| 110 | + return 0; |
| 111 | + } |
| 112 | + ptrace(PTRACE_SETREGS, pid, 0, ®s); |
| 113 | + return 1; |
| 114 | + } |
| 115 | + return 0; |
| 116 | +} |
| 117 | + |
| 118 | +int main(int argc, char *argv[]) { |
| 119 | + pid_t pid, child_pid = 0; |
| 120 | + int wstatus, child_wstatus = 0; |
| 121 | + |
| 122 | + if (argc < 2) { |
| 123 | + printf("usage: vsyscall_trace -p <pid>...\n"); |
| 124 | + printf(" vsyscall_trace <cmd> [args...]\n"); |
| 125 | + return 1; |
| 126 | + } |
| 127 | + |
| 128 | + /* Seize all the processes via ptrace. We don't need to track |
| 129 | + * them, we only need to call wait(), and the options we're |
| 130 | + * passing to PTRACE_SEIZE will cause us to silently pick up |
| 131 | + * child processes too. */ |
| 132 | + if (strcmp(argv[1], "-p") == 0) { |
| 133 | + int i; |
| 134 | + for (i = 2; i < argc; i++) { |
| 135 | + pid = atoi(argv[i]); |
| 136 | + if (ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_TRACEFORK | PTRACE_O_TRACEVFORK | PTRACE_O_TRACECLONE) != 0) { |
| 137 | + perror("PTRACE_SEIZE"); |
| 138 | + return 1; |
| 139 | + } |
| 140 | + } |
| 141 | + } else { |
| 142 | + /* Test to see if vsyscalls work on this machine. If so, |
| 143 | + * we don't need to do anything - exec the given command |
| 144 | + * so we get entirely out of the way and don't risk |
| 145 | + * breaking the process. */ |
| 146 | + child_pid = fork(); |
| 147 | + if (child_pid == -1) { |
| 148 | + perror("fork"); |
| 149 | + return 1; |
| 150 | + } else if (child_pid == 0) { |
| 151 | + ((time_t (*)(time_t *))VSYS_time)(NULL); |
| 152 | + return 0; |
| 153 | + } else { |
| 154 | + waitpid(child_pid, &wstatus, 0); |
| 155 | + /* If the child process segfaulted, it will show |
| 156 | + * up as WIFSIGNALED instead of WIFEXITED. */ |
| 157 | + if (WIFEXITED(wstatus)) { |
| 158 | + execvp(argv[1], &argv[1]); |
| 159 | + perror("execvp"); |
| 160 | + return 1; |
| 161 | + } |
| 162 | + } |
| 163 | + |
| 164 | + /* Actually start the child process. */ |
| 165 | + child_pid = fork(); |
| 166 | + if (child_pid == -1) { |
| 167 | + perror("fork"); |
| 168 | + return 1; |
| 169 | + } else if (child_pid == 0) { |
| 170 | + /* Allow the parent process to run PTRACE_SEIZE |
| 171 | + * before continuing. */ |
| 172 | + raise(SIGSTOP); |
| 173 | + execvp(argv[1], &argv[1]); |
| 174 | + perror("execvp"); |
| 175 | + return 1; |
| 176 | + } else { |
| 177 | + if (ptrace(PTRACE_SEIZE, child_pid, 0, PTRACE_O_TRACEFORK | PTRACE_O_TRACEVFORK | PTRACE_O_TRACECLONE) != 0) { |
| 178 | + if (errno == EPERM) { |
| 179 | + fprintf(stderr, "Error: no kernel vsyscall support and ptrace is disabled.\n"); |
| 180 | + fprintf(stderr, "Your kernel does not provide vsyscall emulation, and we cannot\n"); |
| 181 | + fprintf(stderr, "work around this because ptrace is prohibited inside this container.\n"); |
| 182 | + fprintf(stderr, "Either permit ptrace for this container (e.g., for Docker, use\n"); |
| 183 | + fprintf(stderr, "docker run --security-opt=seccomp:unconfined) or boot your kernel\n"); |
| 184 | + fprintf(stderr, "with vsyscall=emulate.\n"); |
| 185 | + } else { |
| 186 | + perror("PTRACE_SEIZE"); |
| 187 | + } |
| 188 | + kill(child_pid, SIGKILL); |
| 189 | + return 1; |
| 190 | + } |
| 191 | + |
| 192 | + fprintf(stderr, "Warning: using ptrace-based vsyscall emulation.\n"); |
| 193 | + fprintf(stderr, "This container contains old binaries which require the use of the legacy\n"); |
| 194 | + fprintf(stderr, "'vsyscall' feature of the Linux kernel, and your kernel does not provide\n"); |
| 195 | + fprintf(stderr, "vsyscall emulation. We will attempt to emulate vsyscalls ourselves using\n"); |
| 196 | + fprintf(stderr, "ptrace, but performance may suffer and other tools that use ptrace (e.g.,\n"); |
| 197 | + fprintf(stderr, "gdb and strace) will not work.\n"); |
| 198 | + fprintf(stderr, "To avoid this emulation, please boot your kernel with vsyscall=emulate.\n"); |
| 199 | + kill(child_pid, SIGCONT); |
| 200 | + } |
| 201 | + } |
| 202 | + |
| 203 | + /* The vDSO shows up as an object in our address space naemd |
| 204 | + * "linux-vdso.so.1" that's already been loaded. */ |
| 205 | + void *vdso = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_NOLOAD); |
| 206 | + VDSO_gettimeofday = (unsigned long)dlsym(vdso, "__vdso_gettimeofday") & 0xfff; |
| 207 | + VDSO_time = (unsigned long)dlsym(vdso, "__vdso_time") & 0xfff; |
| 208 | + VDSO_getcpu = (unsigned long)dlsym(vdso, "__vdso_getcpu") & 0xfff; |
| 209 | + |
| 210 | + while ((pid = waitpid(-1, &wstatus, 0)) != -1) { |
| 211 | + if (WIFSTOPPED(wstatus)) { |
| 212 | + if (WSTOPSIG(wstatus) == SIGSEGV && handle_vsyscall(pid)) { |
| 213 | + /* The last argument to PTRACE_CONT is |
| 214 | + * the signal to send - passing 0 means |
| 215 | + * to suppress the signal. */ |
| 216 | + ptrace(PTRACE_CONT, pid, 0, 0); |
| 217 | + } else { |
| 218 | + ptrace(PTRACE_CONT, pid, 0, WSTOPSIG(wstatus)); |
| 219 | + } |
| 220 | + } else if (pid == child_pid && WIFEXITED(wstatus)) { |
| 221 | + /* Save this exit status so we can use it as our |
| 222 | + * own exit status. But don't exit yet if there |
| 223 | + * are further descendant processes still |
| 224 | + * running. */ |
| 225 | + child_wstatus = wstatus; |
| 226 | + } |
| 227 | + } |
| 228 | + if (errno != ECHILD) { |
| 229 | + perror("waitpid"); |
| 230 | + return 1; |
| 231 | + } |
| 232 | + if (WIFSIGNALED(wstatus)) { |
| 233 | + /* Send ourselves the same signal that killed the child |
| 234 | + * process, so our own parent process reports the right |
| 235 | + * exit status. */ |
| 236 | + raise(WTERMSIG(wstatus)); |
| 237 | + /* In case that signal is not fatal, return nonzero. */ |
| 238 | + return 1; |
| 239 | + } else { |
| 240 | + return WEXITSTATUS(wstatus); |
| 241 | + } |
| 242 | +} |
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