#include <unistd.h>
int execve(const char *pathname, char *const argv[],
char *const envp[]);
pathname must be either a binary executable, or a script starting with a line of the form:
#!interpreter [optional-arg]
For details of the latter case, see "Interpreter scripts" below.
argv is an array of pointers to strings passed to the new program as its command-line arguments. By convention, the first of these strings (i.e., argv[0]) should contain the filename associated with the file being executed. The argv array must be terminated by a NULL pointer. (Thus, in the new program, argv[argc] will be NULL.)
envp is an array of pointers to strings, conventionally of the form key=value, which are passed as the environment of the new program. The envp array must be terminated by a NULL pointer.
The argument vector and environment can be accessed by the new program's main function, when it is defined as:
int main(int argc, char *argv[], char *envp[])
Note, however, that the use of a third argument to the main function is not specified in POSIX.1; according to POSIX.1, the environment should be accessed via the external variable environ(7).
execve() does not return on success, and the text, initialized data, uninitialized data (bss), and stack of the calling process are overwritten according to the contents of the newly loaded program.
If the current program is being ptraced, a SIGTRAP signal is sent to it after a successful execve().
If the set-user-ID bit is set on the program file referred to by pathname, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, if the set-group-ID bit is set on the program file, then the effective group ID of the calling process is set to the group of the program file.
The aforementioned transformations of the effective IDs are not performed (i.e., the set-user-ID and set-group-ID bits are ignored) if any of the following is true:
The capabilities of the program file (see capabilities(7)) are also ignored if any of the above are true.
The effective user ID of the process is copied to the saved set-user-ID; similarly, the effective group ID is copied to the saved set-group-ID. This copying takes place after any effective ID changes that occur because of the set-user-ID and set-group-ID mode bits.
The process's real UID and real GID, as well its supplementary group IDs, are unchanged by a call to execve().
If the executable is an a.out dynamically linked binary executable containing shared-library stubs, the Linux dynamic linker ld.so(8) is called at the start of execution to bring needed shared objects into memory and link the executable with them.
If the executable is a dynamically linked ELF executable, the interpreter named in the PT_INTERP segment is used to load the needed shared objects. This interpreter is typically /lib/ld-linux.so.2 for binaries linked with glibc (see ld-linux.so(8)).
The process attributes in the preceding list are all specified in POSIX.1. The following Linux-specific process attributes are also not preserved during an execve():
Note the following further points:
#!interpreter [optional-arg]
The interpreter must be a valid pathname for an executable file.
If the pathname argument of execve() specifies an interpreter script, then interpreter will be invoked with the following arguments:
interpreter [optional-arg] pathname arg...
where pathname is the absolute pathname of the file specified as the first argument of execve(), and arg... is the series of words pointed to by the argv argument of execve(), starting at argv[1]. Note that there is no way to get the argv[0] that was passed to the execve() call.
For portable use, optional-arg should either be absent, or be specified as a single word (i.e., it should not contain white space); see NOTES below.
Since Linux 2.6.28, the kernel permits the interpreter of a script to itself be a script. This permission is recursive, up to a limit of four recursions, so that the interpreter may be a script which is interpreted by a script, and so on.
On Linux prior to kernel 2.6.23, the memory used to store the environment and argument strings was limited to 32 pages (defined by the kernel constant MAX_ARG_PAGES). On architectures with a 4-kB page size, this yields a maximum size of 128 kB.
On kernel 2.6.23 and later, most architectures support a size limit derived from the soft RLIMIT_STACK resource limit (see getrlimit(2)) that is in force at the time of the execve() call. (Architectures with no memory management unit are excepted: they maintain the limit that was in effect before kernel 2.6.23.) This change allows programs to have a much larger argument and/or environment list. For these architectures, the total size is limited to 1/4 of the allowed stack size. (Imposing the 1/4-limit ensures that the new program always has some stack space.) Additionally, the total size is limited to 3/4 of the value of the kernel constant _STK_LIM (8 MiB). Since Linux 2.6.25, the kernel also places a floor of 32 pages on this size limit, so that, even when RLIMIT_STACK is set very low, applications are guaranteed to have at least as much argument and environment space as was provided by Linux 2.6.22 and earlier. (This guarantee was not provided in Linux 2.6.23 and 2.6.24.) Additionally, the limit per string is 32 pages (the kernel constant MAX_ARG_STRLEN), and the maximum number of strings is 0x7FFFFFFF.
