.\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk
.\" Add text describing limits on command-line arguments + environment
.\"
-.TH EXECVE 2 2014-01-08 "Linux" "Linux Programmer's Manual"
+.TH EXECVE 2 2019-05-09 "Linux" "Linux Programmer's Manual"
.SH NAME
execve \- execute program
.SH SYNOPSIS
.B #include <unistd.h>
-.sp
-.BI "int execve(const char *" filename ", char *const " argv "[], "
+.PP
+.BI "int execve(const char *" pathname ", char *const " argv "[], "
.br
.BI " char *const " envp []);
.SH DESCRIPTION
.BR execve ()
-executes the program pointed to by \fIfilename\fP.
-\fIfilename\fP must be either a binary executable, or a script
+executes the program referred to by \fIpathname\fP.
+This causes the program that is currently being run by the calling process
+to be replaced with a new program, with newly initialized stack, heap,
+and (initialized and uninitialized) data segments.
+.PP
+\fIpathname\fP must be either a binary executable, or a script
starting with a line of the form:
-
+.PP
.in +4n
-.nf
+.EX
\fB#!\fP \fIinterpreter \fP[optional-arg]
-.fi
+.EE
.in
-
+.PP
For details of the latter case, see "Interpreter scripts" below.
-
+.PP
\fIargv\fP is an array of argument strings passed to the new program.
-By convention, the first of these strings should contain the filename
-associated with the file being executed.
+By convention, the first of these strings (i.e.,
+.IR argv[0] )
+should contain the filename associated with the file being executed.
\fIenvp\fP is an array of strings, conventionally of the form
\fBkey=value\fP, which are passed as environment to the new program.
-Both \fIargv\fP and \fIenvp\fP must be terminated by a null pointer.
+The \fIargv\fP and \fIenvp\fP arrays must each include a null pointer
+at the end of the array.
+.PP
The argument vector and environment can be accessed by the
called program's main function, when it is defined as:
-
+.PP
.in +4n
-.nf
+.EX
int main(int argc, char *argv[], char *envp[])
-.fi
+.EE
.in
-
+.PP
+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
+.BR environ (7).
+.PP
.BR execve ()
-does not return on success, and the text, data, bss, and
-stack of the calling process are overwritten by that of the program
-loaded.
-
-If the current program is being ptraced, a \fBSIGTRAP\fP is sent to it
+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.
+.PP
+If the current program is being ptraced, a \fBSIGTRAP\fP signal is sent to it
after a successful
.BR execve ().
-
-If the set-user-ID bit is set on the program file pointed to by
-\fIfilename\fP,
-and the underlying filesystem is not mounted
-.I nosuid
-(the
-.B MS_NOSUID
-flag for
-.BR mount (2)),
-and the calling process is not being ptraced,
+.PP
+If the set-user-ID bit is set on the program file referred to by
+\fIpathname\fP,
then the effective user ID of the calling process is changed
to that of the owner of the program file.
Similarly, when the set-group-ID
bit of the program file is set the effective group ID of the calling
process is set to the group of the program file.
-
+.PP
+The aforementioned transformations of the effective IDs are
+.I not
+performed (i.e., the set-user-ID and set-group-ID bits are ignored)
+if any of the following is true:
+.IP * 3
+the
+.I no_new_privs
+attribute is set for the calling thread (see
+.BR prctl (2));
+.IP *
+the underlying filesystem is mounted
+.I nosuid
+(the
+.B MS_NOSUID
+flag for
+.BR mount (2));
+or
+.IP *
+the calling process is being ptraced.
+.PP
+The capabilities of the program file (see
+.BR capabilities (7))
+are also ignored if any of the above are true.
+.PP
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 permission bits.
-
+because of the set-user-ID and set-group-ID mode bits.
+.PP
+The process's real UID and real GID, as well its supplementary group IDs,
+are unchanged by a call to
+.BR execve ().
