.\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk
.\" Add text describing limits on command-line arguments + environment
.\"
-.TH EXECVE 2 2016-03-15 "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 (i.e.,
.IR argv[0] )
\fBkey=value\fP, which are passed as environment to the new program.
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, inititialized data,
+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 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
is called at the start of execution to bring
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 objects.
.I /lib/ld-linux.so.2
for binaries linked with glibc (see
.BR ld-linux.so (8)).
-
+.PP
All process attributes are preserved during an
.BR execve (),
except the following:
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
+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.
.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 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.
.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).)
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 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
.TP
.B ENOENT
The file
-.I filename
+.I pathname
or a script or ELF interpreter does not exist, or a shared library
.\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241
needed for the file or interpreter cannot be found.
.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
.\" 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.
-
+.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
.\" 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.
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,
.\"
.SS Interpreter scripts
A maximum line length of 127 characters is allowed for the first line in
-an interpreter scripts.
-
+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.
-
+.PP
Linux ignores the set-user-ID and set-group-ID bits on scripts.
.\"
.\" .SH BUGS
error that can occur (since Linux 3.1) when calling
.BR execve ()
is as follows.
-
+.PP
The
.BR EAGAIN
error can occur when a
.\" 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,
.BR set*uid ()
+
.BR execve ().
-
+.PP
If the resource limit was not still exceeded at the time of the
.BR execve ()
call
.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);
}
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 "$" " ./execve ./myecho"
argv[0]: ./myecho
argv[1]: hello
argv[2]: world
-.fi
+.EE
.in
-
+.PP
We can also use these programs to demonstrate the use of a script
interpreter.
To do this we create a script whose "interpreter" is our
.I myecho
program:
-
+.PP
.in +4n
-.nf
+.EX
.RB "$" " cat > script"
.B #!./myecho script-arg
.B ^D
.RB "$" " chmod +x script"
-.fi
+.EE
.in
-
+.PP
We can then use our program to exec the script:
-
+.PP
.in +4n
-.nf
+.EX
.RB "$" " ./execve ./script"
argv[0]: ./myecho
argv[1]: script-arg
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),
projects / thirdparty / man-pages.git / blobdiff