src/c/environment-exec.c

1/* SPDX-License-Identifier: BSD-3-Clause */

2/* Replace an exec-only child's environment without interpreting its data.

3 * Values arrive in a private NUL file or numbered environment entries. */

4#define _GNU_SOURCE

5#include <errno.h>

6#include <fcntl.h>

7#include <signal.h>

8#include <stdio.h>

9#include <stdlib.h>

10#include <string.h>

11#include <sys/stat.h>

12#include <unistd.h>

14static void fail(const char *message) {

15 fprintf(stderr,"environment-exec: %s\n",message);

16 exit(126);

17}

19int main(int argc,char **argv) {

20 if (argc<5) fail("invalid invocation");

21 char *end;

22 errno=0;

23 long count=strtol(argv[2],&end,10);

24 if (errno || *end || count<0 || count>argc-5 ||

25 strcmp(argv[3+count],"--")) fail("invalid override count");

26 char **values=calloc((size_t)count+1,sizeof(*values));

27 if (!values) fail("allocation failed");

28 for (long i=0;i<count;i++) {

29 char key[64];

30 snprintf(key,sizeof(key),"SIGIL_EXEC_VALUE_%ld",i);

31 const char *value=getenv(key);

32 if (!value || !(values[i]=strdup(value))) fail("missing override");

33 }

34 int fd=open(argv[1],O_RDONLY|O_CLOEXEC|O_NOFOLLOW);

35 struct stat status;

36 if (fd<0 || fstat(fd,&status) || !S_ISREG(status.st_mode) ||

37 status.st_uid!=geteuid() || (status.st_mode&077) ||

38 status.st_size>16*1024*1024) fail("invalid private snapshot");

39 FILE *input=fdopen(fd,"r");

40 if (!input || clearenv()) fail("cannot replace environment");

41 char *entry=NULL;

42 size_t capacity=0;

43 ssize_t length;

44 while ((length=getdelim(&entry,&capacity,0,input))>=0) {

45 if (!length || entry[length-1]) fail("unterminated snapshot entry");

46 char *separator=strchr(entry,'=');

47 if (!separator || separator==entry) fail("invalid snapshot entry");

48 *separator=0;

49 if (setenv(entry,separator+1,1)) fail("cannot restore entry");

50 }

51 if (ferror(input)) fail("cannot read snapshot");

52 fclose(input);

53 free(entry);

54 for (long i=0;i<count;i++) {

55 if (setenv(argv[3+i],values[i],1)) fail("cannot apply override");

56 free(values[i]);

57 }

58 free(values);

59 /* This exec-only child must not export its launcher's ignored signals

60 * or blocked mask to the client. Unsupported signal numbers are skipped. */

61 struct sigaction action={.sa_handler=SIG_DFL};

62 if(sigemptyset(&action.sa_mask)) fail("cannot initialize signal defaults");

63 for(int signum=1;signum<NSIG;signum++) {

64 if(signum==SIGKILL || signum==SIGSTOP) continue;

65 if(sigaction(signum,&action,NULL) && errno!=EINVAL)

66 fail("cannot restore signal disposition");

67 }

68 sigset_t unblocked;

69 if(sigemptyset(&unblocked) || sigprocmask(SIG_SETMASK,&unblocked,NULL))

70 fail("cannot restore signal mask");

71 execvp(argv[4+count],argv+4+count);

72 return errno==ENOENT ? 127 : 126;

73}