+.PP
If the executable is an a.out dynamically linked
binary executable containing
shared-library stubs, the Linux dynamic linker
.BR ld.so (8)
is called at the start of execution to bring
-needed shared libraries into memory
+needed shared objects into memory
and link the executable with them.
-
+.PP
If the executable is a dynamically linked ELF executable, the
interpreter named in the PT_INTERP segment is used to load the needed
-shared libraries.
+shared objects.
This interpreter is typically
.I /lib/ld-linux.so.2
-for binaries linked with glibc 2.
-(For binaries linked with the old Linux libc5, the interpreter was typically
-.IR /lib/ld-linux.so.1 .)
-
+for binaries linked with glibc (see
+.BR ld-linux.so (8)).
+.PP
All process attributes are preserved during an
.BR execve (),
except the following:
.BR fenv (3)).
.PP
The process attributes in the preceding list are all specified
-in POSIX.1-2001.
+in POSIX.1.
The following Linux-specific process attributes are also
not preserved during an
.BR execve ():
.B SIGCHLD
(see
.BR clone (2)).
+.IP *
+The file descriptor table is unshared, undoing the effect of the
+.B CLONE_FILES
+flag of
+.BR clone (2).
.PP
Note the following further points:
.IP * 3
The equivalent of \fIsetlocale(LC_ALL, "C")\fP
is executed at program start-up.
.IP *
-POSIX.1-2001 specifies that the dispositions of any signals that
+POSIX.1 specifies that the dispositions of any signals that
are ignored or set to the default are left unchanged.
-POSIX.1-2001 specifies one exception: if
+POSIX.1 specifies one exception: if
.B SIGCHLD
is being ignored,
then an implementation may leave the disposition unchanged or
See
.BR fcntl (2)
for details.)
-POSIX.1-2001 says that if file descriptors 0, 1, and 2 would
+POSIX.1 says that if file descriptors 0, 1, and 2 would
otherwise be closed after a successful
.BR execve (),
-and the process would gain privilege because the set-user_ID or
-set-group_ID permission bit was set on the executed file,
+and the process would gain privilege because the set-user-ID or
+set-group_ID mode bit was set on the executed file,
then the system may open an unspecified file for each of these
file descriptors.
As a general principle, no portable program, whether privileged or not,
.SS Interpreter scripts
An interpreter script is a text file that has execute
permission enabled and whose first line is of the form:
-
+.PP
.in +4n
-.nf
+.EX
\fB#!\fP \fIinterpreter \fP[optional-arg]
-.fi
+.EE
.in
-
+.PP
The
.I interpreter
-must be a valid pathname for an
-executable which is not itself a script.
+must be a valid pathname for an executable file.
If the
-.I filename
+.I pathname
argument of
.BR execve ()
specifies an interpreter script, then
.I interpreter
will be invoked with the following arguments:
-
+.PP
.in +4n
-.nf
-\fIinterpreter\fP [optional-arg] \fIfilename\fP arg...
-.fi
+.EX
+\fIinterpreter\fP [optional-arg] \fIpathname\fP arg...
+.EE
.in
-
+.PP
where
.I arg...
is the series of words pointed to by the
.BR execve (),
starting at
.IR argv [1].
-
+.PP
For portable use,
.I optional-arg
should either be absent, or be specified as a single word (i.e., it
should not contain white space); see NOTES below.
+.PP
+Since Linux 2.6.28,
+.\" commit bf2a9a39639b8b51377905397a5005f444e9a892
+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.
.SS Limits on size of arguments and environment
Most UNIX implementations impose some limit on the total size
of the command-line argument
.I <limits.h>
or available at run time using the call
.IR "sysconf(_SC_ARG_MAX)" ).
-
+.PP
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
.BR MAX_ARG_PAGES ).
On architectures with a 4-kB page size,
-this yields a maximum size of 128 kB.
-
+this yields a maximum size of 128\ kB.
+.PP
On kernel 2.6.23 and later, most architectures support a size limit
derived from the soft
.B RLIMIT_STACK
ensures that the new program always has some stack space.)
