Index: head/libexec/rtld-elf/rtld.1
===================================================================
--- head/libexec/rtld-elf/rtld.1	(revision 319137)
+++ head/libexec/rtld-elf/rtld.1	(revision 319138)
@@ -1,303 +1,374 @@
 .\" Copyright (c) 1995 Paul Kranenburg
 .\" All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\" 3. All advertising materials mentioning features or use of this software
 .\"    must display the following acknowledgment:
 .\"      This product includes software developed by Paul Kranenburg.
 .\" 3. The name of the author may not be used to endorse or promote products
 .\"    derived from this software without specific prior written permission
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
 .\" $FreeBSD$
 .\"
 .Dd May 20, 2017
 .Dt RTLD 1
 .Os
 .Sh NAME
 .Nm ld-elf.so.1 ,
 .Nm ld.so ,
 .Nm rtld
 .Nd run-time link-editor
 .Sh DESCRIPTION
 The
 .Nm
 utility is a self-contained shared object providing run-time
 support for loading and link-editing shared objects into a process'
 address space.
 It is also commonly known as the dynamic linker.
 It uses the data structures
 contained within dynamically linked programs to determine which shared
 libraries are needed and loads them using the
 .Xr mmap 2
 system call.
 .Pp
 After all shared libraries have been successfully loaded,
 .Nm
 proceeds to resolve external references from both the main program and
 all objects loaded.
 A mechanism is provided for initialization routines
 to be called on a per-object basis, giving a shared object an opportunity
 to perform any extra set-up before execution of the program proper begins.
 This is useful for C++ libraries that contain static constructors.
 .Pp
 When resolving dependencies for the loaded objects,
 .Nm
 translates dynamic token strings in rpath and soname.
 If the
 .Fl "z origin"
 option of the static linker was set when linking the binary,
 the token expansion is performed at the object load time, see
 .Xr ld 1 .
 The following strings are recognized now:
 .Bl -tag -width ".Pa $PLATFORM"
 .It Pa $ORIGIN
 Translated to the full path of the loaded object.
 .It Pa $OSNAME
 Translated to the name of the operating system implementation.
 .It Pa $OSREL
 Translated to the release level of the operating system.
 .It Pa $PLATFORM
 Translated to the machine hardware platform.
 .El
 .Pp
 The
 .Nm
 utility itself is loaded by the kernel together with any dynamically-linked
 program that is to be executed.
 The kernel transfers control to the
 dynamic linker.
 After the dynamic linker has finished loading,
 relocating, and initializing the program and its required shared
 objects, it transfers control to the entry point of the program.
 The following search order is used to locate required shared objects:
 .Pp
 .Bl -enum -offset indent -compact
 .It
 .Dv DT_RPATH
 of the referencing object unless that object also contains a
 .Dv DT_RUNPATH
 tag
 .It
 .Dv DT_RPATH
 of the program unless the referencing object contains a
 .Dv DT_RUNPATH
 tag
 .It
 Path indicated by
 .Ev LD_LIBRARY_PATH
 environment variable
 .It
 .Dv DT_RUNPATH
 of the referencing object
 .It
 Hints file produced by the
 .Xr ldconfig 8
 utility
 .It
 The
 .Pa /lib
 and
 .Pa /usr/lib
 directories, unless the referencing object was linked using the
 .Dq Fl z Ar nodefaultlib
 option
 .El
 .Pp
 The
 .Nm
 utility
 recognizes a number of environment variables that can be used to modify
 its behaviour.
 On 64-bit architectures, the linker for 32-bit objects recognizes
 all the environment variables listed below, but is being prefixed with
 .Ev LD_32_ ,
 for example:
 .Ev LD_32_TRACE_LOADED_OBJECTS .
 .Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
 .It Ev LD_DUMP_REL_POST
 If set,
 .Nm
 will print a table containing all relocations after symbol
 binding and relocation.
 .It Ev LD_DUMP_REL_PRE
 If set,
 .Nm
 will print a table containing all relocations before symbol
 binding and relocation.
 .It Ev LD_LIBMAP
 A library replacement list in the same format as
 .Xr libmap.conf 5 .
 For convenience, the characters
 .Ql =
 and
 .Ql \&,
 can be used instead of a space and a newline.
 This variable is parsed after
 .Xr libmap.conf 5 ,
 and will override its entries.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_LIBMAP_DISABLE
 If set, disables the use of
 .Xr libmap.conf 5
 and
 .Ev LD_LIBMAP .
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_ELF_HINTS_PATH
 This variable will override the default location of
 .Dq hints
 file.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_LIBRARY_PATH
 A colon separated list of directories, overriding the default search path
 for shared libraries.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_LIBRARY_PATH_RPATH
 If the variable is specified and has a value starting with
 any of \'y\', \'Y\' or \'1\' symbols, the path specified by
 .Ev LD_LIBRARY_PATH
 variable is allowed to override the path from
 .Dv DT_RPATH
 for binaries which does not contain
 .Dv DT_RUNPATH
 tag.
 For such binaries, when the variable
 .Ev LD_LIBRARY_PATH_RPATH
 is set,
 .Dq Fl z Ar nodefaultlib
 link-time option is ignored as well.
 .It Ev LD_PRELOAD
 A list of shared libraries, separated by colons and/or white space,
 to be linked in before any
 other shared libraries.
 If the directory is not specified then
 the directories specified by
 .Ev LD_LIBRARY_PATH
 will be searched first
 followed by the set of built-in standard directories.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_LIBRARY_PATH_FDS
 A colon separated list of file descriptor numbers for library directories.
