HardenedBSD/lib/libsys/execve.2

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.Dd January 26, 2022
.Dt EXECVE 2
.Os
.Sh NAME
.Nm execve ,
.Nm fexecve
.Nd execute a file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn execve "const char *path" "char *const argv[]" "char *const envp[]"
.Ft int
.Fn fexecve "int fd" "char *const argv[]" "char *const envp[]"
.Sh DESCRIPTION
The
.Fn execve
system call
transforms the calling process into a new process.
The new process is constructed from an ordinary file,
whose name is pointed to by
.Fa path ,
called the
.Em new process file .
The
.Fn fexecve
system call is equivalent to
.Fn execve
except that the file to be executed is determined by the file
descriptor
.Fa fd
instead of a
.Fa path .
This file is either an executable object file,
or a file of data for an interpreter.
An executable object file consists of an identifying header,
followed by pages of data representing the initial program (text)
and initialized data pages.
Additional pages may be specified
by the header to be initialized with zero data; see
.Xr elf 5
and
.Xr a.out 5 .
.Pp
An interpreter file begins with a line of the form:
.Pp
.Bd -ragged -offset indent -compact
.Sy \&#!
.Em interpreter
.Bq Em arg
.Ed
.Pp
When an interpreter file is
.Sy execve Ap d ,
the system actually
.Sy execve Ap s
the specified
.Em interpreter .
If the optional
.Em arg
is specified, it becomes the first argument to the
.Em interpreter ,
and the name of the originally
.Sy execve Ap d
file becomes the second argument;
otherwise, the name of the originally
.Sy execve Ap d
file becomes the first argument.
The original arguments are shifted over to
become the subsequent arguments.
The zeroth argument is set to the specified
.Em interpreter .
.Pp
The argument
.Fa argv
is a pointer to a null-terminated array of
character pointers to null-terminated character strings.
These strings construct the argument list to be made available to the new
process.
At least one argument must be present in
the array; by custom, the first element should be
the name of the executed program (for example, the last component of
.Fa path ) .
.Pp
The argument
.Fa envp
is also a pointer to a null-terminated array of
character pointers to null-terminated strings.
A pointer to this array is normally stored in the global variable
.Va environ .
These strings pass information to the
new process that is not directly an argument to the command (see
.Xr environ 7 ) .
.Pp
File descriptors open in the calling process image remain open in
the new process image, except for those for which the close-on-exec
flag is set (see
.Xr close 2
and
.Xr fcntl 2 ) .
Descriptors that remain open are unaffected by
.Fn execve .
If any of the standard descriptors (0, 1, and/or 2) are closed at the
time
.Fn execve
is called, and the process will gain privilege as a result of set-id
semantics, those descriptors will be re-opened automatically.
No programs, whether privileged or not, should assume that these descriptors
will remain closed across a call to
.Fn execve .
.Pp
Signals set to be ignored in the calling process are set to be ignored in
the
new process.
Signals which are set to be caught in the calling process image
are set to default action in the new process image.
Blocked signals remain blocked regardless of changes to the signal action.
The signal stack is reset to be undefined (see
.Xr sigaction 2
for more information).
.Pp
If the set-user-ID mode bit of the new process image file is set
(see
.Xr chmod 2 ) ,
the effective user ID of the new process image is set to the owner ID
of the new process image file.
If the set-group-ID mode bit of the new process image file is set,
the effective group ID of the new process image is set to the group ID
of the new process image file.
(The effective group ID is the first element of the group list.)
The real user ID, real group ID and
other group IDs of the new process image remain the same as the calling
process image.
After any set-user-ID and set-group-ID processing,
the effective user ID is recorded as the saved set-user-ID,
and the effective group ID is recorded as the saved set-group-ID.
These values may be used in changing the effective IDs later (see
.Xr setuid 2 ) .
.Pp
The set-ID bits are not honored if the respective file system has the
.Cm nosuid
option enabled or if the new process file is an interpreter file.
Syscall
tracing is disabled if effective IDs are changed.