.\" Ollie: That doesn't include the lists of pointers, though,
.\" so the actual usage is a bit higher (1 pointer per argument).
+Additionally, the total size is limited to 3/4 of the value
+of the kernel constant
+.B _STK_LIM
+(8 Mibibytes).
Since Linux 2.6.25,
-the kernel places a floor of 32 pages on this size limit,
+the kernel also places a floor of 32 pages on this size limit,
so that, even when
.BR RLIMIT_STACK
is set very low,
.TP
.B EACCES
Search permission is denied on a component of the path prefix of
-.I filename
+.I pathname
or the name of a script interpreter.
(See also
.BR path_resolution (7).)
The filesystem is mounted
.IR noexec .
.TP
+.BR EAGAIN " (since Linux 3.1)"
+.\" commit 72fa59970f8698023045ab0713d66f3f4f96945c
+Having changed its real UID using one of the
+.BR set*uid ()
+calls, the caller was\(emand is now still\(emabove its
+.BR RLIMIT_NPROC
+resource limit (see
+.BR setrlimit (2)).
+For a more detailed explanation of this error, see NOTES.
+.TP
.B EFAULT
-.I filename
+.I pathname
or one of the pointers in the vectors
.I argv
or
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
-.I filename
+.I pathname
or the name of a script or ELF interpreter.
.TP
+.B ELOOP
+The maximum recursion limit was reached during recursive script
+interpretation (see "Interpreter scripts", above).
+Before Linux 3.8,
+.\" commit d740269867021faf4ce38a449353d2b986c34a67
+the error produced for this case was
+.BR ENOEXEC .
+.TP
.B EMFILE
-The process has the maximum number of files open.
+The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENAMETOOLONG
-.I filename
+.I pathname
is too long.
.TP
.B ENFILE
-The system limit on the total number of open files has been reached.
+The system-wide limit on the total number of open files has been reached.
.TP
.B ENOENT
The file
-.I filename
+.I pathname
or a script or ELF interpreter does not exist, or a shared library
-needed for file or interpreter cannot be found.
+.\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241
+needed for the file or interpreter cannot be found.
.TP
.B ENOEXEC
An executable is not in a recognized format, is for the wrong
.TP
.B ENOTDIR
A component of the path prefix of
-.I filename
+.I pathname
or a script or ELF interpreter is not a directory.
.TP
.B EPERM
The process is being traced, the user is not the superuser and the
file has the set-user-ID or set-group-ID bit set.
.TP
+.B EPERM
+A "capability-dumb" applications would not obtain the full set of
+permitted capabilities granted by the executable file.
+See
+.BR capabilities (7).
+.TP
.B ETXTBSY
-Executable was open for writing by one or more processes.
+The specified executable was open for writing by one or more processes.
.SH CONFORMING TO
-SVr4, 4.3BSD, POSIX.1-2001.
-POSIX.1-2001 does not document the #! behavior
-but is otherwise compatible.
+POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
+POSIX does not document the #! behavior, but it exists
+(with some variations) on other UNIX systems.
.\" SVr4 documents additional error
.\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
.\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
.\" EISDIR or ELIBBAD error conditions.
.SH NOTES
+One sometimes sees
+.BR execve ()
+(and the related functions described in
+.BR exec (3))
+described as "executing a
+.I new
+process" (or similar).
+This is a highly misleading description:
+there is no new process;
+many attributes of the calling process remain unchanged
+(in particular, its PID).
+All that
+.BR execve (2)
+does is arrange for an existing process (the calling process)
+to execute a new program.
+.PP
Set-user-ID and set-group-ID processes can not be
.BR ptrace (2)d.
-
-Linux ignores the set-user-ID and set-group-ID bits on scripts.
-
+.PP
The result of mounting a filesystem
.I 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 she did not have already (and return
+give the user powers they did not have already (and return
.BR EPERM ),
some will just ignore the set-user-ID and set-group-ID bits and
.BR exec ()
successfully.