 This is intended for use within
 .Xr capsicum 4
 sandboxes, when global namespaces such as the filesystem are unavailable.
 It is consulted just after LD_LIBRARY_PATH.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_BIND_NOT
 When set to a nonempty string, prevents modifications of the PLT slots when
 doing bindings.
 As result, each call of the PLT-resolved function is resolved.
 In combination with debug output, this provides complete account of
 all bind actions at runtime.
 This variable is unset for set-user-ID and set-group-ID programs.
 .It Ev LD_BIND_NOW
 When set to a nonempty string, causes
 .Nm
 to relocate all external function calls before starting execution of the
 program.
 Normally, function calls are bound lazily, at the first call
 of each function.
 .Ev LD_BIND_NOW
 increases the start-up time of a program, but it avoids run-time
 surprises caused by unexpectedly undefined functions.
 .It Ev LD_TRACE_LOADED_OBJECTS
 When set to a nonempty string, causes
 .Nm
 to exit after loading the shared objects and printing a summary which includes
 the absolute pathnames of all objects, to standard output.
 .It Ev LD_TRACE_LOADED_OBJECTS_ALL
 When set to a nonempty string, causes
 .Nm
 to expand the summary to indicate which objects caused each object to
 be loaded.
 .It Ev LD_TRACE_LOADED_OBJECTS_FMT1
 .It Ev LD_TRACE_LOADED_OBJECTS_FMT2
 When set, these variables are interpreted as format strings a la
 .Xr printf 3
 to customize the trace output and are used by
 .Xr ldd 1 Ns 's
 .Fl f
 option and allows
 .Xr ldd 1
 to be operated as a filter more conveniently.
 If the dependency name starts with string
 .Pa lib ,
 .Ev LD_TRACE_LOADED_OBJECTS_FMT1
 is used, otherwise
 .Ev LD_TRACE_LOADED_OBJECTS_FMT2
 is used.
 The following conversions can be used:
 .Bl -tag -width 4n
 .It Li %a
 The main program's name
 (also known as
 .Dq __progname ) .
 .It Li \&%A
 The value of the environment variable
 .Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
 Typically used to print both the names of programs and shared libraries
 being inspected using
 .Xr ldd 1 .
 .It Li %o
 The library name.
 .It Li %p
 The full pathname as determined by
 .Nm rtld Ns 's
 library search rules.
 .It Li %x
 The library's load address.
 .El
 .Pp
 Additionally,
 .Ql \en
 and
 .Ql \et
 are recognized and have their usual meaning.
 .It Ev LD_UTRACE
 If set,
 .Nm
 will log events such as the loading and unloading of shared objects via
 .Xr utrace 2 .
 .It Ev LD_LOADFLTR
 If set,
 .Nm
 will process the filtee dependencies of the loaded objects immediately,
 instead of postponing it until required.
 Normally, the filtees are opened at the time of the first symbol resolution
 from the filter object.
 .El
+.Sh DIRECT EXECUTION MODE
+.Nm
+is typically used implicitly, loaded by the kernel as requested by the
+.Dv PT_INTERP
+program header of the executed binary.
+.Fx
+also supports a direct execution mode for the dynamic linker.
+In this mode, the user explicitly executes
+.Nm
+and provides the path of the program to be linked and executed as
+an argument.
+This mode allows use of a non-standard dynamic linker for a program
+activation without changing the binary or without changing
+the installed dynamic linker.
+Execution options may be specified.
+.Pp
+The syntax of the direct invocation is
+.Bd -ragged -offset indent
+.Pa /libexec/ld-elf.so.1
+.Op Fl f Ar fd
+.Op Fl p
+.Op Fl -
+.Pa image_path
+.Op Ar image arguments
+.Ed
+.Pp
+The options are as follows:
+.Bl -tag -width indent
+.It Fl f Ar fd
+File descriptor
+.Ar fd
+references the binary to be activated by
+.Nm .
+It must already be opened in the process when executing
+.Nm .
+If this option is specified,
+.Ar image_path
+is only used to provide the
+.Va argv[0]
+value to the program.
+.It Fl p
+If the
+.Pa image_path
+argument specifies a name which does not contain a slash
+.Dq Li /
+character,
+.Nm
+uses the search path provided by the environment variable
+.Dv PATH
+to find the binary to execute.
+.It Fl -
+Ends the
+.Nm
+options.
+The argument following
+.Fl -
+is interpreted as the path of binary to execute.
+.El
+.Pp
+To conform to user expectation to not break some naively restricted
+execution environments, in the direct execution mode
+.Nm
+emulates verification of the binary execute permission
+for current user.
+The verification only uses Unix
+.Dv DACs ,
+ignores
+.Dv ACLs
+and is racy by its nature.
+The environments which rely on such restrictions are weak
+and breakable on its own.
 .Sh FILES
 .Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact
 .It Pa /var/run/ld-elf.so.hints
 Hints file.
 .It Pa /var/run/ld-elf32.so.hints
 Hints file for 32-bit binaries on 64-bit system.
 .It Pa /etc/libmap.conf
 The libmap configuration file.
 .It Pa /etc/libmap32.conf
 The libmap configuration file for 32-bit binaries on 64-bit system.
 .El
 .Sh SEE ALSO
 .Xr ld 1 ,
 .Xr ldd 1 ,
 .Xr capsicum 4 ,
 .Xr elf 5 ,
 .Xr libmap.conf 5 ,
 .Xr ldconfig 8