Set-user-ID and set-group-ID processes can not be ptrace(2)d.
The result of mounting a filesystem nosuid varies across Linux kernel versions: some will refuse execution of set-user-ID and set-group-ID executables when this would give the user powers they did not have already (and return EPERM), some will just ignore the set-user-ID and set-group-ID bits and exec() successfully.
On Linux, argv and envp can be specified as NULL. In both cases, this has the same effect as specifying the argument as a pointer to a list containing a single null pointer. Do not take advantage of this nonstandard and nonportable misfeature! On many other UNIX systems, specifying argv as NULL will result in an error (EFAULT). Some other UNIX systems treat the envp==NULL case the same as Linux.
POSIX.1 says that values returned by sysconf(3) should be invariant over the lifetime of a process. However, since Linux 2.6.23, if the RLIMIT_STACK resource limit changes, then the value reported by _SC_ARG_MAX will also change, to reflect the fact that the limit on space for holding command-line arguments and environment variables has changed.
In most cases where execve() fails, control returns to the original executable image, and the caller of execve() can then handle the error. However, in (rare) cases (typically caused by resource exhaustion), failure may occur past the point of no return: the original executable image has been torn down, but the new image could not be completely built. In such cases, the kernel kills the process with a SIGSEGV (SIGKILL until Linux 3.17) signal.
The semantics of the optional-arg argument of an interpreter script vary across implementations. On Linux, the entire string following the interpreter name is passed as a single argument to the interpreter, and this string can include white space. However, behavior differs on some other systems. Some systems use the first white space to terminate optional-arg. On some systems, an interpreter script can have multiple arguments, and white spaces in optional-arg are used to delimit the arguments.
Linux (like most other modern UNIX systems) ignores the set-user-ID and set-group-ID bits on scripts.
The EAGAIN error can occur when a preceding call to setuid(2), setreuid(2), or setresuid(2) caused the real user ID of the process to change, and that change caused the process to exceed its RLIMIT_NPROC resource limit (i.e., the number of processes belonging to the new real UID exceeds the resource limit). From Linux 2.6.0 to 3.0, this caused the set*uid() call to fail. (Prior to 2.6, the resource limit was not imposed on processes that changed their user IDs.)
Since Linux 3.1, the scenario just described no longer causes the set*uid() call to fail, because it too often led to security holes where buggy applications didn't check the return status and assumed that---if the caller had root privileges---the call would always succeed. Instead, the set*uid() calls now successfully change the real UID, but the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to note that the RLIMIT_NPROC resource limit has been exceeded. If the PF_NPROC_EXCEEDED flag is set and the resource limit is still exceeded at the time of a subsequent execve() call, that call fails with the error EAGAIN. This kernel logic ensures that the RLIMIT_NPROC resource limit is still enforced for the common privileged daemon workflow---namely, fork(2) + set*uid() + execve().
If the resource limit was not still exceeded at the time of the execve() call (because other processes belonging to this real UID terminated between the set*uid() call and the execve() call), then the execve() call succeeds and the kernel clears the PF_NPROC_EXCEEDED process flag. The flag is also cleared if a subsequent call to fork(2) by this process succeeds.
/* myecho.c */
#include <stdio.h> #include <stdlib.h>
int
main(int argc, char *argv[])
{
for (int j = 0; j < argc; j++)
printf("argv[%d]: %s\n", j, argv[j]);
exit(EXIT_SUCCESS);
}
This program can be used to exec the program named in its command-line argument:
/* execve.c */
#include <stdio.h> #include <stdlib.h> #include <unistd.h>
int
main(int argc, char *argv[])
{
char *newargv[] = { NULL, "hello", "world", NULL };
char *newenviron[] = { NULL };
if (argc != 2) {
fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);
exit(EXIT_FAILURE);
}
newargv[0] = argv[1];
execve(argv[1], newargv, newenviron);
perror("execve"); /* execve() returns only on error */
exit(EXIT_FAILURE);
}
We can use the second program to exec the first as follows:
$ cc myecho.c -o myecho $ cc execve.c -o execve $ ./execve ./myecho argv[0]: ./myecho argv[1]: hello argv[2]: world
We can also use these programs to demonstrate the use of a script interpreter. To do this we create a script whose "interpreter" is our myecho program:
$ cat > script #!./myecho script-arg haD $ chmod +x script
We can then use our program to exec the script:
$ ./execve ./script argv[0]: ./myecho argv[1]: script-arg argv[2]: ./script argv[3]: hello argv[4]: world