-
+.PP
+On Linux,
+.I argv
+and
+.I 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.
+.B "Do not take advantage of this nonstandard and nonportable misfeature!"
+On many other UNIX systems, specifying
+.I argv
+as NULL will result in an error
+.RB ( EFAULT ).
+.I Some
+other UNIX systems treat the
+.I envp==NULL
+case the same as Linux.
+.\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
+.\" HP-UX 11 is like Linux -- mtk, Apr 2007
+.\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
+.\" Bug rejected (because fix would constitute an ABI change).
+.\"
+.PP
+POSIX.1 says that values returned by
+.BR sysconf (3)
+should be invariant over the lifetime of a process.
+However, since Linux 2.6.23, if the
+.BR RLIMIT_STACK
+resource limit changes, then the value reported by
+.B _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.
+.PP
+In most cases where
+.BR execve ()
+fails, control returns to the original executable image,
+and the caller of
+.BR 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
+.BR SIGKILL
+signal.
+.\"
+.SS Interpreter scripts
A maximum line length of 127 characters is allowed for the first line in
-a #! executable shell script.
-
+an interpreter script.
+.PP
The semantics of the
.I optional-arg
argument of an interpreter script vary across implementations.
and white spaces in
.I optional-arg
are used to delimit the arguments.
-
-On Linux, either
-.I argv
-or
-.I envp
-can be specified as NULL,
-which has the same effect as specifying these arguments
-as a pointer to a list containing a single null pointer.
-.B "Do not take advantage of this misfeature!"
-It is nonstandard and nonportable:
-on most other UNIX systems doing this will result in an error
-.RB ( EFAULT ).
-.\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
-.\" HP-UX 11 is like Linux -- mtk, Apr 2007
-.\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
-.\" Bug rejected (because fix would constitute an ABI change).
-.\"
-
-POSIX.1-2001 says that values returned by
-.BR sysconf (3)
-should be invariant over the lifetime of a process.
-However, since Linux 2.6.23, if the
-.BR RLIMIT_STACK
-resource limit changes, then the value reported by
-.B _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.
+.PP
+Linux ignores the set-user-ID and set-group-ID bits on scripts.
.\"
.\" .SH BUGS
.\" Some Linux versions have failed to check permissions on ELF
.\" .BR execve ()
.\" that could be exploited for denial of service by a suitably crafted
.\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
+.SS execve() and EAGAIN
+A more detailed explanation of the
+.BR EAGAIN
+error that can occur (since Linux 3.1) when calling
+.BR execve ()
+is as follows.
+.PP
+The
+.BR EAGAIN
+error can occur when a
+.I preceding
+call to
+.BR setuid (2),
+.BR setreuid (2),
+or
+.BR setresuid (2)
+caused the real user ID of the process to change,
+and that change caused the process to exceed its
+.BR 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
+.BR set*uid ()
+call to fail.
+(Prior to 2.6,
+.\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
+the resource limit was not imposed on processes that
+changed their user IDs.)
+.PP
+Since Linux 3.1, the scenario just described no longer causes the
+.BR 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\(emif the caller had root privileges\(emthe call would always succeed.
+Instead, the
+.BR set*uid ()
+calls now successfully change the real UID,
+but the kernel sets an internal flag, named
+.BR PF_NPROC_EXCEEDED ,
+to note that the
+.BR RLIMIT_NPROC
+resource limit has been exceeded.
+If the
+.BR PF_NPROC_EXCEEDED
+flag is set and the resource limit is still
+exceeded at the time of a subsequent
+.BR execve ()
+call, that call fails with the error
+.BR EAGAIN .
+This kernel logic ensures that the
+.BR RLIMIT_NPROC
+resource limit is still enforced for the
+common privileged daemon workflow\(emnamely,
+.BR fork (2)
++
+.BR set*uid ()
++
+.BR execve ().
