61 lines
1.8 KiB
C
61 lines
1.8 KiB
C
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// Shows dangers of using a non-reentrant function with signal
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// handling. Strange things can happen due to the use of the
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// non-reentrant function getpwnam() being called both in the main
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// loop and signal handler. Uncommenting the getpwnam() in the signal
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// handler causes unpredictable things to happen like a permanent
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// stall of the program on Linux boxes.
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//
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// Adapted from Stevens and Rago Fig 10.5.
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#include <stdio.h>
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#include <signal.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <fcntl.h>
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#include <errno.h>
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#include <pwd.h>
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#include <string.h>
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#define USERNAME "kauffman"
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static void alarm_handler(int signo) {
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printf("in signal handler\n");
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struct passwd *rootptr = NULL;
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// NON-REENTRANT FUNCTION CALL IN SIGNAL HANDLER
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rootptr = getpwnam("root"); // fetch record associated with root password
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if (rootptr == NULL){ // check for presence of root
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printf("no root, that's weird...\n");
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}
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else{
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printf("root looks okay\n");
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}
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alarm(1); // reset the alarm
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printf("leaving signal handler\n");
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}
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int main(void) {
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signal(SIGALRM, alarm_handler); // xhandle alarms in with the above function
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alarm(1);
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printf("Repeatedly checking on user '%s'\n",USERNAME);
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while(1){
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struct passwd *ptr = getpwnam(USERNAME); // non-reentrant call
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if(ptr == NULL){
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perror("couldn't find user");
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exit(1);
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}
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if(strcmp(ptr->pw_name, USERNAME) != 0){ // should always be equal to USERNAME
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printf("return value corrupted!, pw_name = %s\n",
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ptr->pw_name);
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exit(1);
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}
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// else{
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// printf("%s looks okay\n",USERNAME); // verbose output
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// }
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}
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exit(0);
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}
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