.Pp
The new process also inherits the following attributes from
the calling process:
.Pp
.Bl -column parent_process_ID -offset indent -compact
.It process ID Ta see Xr getpid 2
.It parent process ID Ta see Xr getppid 2
.It process group ID Ta see Xr getpgrp 2
.It access groups Ta see Xr getgroups 2
.It working directory Ta see Xr chdir 2
.It root directory Ta see Xr chroot 2
.It control terminal Ta see Xr termios 4
.It resource usages Ta see Xr getrusage 2
.It interval timers Ta see Xr getitimer 2
.It resource limits Ta see Xr getrlimit 2
.It file mode mask Ta see Xr umask 2
.It signal mask Ta see Xr sigaction 2 ,
.Xr sigprocmask 2
.El
.Pp
When a program is executed as a result of an
.Fn execve
system call, it is entered as follows:
.Bd -literal -offset indent
main(argc, argv, envp)
int argc;
char **argv, **envp;
.Ed
.Pp
where
.Fa argc
is the number of elements in
.Fa argv
(the ``arg count'')
and
.Fa argv
points to the array of character pointers
to the arguments themselves.
.Pp
The
.Fn fexecve
ignores the file offset of
.Fa fd .
Since execute permission is checked by
.Fn fexecve ,
the file descriptor
.Fa fd
need not have been opened with the
.Dv O_EXEC
flag.
However, if the file to be executed denies read permission for the process
preparing to do the exec, the only way to provide the
.Fa fd
to
.Fn fexecve
is to use the
.Dv O_EXEC
flag when opening
.Fa fd .
Note that the file to be executed can not be open for writing.
.Sh RETURN VALUES
As the
.Fn execve
system call overlays the current process image
with a new process image the successful call
has no process to return to.
If
.Fn execve
does return to the calling process an error has occurred; the
return value will be -1 and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn execve
system call
will fail and return to the calling process if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOEXEC
When invoking an interpreted script, the length of the first line,
inclusive of the
.Sy \&#!
prefix and terminating newline, exceeds
.Dv MAXSHELLCMDLEN
characters.
.It Bq Er ENOENT
The new process file does not exist.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.It Bq Er EACCES
The new process file is not an ordinary file.
.It Bq Er EACCES
The new process file mode denies execute permission.
.It Bq Er EINVAL
.Fa argv
did not contain at least one element.
.It Bq Er ENOEXEC
The new process file has the appropriate access
permission, but has an invalid magic number in its header.
.It Bq Er ETXTBSY
The new process file is a pure procedure (shared text)
file that is currently open for writing by some process.
.It Bq Er ENOMEM
The new process requires more virtual memory than
is allowed by the imposed maximum
.Pq Xr getrlimit 2 .
.It Bq Er E2BIG
The number of bytes in the new process' argument list
is larger than the system-imposed limit.
This limit is specified by the
.Xr sysctl 3
MIB variable
.Dv KERN_ARGMAX .
.It Bq Er EFAULT
The new process file is not as long as indicated by
the size values in its header.
.It Bq Er EFAULT
The
.Fa path ,
.Fa argv ,
or
.Fa envp
arguments
point
to an illegal address.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.El
.Pp
In addition, the
.Fn fexecve
will fail and return to the calling process if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument is not a valid file descriptor open for executing.
.El
.Sh SEE ALSO
.Xr ktrace 1 ,
.Xr _exit 2 ,
.Xr fork 2 ,
.Xr open 2 ,
.Xr execl 3 ,
.Xr exit 3 ,
.Xr sysctl 3 ,
.Xr a.out 5 ,
.Xr elf 5 ,
.Xr fdescfs 5 ,
.Xr environ 7 ,
.Xr mount 8
.Sh STANDARDS
The
.Fn execve
system call conforms to
.St -p1003.1-2001 ,
with the exception of reopening descriptors 0, 1, and/or 2 in certain
circumstances.
A future update of the Standard is expected to require this behavior,
and it may become the default for non-privileged processes as well.
.\" NB: update this caveat when TC1 is blessed.
The support for executing interpreted programs is an extension.
The
.Fn fexecve
system call conforms to The Open Group Extended API Set 2 specification.
.Sh HISTORY
The
.Fn execve
system call appeared in
.At v7 .
The
.Fn fexecve
system call appeared in
.Fx 8.0 .
.Sh CAVEATS
If a program is
.Em setuid
to a non-super-user, but is executed when
the real
.Em uid
is ``root'', then the program has some of the powers
of a super-user as well.
.Pp
When executing an interpreted program through
.Fn fexecve ,
kernel supplies
.Pa /dev/fd/n
as a second argument to the interpreter,
where
.Ar n
is the file descriptor passed in the
.Fa fd
argument to
.Fn fexecve .
For this construction to work correctly, the
.Xr fdescfs 5
filesystem shall be mounted on
.Pa /dev/fd .