+.PP
+If the resource limit was not still exceeded at the time of the
+.BR execve ()
+call
+(because other processes belonging to this real UID terminated between the
+.BR set*uid ()
+call and the
+.BR execve ()
+call), then the
+.BR execve ()
+call succeeds and the kernel clears the
+.BR PF_NPROC_EXCEEDED
+process flag.
+The flag is also cleared if a subsequent call to
+.BR fork (2)
+by this process succeeds.
.SS Historical
-With UNIX V6 the argument list of an
+With UNIX\ V6, the argument list of an
.BR exec ()
call was ended by 0,
while the argument list of
Thus, this argument list was not directly usable in a further
.BR exec ()
call.
-Since UNIX V7 both are NULL.
+Since UNIX\ V7, both are NULL.
+.\"
+.\" .SH BUGS
+.\" Some Linux versions have failed to check permissions on ELF
+.\" interpreters. This is a security hole, because it allows users to
+.\" open any file, such as a rewinding tape device, for reading. Some
+.\" Linux versions have also had other security holes in
+.\" .BR execve ()
+.\" that could be exploited for denial of service by a suitably crafted
+.\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
.SH EXAMPLE
The following program is designed to be execed by the second program below.
It just echoes its command-line arguments, one per line.
-
+.PP
.in +4n
-.nf
+.EX
/* myecho.c */
#include <stdio.h>
int j;
for (j = 0; j < argc; j++)
- printf("argv[%d]: %s\\n", j, argv[j]);
+ printf("argv[%d]: %s\en", j, argv[j]);
exit(EXIT_SUCCESS);
}
-.fi
+.EE
.in
-
+.PP
This program can be used to exec the program named in its command-line
argument:
+.PP
.in +4n
-.nf
-
+.EX
/* execve.c */
#include <stdio.h>
char *newenviron[] = { NULL };
if (argc != 2) {
- fprintf(stderr, "Usage: %s <file\-to\-exec>\\n", argv[0]);
+ fprintf(stderr, "Usage: %s <file\-to\-exec>\en", argv[0]);
exit(EXIT_FAILURE);
}
newargv[0] = argv[1];
execve(argv[1], newargv, newenviron);
- perror("execve"); /* execve() only returns on error */
+ perror("execve"); /* execve() returns only on error */
exit(EXIT_FAILURE);
}
-.fi
+.EE
.in
-
+.PP
We can use the second program to exec the first as follows:
-
+.PP
.in +4n
-.nf
+.EX
.RB "$" " cc myecho.c \-o myecho"
.RB "$" " cc execve.c \-o execve"
.RB "$" " ./execve ./myecho"
argv[0]: ./myecho
argv[1]: hello
argv[2]: world
-.fi
+.EE
.in
-
+.PP
We can also use these programs to demonstrate the use of a script
interpreter.
To do this we create a script whose "interpreter" is our
.I myecho
program:
-
+.PP
.in +4n
-.nf
-.RB "$" " cat > script.sh"
-.B #! ./myecho script-arg
+.EX
+.RB "$" " cat > script"
+.B #!./myecho script-arg
.B ^D
-.RB "$" " chmod +x script.sh"
-.fi
+.RB "$" " chmod +x script"
+.EE
.in
-
+.PP
We can then use our program to exec the script:
-
+.PP
.in +4n
-.nf
-.RB "$" " ./execve ./script.sh"
+.EX
+.RB "$" " ./execve ./script"
argv[0]: ./myecho
argv[1]: script-arg
-argv[2]: ./script.sh
+argv[2]: ./script
argv[3]: hello
argv[4]: world
-.fi
+.EE
.in
.SH SEE ALSO
.BR chmod (2),
+.BR execveat (2),
.BR fork (2),
+.BR get_robust_list (2),
.BR ptrace (2),
-.BR execl (3),
+.BR exec (3),
.BR fexecve (3),
.BR getopt (3),
+.BR system (3),
.BR credentials (7),
.BR environ (7),
.BR path_resolution (7),
projects / thirdparty / man-pages.git / blobdiff